[PATCH v4 00/19] selftests/mm: improve khugepaged coverage

[PATCH v4 00/19] selftests/mm: improve khugepaged coverage

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

mTHP collapse went in for 7.2 with no functional selftest coverage. Every
khugepaged collapse case is PMD-shaped, down to the detection: it reads
smaps AnonHugePages, which cannot see anything below the PMD order.

I am also reworking khugepaged's collapse mechanism, and that wants a suite
worth trusting before the mechanism changes underneath it. Nothing here
depends on the rework: everything passes on an unmodified kernel and
documents what khugepaged already does.

The table of cases is raised to 256 entries first. It holds 64, and a full
invocation -- every context crossed with every memory type, which needs a
directory for the file and shmem ones -- already registers 63, so the next
case added anywhere aborts the whole suite before a single test runs.

Five existing cases fail on the environment rather than on the kernel:

 - The collapse wait is a fixed three seconds, whatever a huge page costs
   to build. Scale it with the PMD size.

 - collapse_compound_extreme() wants a 512M folio from the fault path. Skip
   it where the PMD is that large.

 - The page cache caps folio order below the PMD order where a PMD is 512M,
   so nothing can hold a PMD-sized page cache folio. Skip the shmem and
   file cases there.

 - The swap cases ask a best-effort MADV_PAGEOUT for an exact swap count,
   and khugepaged reads the pages back in while they ask. Retry, hold the
   range out of the daemon's reach while retrying, and skip if the pages
   still will not go.

 - The MADV_COLLAPSE cases leave khugepaged running beside them, so a case
   can pass on the daemon's work rather than on its own.

This is why the suite now runs on arm64 with 64K pages.

Then the new coverage:

 - Folio-order helpers in vm_util, and folio_order_check to validate them
   against the kernel before any collapse test trusts them.

 - Four order-parameterized anon collapse cases, plus mixed sources by
   source order, at every supported order by default.

 - A shared-source write race: a co-sharer writing throughout a collapse
   and the collapsing side must not see each other's pages.

 - khugepaged_full_pass(), which steps the daemon one scan at a time
   through the sysfs wake path, and khugepaged_sync_check to hold it to
   exactly one attributed collapse per step.

 - khugepaged_race, racing faults, MADV_DONTNEED, pins, fork, mremap,
   reclaim and compaction against collapse, at both ends of the occupancy
   scale and over whole-table zaps, checked by content and by KASAN and
   lockdep.

Changes since v3:

 - New patch 6 stops khugepaged during the MADV_COLLAPSE cases: the
   per-order controls have to be cleared too, not only the global ones.

 - The shared-source write race did not race. The two sides now hand off,
   and the co-sharer unshares one page at a time (Sashiko).

 - khugepaged_sync_check no longer counts a tracepoint only the collapse
   rework emits.

 - khugepaged_race loses -z and -p: both occupancy limits and both
   pressure states always run.

 - -c refused the PMD order it advertised.

 - Smaller: two unchecked madvise() calls, and thread mask bits that had
   to be renumbered by hand.

 - The changelogs are rewritten throughout (Mike Rapoport).

Tested on an unmodified mm-new (33f61b12d297), with KASAN, lockdep,
DEBUG_VM and page_table_check, in 16G guests:

  x86-64 4K     181 pass, 1 skip, 0 fail
  arm64 64K     215 pass, 5 skip, 0 fail

The skips are structural: mixed sources declines at the smallest order, and
at 64K collapse_compound_extreme() skips as above. arm64 runs fewer cases
than x86-64 rather than skipping them: with no PMD-order page cache folio
the file and shmem contexts are never registered there.

Kiryl Shutsemau (Meta) (19):
  selftests/mm: raise the khugepaged test-case cap
  selftests/mm: skip collapse_compound_extreme() where the PMD is too
    large
  selftests/mm: scale khugepaged's collapse wait with the PMD size
  selftests/mm: skip khugepaged page cache cases without a PMD folio
  selftests/mm: make the swap cases' swapout reliable
  selftests/mm: stop khugepaged during the MADV_COLLAPSE cases
  selftests/mm: move is_backed_by_folio() into vm_util
  selftests/mm: add folio-order check for address ranges
  selftests/mm: add folio-order detection self-check
  selftests/mm: add khugepaged completion barrier helper
  selftests/mm: add order-parameterized khugepaged collapse cases
  selftests/mm: parameterize the mixed-source collapse case by source
    order
  selftests/mm: cover a shared-source collapse write race
  selftests/mm: run every supported collapse order by default
  selftests/mm: check that one khugepaged pass collapses one window
  selftests/mm: add khugepaged race harness
  selftests/mm: race collapse of windows with holes
  selftests/mm: add memory-pressure threads to the khugepaged race
    harness
  selftests/mm: zap whole PTE tables in the khugepaged race harness

 tools/testing/selftests/mm/Makefile           |   3 +
 .../testing/selftests/mm/folio_order_check.c  | 137 ++++
 tools/testing/selftests/mm/hmm-tests.c        |   1 -
 .../testing/selftests/mm/hugepage_settings.c  |  74 ++-
 .../testing/selftests/mm/hugepage_settings.h  |   3 +
 tools/testing/selftests/mm/khugepaged.c       | 524 +++++++++++++++-
 tools/testing/selftests/mm/khugepaged_race.c  | 589 ++++++++++++++++++
 .../selftests/mm/khugepaged_sync_check.c      | 196 ++++++
 tools/testing/selftests/mm/migration.c        |   1 -
 tools/testing/selftests/mm/run_vmtests.sh     |   8 +-
 .../selftests/mm/split_huge_page_test.c       |  62 --
 tools/testing/selftests/mm/vm_util.c          | 145 +++++
 tools/testing/selftests/mm/vm_util.h          |  10 +
 13 files changed, 1651 insertions(+), 102 deletions(-)
 create mode 100644 tools/testing/selftests/mm/folio_order_check.c
 create mode 100644 tools/testing/selftests/mm/khugepaged_race.c
 create mode 100644 tools/testing/selftests/mm/khugepaged_sync_check.c


base-commit: 33f61b12d297562321533c048e034b1fb21c1cf3
-- 
2.54.0

[PATCH v4 01/19] selftests/mm: raise the khugepaged test-case cap

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

TEST() ends the run with "MAX_TEST_CASES is too small" when the table
fills, and the table holds 64.  A full invocation already registers 63, so
the next case added anywhere aborts the whole suite before a single test
runs.

Raise the cap to 256.  The table is a static array of small structs, so the
room costs nothing worth counting.

Assisted-by: Claude-Code:claude-opus-5
Acked-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index d3a53673e1f9..6cfec2b940ac 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1290,7 +1290,7 @@ struct test_case {
 	test_fn fn;
 };
 
-#define MAX_TEST_CASES 64
+#define MAX_TEST_CASES 256
 static struct test_case test_cases[MAX_TEST_CASES];
 static int nr_test_cases;
 
-- 
2.54.0

Re: [PATCH v4 01/19] selftests/mm: raise the khugepaged test-case cap

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:43AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> TEST() ends the run with "MAX_TEST_CASES is too small" when the table
> fills, and the table holds 64.  A full invocation already registers 63, so
> the next case added anywhere aborts the whole suite before a single test
> runs.
>
> Raise the cap to 256.  The table is a static array of small structs, so the
> room costs nothing worth counting.
>
> Assisted-by: Claude-Code:claude-opus-5
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index d3a53673e1f9..6cfec2b940ac 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -1290,7 +1290,7 @@ struct test_case {
>  	test_fn fn;
>  };
>
> -#define MAX_TEST_CASES 64
> +#define MAX_TEST_CASES 256
>  static struct test_case test_cases[MAX_TEST_CASES];
>  static int nr_test_cases;
>
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 02/19] selftests/mm: skip collapse_compound_extreme() where the PMD is too large

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

collapse_compound_extreme() builds a PTE table full of distinct PTE-mapped
compound pages by cycling hpage_pmd_nr fault-time THPs through mremap.  It
therefore needs hpage_pmd_nr PMD-order allocations in a row.  That is fine
at a 2M PMD (4K base pages) or a 32M one (16K).  A 512M PMD -- arm64 with
64K base pages -- makes each of those an order-13 allocation, which the
allocator cannot reliably hand out even once, let alone 8192 times.

The failure is not a quiet one: the case calls ksft_exit_fail_msg(), so the
whole binary stops and every case after it is lost.

Skip the case where the PMD is larger than 32M.  The MADV_COLLAPSE cases
still cover PMD-order collapse on those configurations, and 4K and 16K
PMDs are unaffected.

Assisted-by: Claude-Code:claude-opus-5
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 6cfec2b940ac..dd924edd8557 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -940,6 +940,16 @@ static void collapse_compound_extreme(struct collapse_context *c, struct mem_ops
 	void *p;
 	int i;
 
+	/*
+	 * The test needs hpage_pmd_nr PMD-order allocations, which is likely to
+	 * fail for large PMD sizes.  Skip if the PMD size is over 32M.
+	 */
+	if (hpage_pmd_size > (32UL << 20)) {
+		ksft_test_result_skip("%s: PMD too large for fault-time THP construction\n",
+				      __func__);
+		return;
+	}
+
 	p = ops->setup_area(1);
 	ksft_print_msg("Construct PTE page table full of different PTE-mapped compound pages\n");
 	for (i = 0; i < hpage_pmd_nr; i++) {
-- 
2.54.0

Re: [PATCH v4 02/19] selftests/mm: skip collapse_compound_extreme() where the PMD is too large

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:44AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> collapse_compound_extreme() builds a PTE table full of distinct PTE-mapped
> compound pages by cycling hpage_pmd_nr fault-time THPs through mremap.  It
> therefore needs hpage_pmd_nr PMD-order allocations in a row.  That is fine
> at a 2M PMD (4K base pages) or a 32M one (16K).  A 512M PMD -- arm64 with
> 64K base pages -- makes each of those an order-13 allocation, which the
> allocator cannot reliably hand out even once, let alone 8192 times.
>
> The failure is not a quiet one: the case calls ksft_exit_fail_msg(), so the
> whole binary stops and every case after it is lost.
>
> Skip the case where the PMD is larger than 32M.  The MADV_COLLAPSE cases
> still cover PMD-order collapse on those configurations, and 4K and 16K
> PMDs are unaffected.
>
> Assisted-by: Claude-Code:claude-opus-5
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Ah yeah again we are hit with the consequences of assumptions around PMD
size which do not apply for 64 KiB page size :)

Seems sensible to me so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 6cfec2b940ac..dd924edd8557 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -940,6 +940,16 @@ static void collapse_compound_extreme(struct collapse_context *c, struct mem_ops
>  	void *p;
>  	int i;
>
> +	/*
> +	 * The test needs hpage_pmd_nr PMD-order allocations, which is likely to
> +	 * fail for large PMD sizes.  Skip if the PMD size is over 32M.
> +	 */
> +	if (hpage_pmd_size > (32UL << 20)) {
> +		ksft_test_result_skip("%s: PMD too large for fault-time THP construction\n",
> +				      __func__);
> +		return;
> +	}
> +
>  	p = ops->setup_area(1);
>  	ksft_print_msg("Construct PTE page table full of different PTE-mapped compound pages\n");
>  	for (i = 0; i < hpage_pmd_nr; i++) {
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 03/19] selftests/mm: scale khugepaged's collapse wait with the PMD size

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

wait_for_scan() gives every case the same three seconds, whatever the huge
page costs to build.  collapse_full() asks for four of them: 8M at a 2M
PMD, but 2G at a 512M PMD -- arm64 with 64K base pages.

Three seconds is thin at that size rather than generous.  Across 80 runs of
collapse_full() on arm64 with 64K pages the wait was half a second in 73 of
them, with a tail to two seconds.  The case has also timed out in a full
matrix run, reporting a failure for a collapse that was still going.

Keep three seconds as the floor and add a second per 128M collapsed.  A 2M
PMD is unchanged.  A 512M PMD gets 19 seconds.

arm64/64K: khugepaged all:anon 21 pass/1 fail -> 22 pass/0 fail.  x86-64 is
unchanged.

Assisted-by: Claude-Code:claude-opus-5
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index dd924edd8557..c499804a0ec4 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -561,8 +561,10 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
 		int nr_hpages, int collap_order, struct mem_ops *ops)
 {
 	unsigned long hpage_size = page_size << collap_order;
+	/* Three seconds as a floor, plus a second per 128M to collapse */
+	const unsigned long bytes = (unsigned long)nr_hpages * hpage_size;
+	int timeout = 6 + 2 * (bytes / (128UL << 20));
 	int full_scans;
-	int timeout = 6; /* 3 seconds */
 
 	/* Sanity check */
 	if (!ops->check_huge(p, len, 0, hpage_size))
-- 
2.54.0

Re: [PATCH v4 03/19] selftests/mm: scale khugepaged's collapse wait with the PMD size

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:45AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> wait_for_scan() gives every case the same three seconds, whatever the huge
> page costs to build.  collapse_full() asks for four of them: 8M at a 2M
> PMD, but 2G at a 512M PMD -- arm64 with 64K base pages.
>
> Three seconds is thin at that size rather than generous.  Across 80 runs of
> collapse_full() on arm64 with 64K pages the wait was half a second in 73 of
> them, with a tail to two seconds.  The case has also timed out in a full
> matrix run, reporting a failure for a collapse that was still going.
>
> Keep three seconds as the floor and add a second per 128M collapsed.  A 2M
> PMD is unchanged.  A 512M PMD gets 19 seconds.

Yikes that does make this run really slow for 64 KiB page size machines and a
key aim in the selftests is that they don't take too egregiously long to run.

Can this somehow be trimmed some other way?

>
> arm64/64K: khugepaged all:anon 21 pass/1 fail -> 22 pass/0 fail.  x86-64 is
> unchanged.
>
> Assisted-by: Claude-Code:claude-opus-5
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

In general though the patch seems reasonable so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index dd924edd8557..c499804a0ec4 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -561,8 +561,10 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>  		int nr_hpages, int collap_order, struct mem_ops *ops)
>  {
>  	unsigned long hpage_size = page_size << collap_order;
> +	/* Three seconds as a floor, plus a second per 128M to collapse */
> +	const unsigned long bytes = (unsigned long)nr_hpages * hpage_size;
> +	int timeout = 6 + 2 * (bytes / (128UL << 20));
>  	int full_scans;
> -	int timeout = 6; /* 3 seconds */
>
>  	/* Sanity check */
>  	if (!ops->check_huge(p, len, 0, hpage_size))
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 04/19] selftests/mm: skip khugepaged page cache cases without a PMD folio

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The page cache caps folio order at MAX_PAGECACHE_ORDER, which is smaller
than the PMD order on arm64 with 64K pages, where a PMD is 512M.  A
PMD-sized page cache folio is impossible there, so MADV_COLLAPSE answers
-EINVAL and khugepaged passes over the range.  The shmem cases ask for a
PMD-sized folio anyway, so four of them fail and the run bails out in the
middle.

MAX_PAGECACHE_ORDER is not shmem-specific: it caps every file folio.  Skip
both mem types where the cap is below the PMD order.

Anonymous collapse is unaffected: its orders are not capped this way.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index c499804a0ec4..ec5c36a19d92 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1357,6 +1357,30 @@ int main(int argc, char **argv)
 
 	setbuf(stdout, NULL);
 
+	/*
+	 * The page cache caps folio order at MAX_PAGECACHE_ORDER, which is
+	 * below the PMD order on arm64 with 64K pages.  A PMD-sized page cache
+	 * folio is impossible there, so the kernel refuses these collapses by
+	 * design and there is nothing to test.  The cap is not shmem-specific:
+	 * it rules out regular files too, and the per-order shmem_enabled
+	 * controls exist for exactly the orders it allows, which is what makes
+	 * them readable here.
+	 */
+	if (!(thp_shmem_supported_orders() & (1UL << hpage_pmd_order))) {
+		if (shmem_ops) {
+			ksft_print_msg("no PMD-order page cache folio: skipping shmem\n");
+			shmem_ops = NULL;
+		}
+		if (read_only_file_ops) {
+			ksft_print_msg("no PMD-order page cache folio: skipping file\n");
+			read_only_file_ops = NULL;
+			read_write_file_read_ops = NULL;
+			read_write_file_write_ops = NULL;
+		}
+		if (!anon_ops && !shmem_ops && !read_only_file_ops)
+			ksft_exit_skip("Nothing left to collapse into\n");
+	}
+
 	default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
 	default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
 	default_settings.khugepaged.max_ptes_shared = hpage_pmd_nr / 2;
-- 
2.54.0

Re: [PATCH v4 04/19] selftests/mm: skip khugepaged page cache cases without a PMD folio

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:46AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> The page cache caps folio order at MAX_PAGECACHE_ORDER, which is smaller
> than the PMD order on arm64 with 64K pages, where a PMD is 512M.  A
> PMD-sized page cache folio is impossible there, so MADV_COLLAPSE answers
> -EINVAL and khugepaged passes over the range.  The shmem cases ask for a
> PMD-sized folio anyway, so four of them fail and the run bails out in the
> middle.
>
> MAX_PAGECACHE_ORDER is not shmem-specific: it caps every file folio.  Skip
> both mem types where the cap is below the PMD order.
>
> Anonymous collapse is unaffected: its orders are not capped this way.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index c499804a0ec4..ec5c36a19d92 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -1357,6 +1357,30 @@ int main(int argc, char **argv)
>
>  	setbuf(stdout, NULL);
>
> +	/*
> +	 * The page cache caps folio order at MAX_PAGECACHE_ORDER, which is
> +	 * below the PMD order on arm64 with 64K pages.  A PMD-sized page cache
> +	 * folio is impossible there, so the kernel refuses these collapses by
> +	 * design and there is nothing to test.  The cap is not shmem-specific:
> +	 * it rules out regular files too, and the per-order shmem_enabled
> +	 * controls exist for exactly the orders it allows, which is what makes
> +	 * them readable here.
> +	 */

This is a very schloppy comment. Can you trim it please?

> +	if (!(thp_shmem_supported_orders() & (1UL << hpage_pmd_order))) {

Is this inferring file-backed khugepaged behaviour from shmem? That seems iffy.

> +		if (shmem_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping shmem\n");
> +			shmem_ops = NULL;
> +		}
> +		if (read_only_file_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping file\n");
> +			read_only_file_ops = NULL;
> +			read_write_file_read_ops = NULL;
> +			read_write_file_write_ops = NULL;
> +		}
> +		if (!anon_ops && !shmem_ops && !read_only_file_ops)
> +			ksft_exit_skip("Nothing left to collapse into\n");
> +	}
> +
>  	default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
>  	default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
>  	default_settings.khugepaged.max_ptes_shared = hpage_pmd_nr / 2;
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 05/19] selftests/mm: make the swap cases' swapout reliable

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

collapse_swapin_single_pte() and collapse_max_ptes_swap() swap a range out
and then require smaps to report exactly the count they asked for.  Two
things keep that count from arriving.

MADV_PAGEOUT is best effort, so the count often turns up a moment late.

And wait_for_scan() leaves MADV_HUGEPAGE behind, so khugepaged is still
working on the range.  Collapsing a range with up to max_ptes_swap pages
swapped out means reading them back in, so the daemon empties the swap as
fast as the case fills it.  On arm64 with 64K pages max_ptes_swap is 1024
pages, which is 64M a step, and the case loses:

    # Swapout 1024 of 8192 pages... Fail
    not ok 10 collapse_max_ptes_swap

Ask again for up to two seconds, with the range held out of the daemon's
reach while asking.  The collapse each case runs next puts MADV_HUGEPAGE
back, so only the setup is affected.

If the pages still will not go, skip.  A machine with no swap, or swap too
small, full, capped by a memcg or busy with writeback, is not the kernel
under test refusing.  An error from madvise() itself still ends the run.

Assisted-by: Claude-Code:claude-opus-5
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 53 +++++++++++++++++++------
 1 file changed, 41 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index ec5c36a19d92..7eb9db0005a0 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -219,6 +219,41 @@ static bool check_swap(void *addr, unsigned long size)
 	return swap;
 }
 
+/*
+ * Page the range out and wait for the swap count to say so.
+ *
+ * Two things get in the way.  MADV_PAGEOUT is best effort:
+ * shrink_folio_list() leaves a folio alone when it cannot reclaim it right
+ * away, and one still under writeback from an earlier pageout is the common
+ * case, so the count the caller asks for arrives a moment later.  And a range
+ * an earlier collapse left MADV_HUGEPAGE is one khugepaged is still working
+ * on: collapsing a range with up to max_ptes_swap pages swapped out means
+ * reading those pages back in, so the daemon undoes the pageout as fast as it
+ * is asked for.  Keep the range out of its reach; the collapse the caller runs
+ * next puts MADV_HUGEPAGE back.
+ *
+ * Failing to get the pages out is the machine's answer, not the kernel's --
+ * swap too small, swap full, a memcg cap, a folio still under writeback -- so
+ * callers skip rather than fail.  An error from madvise() is different, and
+ * ends the run here.
+ */
+static bool swapout_range(void *p, unsigned long size)
+{
+	int i;
+
+	if (madvise(p, size, MADV_NOHUGEPAGE))
+		ksft_exit_fail_perror("madvise(MADV_NOHUGEPAGE)");
+
+	for (i = 0; i < 40; i++) {
+		if (madvise(p, size, MADV_PAGEOUT))
+			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
+		if (check_swap(p, size))
+			return true;
+		usleep(50 * 1000);
+	}
+	return false;
+}
+
 static void *alloc_mapping(int nr)
 {
 	void *p;
@@ -827,12 +862,10 @@ static void collapse_swapin_single_pte(struct collapse_context *c, struct mem_op
 	ops->fault(p, 0, hpage_pmd_size);
 
 	ksft_print_msg("Swapout one page...");
-	if (madvise(p, page_size, MADV_PAGEOUT))
-		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-	if (check_swap(p, page_size)) {
+	if (swapout_range(p, page_size)) {
 		success("OK");
 	} else {
-		fail("Fail");
+		skip("Could not swap out");
 		goto out;
 	}
 
@@ -853,12 +886,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 	ops->fault(p, 0, hpage_pmd_size);
 
 	ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap + 1, hpage_pmd_nr);
-	if (madvise(p, (max_ptes_swap + 1) * page_size, MADV_PAGEOUT))
-		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-	if (check_swap(p, (max_ptes_swap + 1) * page_size)) {
+	if (swapout_range(p, (max_ptes_swap + 1) * page_size)) {
 		success("OK");
 	} else {
-		fail("Fail");
+		skip("Could not swap out");
 		goto out;
 	}
 
@@ -870,12 +901,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 		ops->fault(p, 0, hpage_pmd_size);
 		ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
 		       hpage_pmd_nr);
-		if (madvise(p, max_ptes_swap * page_size, MADV_PAGEOUT))
-			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-		if (check_swap(p, max_ptes_swap * page_size)) {
+		if (swapout_range(p, max_ptes_swap * page_size)) {
 			success("OK");
 		} else {
-			fail("Fail");
+			skip("Could not swap out");
 			goto out;
 		}
 
-- 
2.54.0

Re: [PATCH v4 05/19] selftests/mm: make the swap cases' swapout reliable

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:47AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> collapse_swapin_single_pte() and collapse_max_ptes_swap() swap a range out
> and then require smaps to report exactly the count they asked for.  Two
> things keep that count from arriving.
>
> MADV_PAGEOUT is best effort, so the count often turns up a moment late.
>
> And wait_for_scan() leaves MADV_HUGEPAGE behind, so khugepaged is still
> working on the range.  Collapsing a range with up to max_ptes_swap pages
> swapped out means reading them back in, so the daemon empties the swap as
> fast as the case fills it.  On arm64 with 64K pages max_ptes_swap is 1024
> pages, which is 64M a step, and the case loses:
>
>     # Swapout 1024 of 8192 pages... Fail
>     not ok 10 collapse_max_ptes_swap
>
> Ask again for up to two seconds, with the range held out of the daemon's
> reach while asking.  The collapse each case runs next puts MADV_HUGEPAGE
> back, so only the setup is affected.
>
> If the pages still will not go, skip.  A machine with no swap, or swap too
> small, full, capped by a memcg or busy with writeback, is not the kernel
> under test refusing.  An error from madvise() itself still ends the run.
>
> Assisted-by: Claude-Code:claude-opus-5
> Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 53 +++++++++++++++++++------
>  1 file changed, 41 insertions(+), 12 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index ec5c36a19d92..7eb9db0005a0 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -219,6 +219,41 @@ static bool check_swap(void *addr, unsigned long size)
>  	return swap;
>  }
>
> +/*
> + * Page the range out and wait for the swap count to say so.
> + *
> + * Two things get in the way.  MADV_PAGEOUT is best effort:
> + * shrink_folio_list() leaves a folio alone when it cannot reclaim it right
> + * away, and one still under writeback from an earlier pageout is the common
> + * case, so the count the caller asks for arrives a moment later.  And a range
> + * an earlier collapse left MADV_HUGEPAGE is one khugepaged is still working
> + * on: collapsing a range with up to max_ptes_swap pages swapped out means
> + * reading those pages back in, so the daemon undoes the pageout as fast as it
> + * is asked for.  Keep the range out of its reach; the collapse the caller runs
> + * next puts MADV_HUGEPAGE back.
> + *
> + * Failing to get the pages out is the machine's answer, not the kernel's --
> + * swap too small, swap full, a memcg cap, a folio still under writeback -- so
> + * callers skip rather than fail.  An error from madvise() is different, and
> + * ends the run here.
> + */

This is a schloppy comment again. Please trim.

Walls of text are not wanted anywhere.

> +static bool swapout_range(void *p, unsigned long size)
> +{
> +	int i;
> +
> +	if (madvise(p, size, MADV_NOHUGEPAGE))
> +		ksft_exit_fail_perror("madvise(MADV_NOHUGEPAGE)");
> +
> +	for (i = 0; i < 40; i++) {
> +		if (madvise(p, size, MADV_PAGEOUT))
> +			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> +		if (check_swap(p, size))
> +			return true;
> +		usleep(50 * 1000);
> +	}
> +	return false;
> +}
> +
>  static void *alloc_mapping(int nr)
>  {
>  	void *p;
> @@ -827,12 +862,10 @@ static void collapse_swapin_single_pte(struct collapse_context *c, struct mem_op
>  	ops->fault(p, 0, hpage_pmd_size);
>
>  	ksft_print_msg("Swapout one page...");
> -	if (madvise(p, page_size, MADV_PAGEOUT))
> -		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> -	if (check_swap(p, page_size)) {
> +	if (swapout_range(p, page_size)) {
>  		success("OK");
>  	} else {
> -		fail("Fail");
> +		skip("Could not swap out");
>  		goto out;
>  	}
>
> @@ -853,12 +886,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
>  	ops->fault(p, 0, hpage_pmd_size);
>
>  	ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap + 1, hpage_pmd_nr);
> -	if (madvise(p, (max_ptes_swap + 1) * page_size, MADV_PAGEOUT))
> -		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> -	if (check_swap(p, (max_ptes_swap + 1) * page_size)) {
> +	if (swapout_range(p, (max_ptes_swap + 1) * page_size)) {
>  		success("OK");
>  	} else {
> -		fail("Fail");
> +		skip("Could not swap out");
>  		goto out;
>  	}
>
> @@ -870,12 +901,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
>  		ops->fault(p, 0, hpage_pmd_size);
>  		ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
>  		       hpage_pmd_nr);
> -		if (madvise(p, max_ptes_swap * page_size, MADV_PAGEOUT))
> -			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> -		if (check_swap(p, max_ptes_swap * page_size)) {
> +		if (swapout_range(p, max_ptes_swap * page_size)) {
>  			success("OK");
>  		} else {
> -			fail("Fail");
> +			skip("Could not swap out");
>  			goto out;
>  		}
>
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

__madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
It clears the global controls only, which is no longer enough.  A per-order
control overrides them, and -s leaves the source order at "always", so
khugepaged collapses the very range the case is working on.  The case then
fails on a collapse that was interfered with rather than refused.

Clear the per-order controls too.  Set them to "inherit", not "never".
khugepaged honours the global never and stays out.  A forced shmem collapse
takes the order it builds from these very controls, and still finds one.

Fixes: 9f0704eae8a4 ("selftests/mm/khugepaged: enlighten for multi-size THP")
Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 7eb9db0005a0..0008862e7cbc 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -550,8 +550,8 @@ static bool is_anon(struct mem_ops *ops)
 static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
 			       struct mem_ops *ops, bool expect)
 {
-	int ret;
 	struct thp_settings settings = *thp_current_settings();
+	int ret, i;
 
 	ksft_print_msg("%s...", msg);
 
@@ -564,9 +564,20 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
 	/*
 	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
 	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
+	 *
+	 * The per-order controls have to go too, not just the global one: a
+	 * source order left at "always" -- which -s does -- lets khugepaged
+	 * collapse the very range the case is working on.  Set them to
+	 * "inherit", not "never".  khugepaged honours the global never and
+	 * stays out.  A forced shmem collapse takes the order it builds from
+	 * these very controls, and still finds one.
 	 */
 	settings.thp_enabled = THP_NEVER;
 	settings.shmem_enabled = SHMEM_NEVER;
+	for (i = 0; i < NR_ORDERS; i++) {
+		settings.hugepages[i].enabled = THP_INHERIT;
+		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
+	}
 	thp_push_settings(&settings);
 
 	/* Clear VM_NOHUGEPAGE */
-- 
2.54.0

Re: [PATCH v4 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:48AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> __madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
> khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
> It clears the global controls only, which is no longer enough.  A per-order
> control overrides them, and -s leaves the source order at "always", so
> khugepaged collapses the very range the case is working on.  The case then
> fails on a collapse that was interfered with rather than refused.
>
> Clear the per-order controls too.  Set them to "inherit", not "never".
> khugepaged honours the global never and stays out.  A forced shmem collapse
> takes the order it builds from these very controls, and still finds one.
>
> Fixes: 9f0704eae8a4 ("selftests/mm/khugepaged: enlighten for multi-size THP")
> Assisted-by: Claude-Code:claude-opus-5
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 7eb9db0005a0..0008862e7cbc 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -550,8 +550,8 @@ static bool is_anon(struct mem_ops *ops)
>  static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>  			       struct mem_ops *ops, bool expect)
>  {
> -	int ret;
>  	struct thp_settings settings = *thp_current_settings();
> +	int ret, i;
>
>  	ksft_print_msg("%s...", msg);
>
> @@ -564,9 +564,20 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>  	/*
>  	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
>  	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
> +	 *
> +	 * The per-order controls have to go too, not just the global one: a
> +	 * source order left at "always" -- which -s does -- lets khugepaged
> +	 * collapse the very range the case is working on.  Set them to
> +	 * "inherit", not "never".  khugepaged honours the global never and
> +	 * stays out.  A forced shmem collapse takes the order it builds from
> +	 * these very controls, and still finds one.

Again far too many words, schloppy. Trim it please. This isn't a chat with
claude it's a comment.

>  	 */
>  	settings.thp_enabled = THP_NEVER;
>  	settings.shmem_enabled = SHMEM_NEVER;
> +	for (i = 0; i < NR_ORDERS; i++) {
> +		settings.hugepages[i].enabled = THP_INHERIT;
> +		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
> +	}
>  	thp_push_settings(&settings);
>
>  	/* Clear VM_NOHUGEPAGE */
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 07/19] selftests/mm: move is_backed_by_folio() into vm_util

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

Checking that an address range is backed by a folio of a given order is
useful to any test that builds or collapses large folios.  mTHP collapse
coverage in the khugepaged selftest needs exactly that.

split_huge_page_test.c already has the building block:
is_backed_by_folio() reads the compound head and tail flags from
/proc/kpageflags to classify the folio behind a page.

Move it into vm_util so other tests can use it.  No functional change.

Assisted-by: Claude-Code:claude-opus-5
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 .../selftests/mm/split_huge_page_test.c       | 62 -------------------
 tools/testing/selftests/mm/vm_util.c          | 62 +++++++++++++++++++
 tools/testing/selftests/mm/vm_util.h          |  2 +
 3 files changed, 64 insertions(+), 62 deletions(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 86a603692826..0adfe7dde7e5 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -42,68 +42,6 @@ const char *kpageflags_proc = "/proc/kpageflags";
 int pagemap_fd;
 int kpageflags_fd;
 
-static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
-		int kpageflags_fd)
-{
-	const uint64_t folio_head_flags = KPF_THP | KPF_COMPOUND_HEAD;
-	const uint64_t folio_tail_flags = KPF_THP | KPF_COMPOUND_TAIL;
-	const unsigned long nr_pages = 1UL << order;
-	unsigned long pfn_head;
-	uint64_t pfn_flags;
-	unsigned long pfn;
-	unsigned long i;
-
-	pfn = pagemap_get_pfn(pagemap_fd, vaddr);
-
-	/* non present page */
-	if (pfn == -1UL)
-		return false;
-
-	if (pageflags_get(pfn, kpageflags_fd, &pfn_flags))
-		goto fail;
-
-	/* check for order-0 pages */
-	if (!order) {
-		if (pfn_flags & (folio_head_flags | folio_tail_flags))
-			return false;
-		return true;
-	}
-
-	/* non THP folio */
-	if (!(pfn_flags & KPF_THP))
-		return false;
-
-	pfn_head = pfn & ~(nr_pages - 1);
-
-	if (pageflags_get(pfn_head, kpageflags_fd, &pfn_flags))
-		goto fail;
-
-	/* head PFN has no compound_head flag set */
-	if ((pfn_flags & folio_head_flags) != folio_head_flags)
-		return false;
-
-	/* check all tail PFN flags */
-	for (i = 1; i < nr_pages; i++) {
-		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
-			goto fail;
-		if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
-			return false;
-	}
-
-	/*
-	 * check the PFN after this folio, but if its flags cannot be obtained,
-	 * assume this folio has the expected order
-	 */
-	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
-		return true;
-
-	/* If we find another tail page, then the folio is larger. */
-	return (pfn_flags & folio_tail_flags) != folio_tail_flags;
-fail:
-	ksft_exit_fail_msg("Failed to get folio info\n");
-	return false;
-}
-
 static int check_after_split_folio_orders(char *vaddr_start, size_t len,
 		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders)
 {
diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 80bc9f597b52..5db1a7774f49 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -494,6 +494,68 @@ int pageflags_get(unsigned long pfn, int kpageflags_fd, uint64_t *flags)
 	return 0;
 }
 
+bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
+			int kpageflags_fd)
+{
+	const uint64_t folio_head_flags = KPF_THP | KPF_COMPOUND_HEAD;
+	const uint64_t folio_tail_flags = KPF_THP | KPF_COMPOUND_TAIL;
+	const unsigned long nr_pages = 1UL << order;
+	unsigned long pfn_head;
+	uint64_t pfn_flags;
+	unsigned long pfn;
+	unsigned long i;
+
+	pfn = pagemap_get_pfn(pagemap_fd, vaddr);
+
+	/* non present page */
+	if (pfn == -1UL)
+		return false;
+
+	if (pageflags_get(pfn, kpageflags_fd, &pfn_flags))
+		goto fail;
+
+	/* check for order-0 pages */
+	if (!order) {
+		if (pfn_flags & (folio_head_flags | folio_tail_flags))
+			return false;
+		return true;
+	}
+
+	/* non THP folio */
+	if (!(pfn_flags & KPF_THP))
+		return false;
+
+	pfn_head = pfn & ~(nr_pages - 1);
+
+	if (pageflags_get(pfn_head, kpageflags_fd, &pfn_flags))
+		goto fail;
+
+	/* head PFN has no compound_head flag set */
+	if ((pfn_flags & folio_head_flags) != folio_head_flags)
+		return false;
+
+	/* check all tail PFN flags */
+	for (i = 1; i < nr_pages; i++) {
+		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
+			goto fail;
+		if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
+			return false;
+	}
+
+	/*
+	 * check the PFN after this folio, but if its flags cannot be obtained,
+	 * assume this folio has the expected order
+	 */
+	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
+		return true;
+
+	/* If we find another tail page, then the folio is larger. */
+	return (pfn_flags & folio_tail_flags) != folio_tail_flags;
+fail:
+	ksft_exit_fail_msg("Failed to get folio info\n");
+	return false;
+}
+
 /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
 int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
 			      bool miss, bool wp, bool minor, uint64_t *ioctls)
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index 9a49af88702e..56a28ce7d029 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -97,6 +97,8 @@ int64_t allocate_transhuge(void *ptr, int pagemap_fd);
 int pageflags_get(unsigned long pfn, int kpageflags_fd, uint64_t *flags);
 int gather_folio_orders(char *vaddr_start, size_t len,
 		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
+bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
+			int kpageflags_fd);
 
 int uffd_register(int uffd, void *addr, uint64_t len,
 		  bool miss, bool wp, bool minor);
-- 
2.54.0

Re: [PATCH v4 07/19] selftests/mm: move is_backed_by_folio() into vm_util

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:49AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> Checking that an address range is backed by a folio of a given order is
> useful to any test that builds or collapses large folios.  mTHP collapse
> coverage in the khugepaged selftest needs exactly that.
>
> split_huge_page_test.c already has the building block:
> is_backed_by_folio() reads the compound head and tail flags from
> /proc/kpageflags to classify the folio behind a page.
>
> Move it into vm_util so other tests can use it.  No functional change.
>
> Assisted-by: Claude-Code:claude-opus-5
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

LGTM so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  .../selftests/mm/split_huge_page_test.c       | 62 -------------------
>  tools/testing/selftests/mm/vm_util.c          | 62 +++++++++++++++++++
>  tools/testing/selftests/mm/vm_util.h          |  2 +
>  3 files changed, 64 insertions(+), 62 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index 86a603692826..0adfe7dde7e5 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -42,68 +42,6 @@ const char *kpageflags_proc = "/proc/kpageflags";
>  int pagemap_fd;
>  int kpageflags_fd;
>
> -static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
> -		int kpageflags_fd)
> -{
> -	const uint64_t folio_head_flags = KPF_THP | KPF_COMPOUND_HEAD;
> -	const uint64_t folio_tail_flags = KPF_THP | KPF_COMPOUND_TAIL;
> -	const unsigned long nr_pages = 1UL << order;
> -	unsigned long pfn_head;
> -	uint64_t pfn_flags;
> -	unsigned long pfn;
> -	unsigned long i;
> -
> -	pfn = pagemap_get_pfn(pagemap_fd, vaddr);
> -
> -	/* non present page */
> -	if (pfn == -1UL)
> -		return false;
> -
> -	if (pageflags_get(pfn, kpageflags_fd, &pfn_flags))
> -		goto fail;
> -
> -	/* check for order-0 pages */
> -	if (!order) {
> -		if (pfn_flags & (folio_head_flags | folio_tail_flags))
> -			return false;
> -		return true;
> -	}
> -
> -	/* non THP folio */
> -	if (!(pfn_flags & KPF_THP))
> -		return false;
> -
> -	pfn_head = pfn & ~(nr_pages - 1);
> -
> -	if (pageflags_get(pfn_head, kpageflags_fd, &pfn_flags))
> -		goto fail;
> -
> -	/* head PFN has no compound_head flag set */
> -	if ((pfn_flags & folio_head_flags) != folio_head_flags)
> -		return false;
> -
> -	/* check all tail PFN flags */
> -	for (i = 1; i < nr_pages; i++) {
> -		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
> -			goto fail;
> -		if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
> -			return false;
> -	}
> -
> -	/*
> -	 * check the PFN after this folio, but if its flags cannot be obtained,
> -	 * assume this folio has the expected order
> -	 */
> -	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
> -		return true;
> -
> -	/* If we find another tail page, then the folio is larger. */
> -	return (pfn_flags & folio_tail_flags) != folio_tail_flags;
> -fail:
> -	ksft_exit_fail_msg("Failed to get folio info\n");
> -	return false;
> -}
> -
>  static int check_after_split_folio_orders(char *vaddr_start, size_t len,
>  		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders)
>  {
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 80bc9f597b52..5db1a7774f49 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -494,6 +494,68 @@ int pageflags_get(unsigned long pfn, int kpageflags_fd, uint64_t *flags)
>  	return 0;
>  }
>
> +bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
> +			int kpageflags_fd)
> +{
> +	const uint64_t folio_head_flags = KPF_THP | KPF_COMPOUND_HEAD;
> +	const uint64_t folio_tail_flags = KPF_THP | KPF_COMPOUND_TAIL;
> +	const unsigned long nr_pages = 1UL << order;
> +	unsigned long pfn_head;
> +	uint64_t pfn_flags;
> +	unsigned long pfn;
> +	unsigned long i;
> +
> +	pfn = pagemap_get_pfn(pagemap_fd, vaddr);
> +
> +	/* non present page */
> +	if (pfn == -1UL)
> +		return false;
> +
> +	if (pageflags_get(pfn, kpageflags_fd, &pfn_flags))
> +		goto fail;
> +
> +	/* check for order-0 pages */
> +	if (!order) {
> +		if (pfn_flags & (folio_head_flags | folio_tail_flags))
> +			return false;
> +		return true;
> +	}
> +
> +	/* non THP folio */
> +	if (!(pfn_flags & KPF_THP))
> +		return false;
> +
> +	pfn_head = pfn & ~(nr_pages - 1);
> +
> +	if (pageflags_get(pfn_head, kpageflags_fd, &pfn_flags))
> +		goto fail;
> +
> +	/* head PFN has no compound_head flag set */
> +	if ((pfn_flags & folio_head_flags) != folio_head_flags)
> +		return false;
> +
> +	/* check all tail PFN flags */
> +	for (i = 1; i < nr_pages; i++) {
> +		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
> +			goto fail;
> +		if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
> +			return false;
> +	}
> +
> +	/*
> +	 * check the PFN after this folio, but if its flags cannot be obtained,
> +	 * assume this folio has the expected order
> +	 */
> +	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
> +		return true;
> +
> +	/* If we find another tail page, then the folio is larger. */
> +	return (pfn_flags & folio_tail_flags) != folio_tail_flags;
> +fail:
> +	ksft_exit_fail_msg("Failed to get folio info\n");
> +	return false;
> +}
> +
>  /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
>  int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
>  			      bool miss, bool wp, bool minor, uint64_t *ioctls)
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index 9a49af88702e..56a28ce7d029 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -97,6 +97,8 @@ int64_t allocate_transhuge(void *ptr, int pagemap_fd);
>  int pageflags_get(unsigned long pfn, int kpageflags_fd, uint64_t *flags);
>  int gather_folio_orders(char *vaddr_start, size_t len,
>  		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
> +bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
> +			int kpageflags_fd);
>
>  int uffd_register(int uffd, void *addr, uint64_t len,
>  		  bool miss, bool wp, bool minor);
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 08/19] selftests/mm: add folio-order check for address ranges

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

An mTHP collapse test needs to know that a range is backed by folios of the
target order, and that they sit where a collapse would put them.  Nothing
answers that today: is_backed_by_folio() classifies the folio behind a
single page, and check_huge_anon() reads smaps AnonHugePages, which only
accounts PMD mappings.

Add is_range_backed_by_folio_orders().  For every order-aligned window of
the range it requires a present head PFN at its natural alignment and a
contiguous PFN run across the window.

A window backed by two smaller folios fails the contiguity check, and a
folio mapped off the window's alignment fails the head check.  The mTHP
cases need both to tell a collapsed window from the one beside it.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/vm_util.c | 42 ++++++++++++++++++++++++++++
 tools/testing/selftests/mm/vm_util.h |  2 ++
 2 files changed, 44 insertions(+)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 5db1a7774f49..c9bd6c92fa41 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -556,6 +556,48 @@ bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 	return false;
 }
 
+/*
+ * Check whether every order-@order window of [start, len) maps exactly one
+ * folio of that order, head to tail.  The address range must be naturally
+ * aligned, each window's PFN run must be contiguous, and a window's first
+ * PFN must be the folio head.
+ *
+ * This is the check "did this range collapse into order-@order folios": a
+ * window assembled from parts of several folios, or mapping a folio shifted
+ * from its natural position, fails.
+ */
+bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
+				     int pagemap_fd, int kpageflags_fd)
+{
+	const unsigned long nr_pages = 1UL << order;
+	const size_t window = nr_pages * psize();
+	char *vaddr;
+
+	if ((uintptr_t)start % window || len % window)
+		return false;
+
+	for (vaddr = start; vaddr < start + len; vaddr += window) {
+		unsigned long pfn = pagemap_get_pfn(pagemap_fd, vaddr);
+		unsigned long i;
+
+		/* Not present, or not mapping the folio head. */
+		if (pfn == -1UL || pfn % nr_pages)
+			return false;
+
+		for (i = 1; i < nr_pages; i++) {
+			if (pagemap_get_pfn(pagemap_fd, vaddr + i * psize()) !=
+			    pfn + i)
+				return false;
+		}
+
+		if (!is_backed_by_folio(vaddr, order, pagemap_fd,
+					kpageflags_fd))
+			return false;
+	}
+
+	return true;
+}
+
 /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
 int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
 			      bool miss, bool wp, bool minor, uint64_t *ioctls)
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index 56a28ce7d029..39dfb18dc10c 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -99,6 +99,8 @@ int gather_folio_orders(char *vaddr_start, size_t len,
 		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
 bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 			int kpageflags_fd);
+bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
+				     int pagemap_fd, int kpageflags_fd);
 
 int uffd_register(int uffd, void *addr, uint64_t len,
 		  bool miss, bool wp, bool minor);
-- 
2.54.0

Re: [PATCH v4 08/19] selftests/mm: add folio-order check for address ranges

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:50AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> An mTHP collapse test needs to know that a range is backed by folios of the
> target order, and that they sit where a collapse would put them.  Nothing
> answers that today: is_backed_by_folio() classifies the folio behind a
> single page, and check_huge_anon() reads smaps AnonHugePages, which only
> accounts PMD mappings.
>
> Add is_range_backed_by_folio_orders().  For every order-aligned window of
> the range it requires a present head PFN at its natural alignment and a
> contiguous PFN run across the window.
>
> A window backed by two smaller folios fails the contiguity check, and a
> folio mapped off the window's alignment fails the head check.  The mTHP
> cases need both to tell a collapsed window from the one beside it.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/vm_util.c | 42 ++++++++++++++++++++++++++++
>  tools/testing/selftests/mm/vm_util.h |  2 ++
>  2 files changed, 44 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 5db1a7774f49..c9bd6c92fa41 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -556,6 +556,48 @@ bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
>  	return false;
>  }
>
> +/*
> + * Check whether every order-@order window of [start, len) maps exactly one

Strange to reference @order in a non-kdoc comment. And then to start referring
to start and len without @? :)

And I'm not sure 'window' is really clear here as a term...

> + * folio of that order, head to tail.  The address range must be naturally
> + * aligned, each window's PFN run must be contiguous, and a window's first
> + * PFN must be the folio head.

Also 'PFN run' is adding a new vague term.

> + *
> + * This is the check "did this range collapse into order-@order folios": a
> + * window assembled from parts of several folios, or mapping a folio shifted
> + * from its natural position, fails.

I think you could simplify this to something like:

	Check that the buffer @start of @len bytes is mTHP-backed by @order
	folios wholly contained within the range.

	This requires that @buffer is aligned to @order and that every folio
	is fully contained head-to-tail.

	Returns: true if wholly mTHP backed, false otherwise.

> + */
> +bool is_range_backed_by_folio_orders(char *start, size_t len, int order,

is_range_mthp_backed() seems a lot clearer?

> +				     int pagemap_fd, int kpageflags_fd)
> +{
> +	const unsigned long nr_pages = 1UL << order;
> +	const size_t window = nr_pages * psize();

folio_size seems clearer?

> +	char *vaddr;
> +
> +	if ((uintptr_t)start % window || len % window)
> +		return false;
> +
> +	for (vaddr = start; vaddr < start + len; vaddr += window) {
> +		unsigned long pfn = pagemap_get_pfn(pagemap_fd, vaddr);

NIT: can const-ify.

> +		unsigned long i;
> +
> +		/* Not present, or not mapping the folio head. */

'Or a tail page' maybe?

> +		if (pfn == -1UL || pfn % nr_pages)
> +			return false;
> +
> +		for (i = 1; i < nr_pages; i++) {
> +			if (pagemap_get_pfn(pagemap_fd, vaddr + i * psize()) !=
> +			    pfn + i)
> +				return false;
> +		}

NIT: can drop the {}.

> +
> +		if (!is_backed_by_folio(vaddr, order, pagemap_fd,
> +					kpageflags_fd))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>  /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
>  int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
>  			      bool miss, bool wp, bool minor, uint64_t *ioctls)
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index 56a28ce7d029..39dfb18dc10c 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -99,6 +99,8 @@ int gather_folio_orders(char *vaddr_start, size_t len,
>  		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
>  bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
>  			int kpageflags_fd);
> +bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
> +				     int pagemap_fd, int kpageflags_fd);
>
>  int uffd_register(int uffd, void *addr, uint64_t len,
>  		  bool miss, bool wp, bool minor);
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 09/19] selftests/mm: add folio-order detection self-check

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The khugepaged mTHP tests detect collapse results with the vm_util
folio-order helpers rather than smaps AnonHugePages, which only sees PMD
mappings.  If those helpers are wrong, every case built on them is wrong
the same way, and nothing says so.

Check them directly.  For every anon THP order the kernel supports, fault
memory in with only that order enabled.  Require the helpers to classify
the backing as exactly that order: not a neighbouring order, and 4K-backed
memory as order 0.

Run it in the thp category, ahead of ./khugepaged, so a broken helper is
reported as itself rather than as a collapse failure.  Verified on x86-64
4K (orders 0, 2-9) and arm64 64K (orders 0, 2-13).

The test needs ALIGN(), which hmm-tests.c and migration.c each defined
privately.  Move it to vm_util.h and drop both copies.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/Makefile           |   1 +
 .../testing/selftests/mm/folio_order_check.c  | 137 ++++++++++++++++++
 tools/testing/selftests/mm/hmm-tests.c        |   1 -
 tools/testing/selftests/mm/migration.c        |   1 -
 tools/testing/selftests/mm/run_vmtests.sh     |   2 +
 tools/testing/selftests/mm/vm_util.h          |   2 +
 6 files changed, 142 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/mm/folio_order_check.c

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index 2d5366196e30..2093fcf6e915 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -104,6 +104,7 @@ TEST_GEN_FILES += guard-regions
 TEST_GEN_FILES += merge
 TEST_GEN_FILES += rmap
 TEST_GEN_FILES += folio_split_race_test
+TEST_GEN_FILES += folio_order_check
 
 ifneq ($(ARCH),arm64)
 TEST_GEN_FILES += soft-dirty
diff --git a/tools/testing/selftests/mm/folio_order_check.c b/tools/testing/selftests/mm/folio_order_check.c
new file mode 100644
index 000000000000..93030a42c3cc
--- /dev/null
+++ b/tools/testing/selftests/mm/folio_order_check.c
@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Self-check for the vm_util folio-order detection helpers,
+ * is_backed_by_folio() and is_range_backed_by_folio_orders().
+ *
+ * For every anon THP order the kernel supports, fault memory in with only
+ * that order enabled and verify the helpers report exactly that order:
+ * not a neighbouring order, and plain 4K memory as order 0. The helpers
+ * are what the khugepaged mTHP tests use to detect collapse results, so
+ * they must agree with the kernel's own idea of the backing before any
+ * collapse test relies on them.
+ */
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+#include "vm_util.h"
+#include "hugepage_settings.h"
+
+static int pagemap_fd;
+static int kpageflags_fd;
+
+/* mmap an anon VMA of exactly @size bytes at a @size-aligned address. */
+static char *alloc_aligned(size_t size)
+{
+	size_t len = size * 2;
+	uintptr_t aligned;
+	char *p;
+
+	p = mmap(NULL, len, PROT_READ | PROT_WRITE,
+		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	if (p == MAP_FAILED)
+		ksft_exit_fail_perror("mmap()");
+
+	aligned = ALIGN((uintptr_t)p, size);
+	if (aligned != (uintptr_t)p)
+		munmap(p, aligned - (uintptr_t)p);
+	if (aligned + size != (uintptr_t)p + len)
+		munmap((char *)aligned + size,
+		       (uintptr_t)p + len - aligned - size);
+
+	return (char *)aligned;
+}
+
+/*
+ * Enable only @order (order 0: nothing), fault one aligned window in and
+ * check the helpers see exactly @order.
+ */
+static void check_order(int order)
+{
+	struct thp_settings settings = *thp_current_settings();
+	size_t size = psize() << order;
+	bool ok = true;
+	char *p;
+	int i;
+
+	for (i = 0; i < NR_ORDERS; i++)
+		settings.hugepages[i].enabled = THP_NEVER;
+	if (order)
+		settings.hugepages[order].enabled = THP_ALWAYS;
+	thp_push_settings(&settings);
+
+	p = alloc_aligned(size);
+	*p = 1;
+
+	if (!is_range_backed_by_folio_orders(p, size, order,
+					     pagemap_fd, kpageflags_fd)) {
+		ksft_print_msg("order %d not detected after fault\n", order);
+		ok = false;
+	}
+
+	/* A lower order must be rejected: the folio is larger. */
+	if (order && is_range_backed_by_folio_orders(p, size, order - 1,
+						     pagemap_fd,
+						     kpageflags_fd)) {
+		ksft_print_msg("order %d also reported as order %d\n",
+			       order, order - 1);
+		ok = false;
+	}
+
+	/* Order 0 pages must not look like any large folio, and vice versa. */
+	if (order && is_range_backed_by_folio_orders(p, size, 0,
+						     pagemap_fd,
+						     kpageflags_fd)) {
+		ksft_print_msg("order %d also reported as order 0\n", order);
+		ok = false;
+	}
+
+	munmap(p, size);
+	thp_pop_settings();
+
+	ksft_test_result(ok, "order %d classified\n", order);
+}
+
+int main(void)
+{
+	struct thp_settings settings;
+	unsigned long orders;
+	int order;
+
+	ksft_print_header();
+
+	if (!thp_available())
+		ksft_exit_skip("Transparent Hugepages not available\n");
+
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		ksft_exit_fail_perror("open(/proc/self/pagemap)");
+	kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
+	if (kpageflags_fd < 0)
+		ksft_exit_skip("open(\"/proc/kpageflags\") requires root\n");
+
+	orders = thp_supported_orders();
+	if (!orders)
+		ksft_exit_skip("No supported THP orders\n");
+
+	ksft_set_plan(__builtin_popcountl(orders) + 1);
+
+	thp_save_settings();
+	thp_read_settings(&settings);
+	/* Base of the settings stack; the bottom entry is never popped. */
+	thp_push_settings(&settings);
+
+	check_order(0);
+	for (order = 1; order < NR_ORDERS; order++) {
+		if (!(orders & (1UL << order)))
+			continue;
+		check_order(order);
+	}
+
+
+	ksft_finished();
+}
diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
index e2642eca0d02..df426f9218e7 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -65,7 +65,6 @@ enum {
 #define HMM_PATH_MAX    64
 #define NTIMES		10
 
-#define ALIGN(x, a) (((x) + (a - 1)) & (~((a) - 1)))
 /* Just the flags we need, copied from mm.h: */
 
 #ifndef FOLL_WRITE
diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
index f19d53c69576..fd35f8a7b5b8 100644
--- a/tools/testing/selftests/mm/migration.c
+++ b/tools/testing/selftests/mm/migration.c
@@ -20,7 +20,6 @@
 
 #define TWOMEG		(2<<20)
 #define RUNTIME		(20)
-#define ALIGN(x, a)	(((x) + (a - 1)) & (~((a) - 1)))
 
 HUGETLB_SETUP_DEFAULT_PAGES(1)
 
diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
index d09f9f6a384e..2652a7920b80 100755
--- a/tools/testing/selftests/mm/run_vmtests.sh
+++ b/tools/testing/selftests/mm/run_vmtests.sh
@@ -402,6 +402,8 @@ CATEGORY="pfnmap" run_test ./pfnmap
 # COW tests
 CATEGORY="cow" run_test ./cow
 
+CATEGORY="thp" run_test ./folio_order_check
+
 CATEGORY="thp" run_test ./khugepaged
 
 CATEGORY="thp" run_test ./khugepaged -s 2
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index 39dfb18dc10c..ce05bce4670d 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -10,6 +10,8 @@
 #include <linux/fs.h>
 
 #define BIT_ULL(nr)                   (1ULL << (nr))
+#define ALIGN(x, a)                   (((x) + (a) - 1) & ~((a) - 1))
+
 #define PM_SOFT_DIRTY                 BIT_ULL(55)
 #define PM_MMAP_EXCLUSIVE             BIT_ULL(56)
 #define PM_UFFD_WP                    BIT_ULL(57)
-- 
2.54.0

Re: [PATCH v4 09/19] selftests/mm: add folio-order detection self-check

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:51AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> The khugepaged mTHP tests detect collapse results with the vm_util
> folio-order helpers rather than smaps AnonHugePages, which only sees PMD
> mappings.  If those helpers are wrong, every case built on them is wrong
> the same way, and nothing says so.
>
> Check them directly.  For every anon THP order the kernel supports, fault
> memory in with only that order enabled.  Require the helpers to classify
> the backing as exactly that order: not a neighbouring order, and 4K-backed
> memory as order 0.
>
> Run it in the thp category, ahead of ./khugepaged, so a broken helper is
> reported as itself rather than as a collapse failure.  Verified on x86-64
> 4K (orders 0, 2-9) and arm64 64K (orders 0, 2-13).
>
> The test needs ALIGN(), which hmm-tests.c and migration.c each defined
> privately.  Move it to vm_util.h and drop both copies.

Also:

uffd-unit-tests defines ALIGN_UP()...

And really you probably want to match the kernel convention of ALIGN()
defaulting to align-up (for some reason) and name this ALIGN_DOWN() to be
consistent and avoid confusion BUT...

Since the tests already do this mistake I guess it's not so bad to share the
define and propagate it for now I suppose.


>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/Makefile           |   1 +
>  .../testing/selftests/mm/folio_order_check.c  | 137 ++++++++++++++++++
>  tools/testing/selftests/mm/hmm-tests.c        |   1 -
>  tools/testing/selftests/mm/migration.c        |   1 -
>  tools/testing/selftests/mm/run_vmtests.sh     |   2 +
>  tools/testing/selftests/mm/vm_util.h          |   2 +
>  6 files changed, 142 insertions(+), 2 deletions(-)
>  create mode 100644 tools/testing/selftests/mm/folio_order_check.c
>
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index 2d5366196e30..2093fcf6e915 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -104,6 +104,7 @@ TEST_GEN_FILES += guard-regions
>  TEST_GEN_FILES += merge
>  TEST_GEN_FILES += rmap
>  TEST_GEN_FILES += folio_split_race_test
> +TEST_GEN_FILES += folio_order_check
>
>  ifneq ($(ARCH),arm64)
>  TEST_GEN_FILES += soft-dirty
> diff --git a/tools/testing/selftests/mm/folio_order_check.c b/tools/testing/selftests/mm/folio_order_check.c
> new file mode 100644
> index 000000000000..93030a42c3cc
> --- /dev/null
> +++ b/tools/testing/selftests/mm/folio_order_check.c
> @@ -0,0 +1,137 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Self-check for the vm_util folio-order detection helpers,
> + * is_backed_by_folio() and is_range_backed_by_folio_orders().
> + *
> + * For every anon THP order the kernel supports, fault memory in with only
> + * that order enabled and verify the helpers report exactly that order:
> + * not a neighbouring order, and plain 4K memory as order 0. The helpers
> + * are what the khugepaged mTHP tests use to detect collapse results, so
> + * they must agree with the kernel's own idea of the backing before any
> + * collapse test relies on them.
> + */
> +#define _GNU_SOURCE
> +#include <fcntl.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/mman.h>
> +#include <unistd.h>
> +
> +#include "kselftest.h"
> +#include "vm_util.h"
> +#include "hugepage_settings.h"
> +
> +static int pagemap_fd;
> +static int kpageflags_fd;
> +
> +/* mmap an anon VMA of exactly @size bytes at a @size-aligned address. */
> +static char *alloc_aligned(size_t size)
> +{
> +	size_t len = size * 2;
> +	uintptr_t aligned;
> +	char *p;
> +
> +	p = mmap(NULL, len, PROT_READ | PROT_WRITE,
> +		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> +	if (p == MAP_FAILED)
> +		ksft_exit_fail_perror("mmap()");
> +
> +	aligned = ALIGN((uintptr_t)p, size);
> +	if (aligned != (uintptr_t)p)
> +		munmap(p, aligned - (uintptr_t)p);
> +	if (aligned + size != (uintptr_t)p + len)
> +		munmap((char *)aligned + size,
> +		       (uintptr_t)p + len - aligned - size);
> +
> +	return (char *)aligned;
> +}
> +
> +/*
> + * Enable only @order (order 0: nothing), fault one aligned window in and
> + * check the helpers see exactly @order.
> + */
> +static void check_order(int order)
> +{
> +	struct thp_settings settings = *thp_current_settings();
> +	size_t size = psize() << order;
> +	bool ok = true;
> +	char *p;
> +	int i;
> +
> +	for (i = 0; i < NR_ORDERS; i++)
> +		settings.hugepages[i].enabled = THP_NEVER;
> +	if (order)
> +		settings.hugepages[order].enabled = THP_ALWAYS;
> +	thp_push_settings(&settings);
> +
> +	p = alloc_aligned(size);
> +	*p = 1;
> +
> +	if (!is_range_backed_by_folio_orders(p, size, order,
> +					     pagemap_fd, kpageflags_fd)) {
> +		ksft_print_msg("order %d not detected after fault\n", order);
> +		ok = false;
> +	}
> +
> +	/* A lower order must be rejected: the folio is larger. */
> +	if (order && is_range_backed_by_folio_orders(p, size, order - 1,
> +						     pagemap_fd,
> +						     kpageflags_fd)) {
> +		ksft_print_msg("order %d also reported as order %d\n",
> +			       order, order - 1);
> +		ok = false;
> +	}
> +
> +	/* Order 0 pages must not look like any large folio, and vice versa. */
> +	if (order && is_range_backed_by_folio_orders(p, size, 0,
> +						     pagemap_fd,
> +						     kpageflags_fd)) {
> +		ksft_print_msg("order %d also reported as order 0\n", order);
> +		ok = false;
> +	}
> +
> +	munmap(p, size);
> +	thp_pop_settings();
> +
> +	ksft_test_result(ok, "order %d classified\n", order);
> +}
> +
> +int main(void)
> +{
> +	struct thp_settings settings;
> +	unsigned long orders;
> +	int order;
> +
> +	ksft_print_header();
> +
> +	if (!thp_available())
> +		ksft_exit_skip("Transparent Hugepages not available\n");
> +
> +	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> +	if (pagemap_fd < 0)
> +		ksft_exit_fail_perror("open(/proc/self/pagemap)");
> +	kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
> +	if (kpageflags_fd < 0)
> +		ksft_exit_skip("open(\"/proc/kpageflags\") requires root\n");
> +
> +	orders = thp_supported_orders();
> +	if (!orders)
> +		ksft_exit_skip("No supported THP orders\n");
> +
> +	ksft_set_plan(__builtin_popcountl(orders) + 1);
> +
> +	thp_save_settings();
> +	thp_read_settings(&settings);
> +	/* Base of the settings stack; the bottom entry is never popped. */
> +	thp_push_settings(&settings);
> +
> +	check_order(0);
> +	for (order = 1; order < NR_ORDERS; order++) {
> +		if (!(orders & (1UL << order)))
> +			continue;
> +		check_order(order);
> +	}
> +
> +
> +	ksft_finished();
> +}
> diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
> index e2642eca0d02..df426f9218e7 100644
> --- a/tools/testing/selftests/mm/hmm-tests.c
> +++ b/tools/testing/selftests/mm/hmm-tests.c
> @@ -65,7 +65,6 @@ enum {
>  #define HMM_PATH_MAX    64
>  #define NTIMES		10
>
> -#define ALIGN(x, a) (((x) + (a - 1)) & (~((a) - 1)))
>  /* Just the flags we need, copied from mm.h: */
>
>  #ifndef FOLL_WRITE
> diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
> index f19d53c69576..fd35f8a7b5b8 100644
> --- a/tools/testing/selftests/mm/migration.c
> +++ b/tools/testing/selftests/mm/migration.c
> @@ -20,7 +20,6 @@
>
>  #define TWOMEG		(2<<20)
>  #define RUNTIME		(20)
> -#define ALIGN(x, a)	(((x) + (a - 1)) & (~((a) - 1)))
>
>  HUGETLB_SETUP_DEFAULT_PAGES(1)
>
> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
> index d09f9f6a384e..2652a7920b80 100755
> --- a/tools/testing/selftests/mm/run_vmtests.sh
> +++ b/tools/testing/selftests/mm/run_vmtests.sh
> @@ -402,6 +402,8 @@ CATEGORY="pfnmap" run_test ./pfnmap
>  # COW tests
>  CATEGORY="cow" run_test ./cow
>
> +CATEGORY="thp" run_test ./folio_order_check
> +
>  CATEGORY="thp" run_test ./khugepaged
>
>  CATEGORY="thp" run_test ./khugepaged -s 2
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index 39dfb18dc10c..ce05bce4670d 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -10,6 +10,8 @@
>  #include <linux/fs.h>
>
>  #define BIT_ULL(nr)                   (1ULL << (nr))
> +#define ALIGN(x, a)                   (((x) + (a) - 1) & ~((a) - 1))
> +
>  #define PM_SOFT_DIRTY                 BIT_ULL(55)
>  #define PM_MMAP_EXCLUSIVE             BIT_ULL(56)
>  #define PM_UFFD_WP                    BIT_ULL(57)
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 10/19] selftests/mm: add khugepaged completion barrier helper

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

Race and functional tests need to drive khugepaged in step: set up a
layout, let one full scan pass over it, check the result.  The khugepaged
selftest already waits for full_scans to advance by two, but only makes
progress if scan_sleep_millisecs happens to be short.

Lift it into khugepaged_full_pass() and drive it through sysfs: any store
to scan_sleep_millisecs wakes the daemon, so the barrier completes whatever
the scan cadence.  A store can be lost when the daemon is between scans, so
it keeps storing until the pass lands; a store to an awake daemon costs
nothing and queues no extra pass.

One wake completes one pass only if the whole mm list fits in a scan batch,
so callers need a large pages_to_scan.

Settings pushes must not start passes either.  A store to either sleep knob
wakes the daemon, so thp_write_settings() now writes a khugepaged knob only
when its value changes.  The other knobs do not wake, but writing them
uniformly costs nothing.  thp_update_num() is exported for tests that want
the same restraint.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 .../testing/selftests/mm/hugepage_settings.c  | 74 ++++++++++++++++---
 .../testing/selftests/mm/hugepage_settings.h  |  3 +
 2 files changed, 68 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/mm/hugepage_settings.c b/tools/testing/selftests/mm/hugepage_settings.c
index d7917dce3aba..992efee17b71 100644
--- a/tools/testing/selftests/mm/hugepage_settings.c
+++ b/tools/testing/selftests/mm/hugepage_settings.c
@@ -183,6 +183,19 @@ void thp_read_settings(struct thp_settings *settings)
 	}
 }
 
+/*
+ * Write only on change: a store to either sleep knob wakes khugepaged --
+ * __sleep_millisecs_store() clears khugepaged_sleep_expire and wakes the
+ * queue -- and settings pushes/pops must not start scan passes nobody
+ * asked for; khugepaged_full_pass() is the only sanctioned wake.  The
+ * other knobs do not wake, but writing them the same way costs nothing.
+ */
+void thp_update_num(const char *name, unsigned long num)
+{
+	if (thp_read_num(name) != num)
+		thp_write_num(name, num);
+}
+
 void thp_write_settings(struct thp_settings *settings)
 {
 	struct khugepaged_settings *khugepaged = &settings->khugepaged;
@@ -198,15 +211,15 @@ void thp_write_settings(struct thp_settings *settings)
 			shmem_enabled_strings[settings->shmem_enabled]);
 	thp_write_num("use_zero_page", settings->use_zero_page);
 
-	thp_write_num("khugepaged/defrag", khugepaged->defrag);
-	thp_write_num("khugepaged/alloc_sleep_millisecs",
-			khugepaged->alloc_sleep_millisecs);
-	thp_write_num("khugepaged/scan_sleep_millisecs",
-			khugepaged->scan_sleep_millisecs);
-	thp_write_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
-	thp_write_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
-	thp_write_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
-	thp_write_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
+	thp_update_num("khugepaged/defrag", khugepaged->defrag);
+	thp_update_num("khugepaged/alloc_sleep_millisecs",
+		       khugepaged->alloc_sleep_millisecs);
+	thp_update_num("khugepaged/scan_sleep_millisecs",
+		       khugepaged->scan_sleep_millisecs);
+	thp_update_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
+	thp_update_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
+	thp_update_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
+	thp_update_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
 
 	if (dev_queue_read_ahead_path[0])
 		write_num(dev_queue_read_ahead_path, settings->read_ahead_kb);
@@ -230,6 +243,49 @@ void thp_write_settings(struct thp_settings *settings)
 	}
 }
 
+/*
+ * Completion barrier for khugepaged: wait until a full scan pass that
+ * started after this call has finished. full_scans must advance by two;
+ * a +1 step may complete a pass that examined this mm before the
+ * caller's setup was in place.
+ *
+ * Any store to scan_sleep_millisecs wakes the daemon, so the barrier works
+ * whatever the configured scan cadence -- but a store can be lost.
+ * __sleep_millisecs_store() clears khugepaged_sleep_expire and wakes the
+ * queue; if the daemon is between scans rather than sleeping, it sets
+ * khugepaged_sleep_expire itself on the way into khugepaged_wait_work() and
+ * then sleeps for the full interval, having never seen the store.  So keep
+ * storing until the pass lands; a store while the daemon is awake costs
+ * nothing and does not queue an extra pass.
+ *
+ * One wake completes one full pass only if the whole mm list fits in
+ * one scan batch, so callers must pair this with a large
+ * pages_to_scan.
+ */
+bool khugepaged_full_pass(unsigned int timeout_s)
+{
+	unsigned long deadline_ms = timeout_s * 1000UL;
+	unsigned long sleep_ms =
+		thp_read_num("khugepaged/scan_sleep_millisecs");
+	unsigned long elapsed_ms = 0;
+	int pass;
+
+	for (pass = 0; pass < 2; pass++) {
+		unsigned long target =
+			thp_read_num("khugepaged/full_scans") + 1;
+
+		while (thp_read_num("khugepaged/full_scans") < target) {
+			if (elapsed_ms >= deadline_ms)
+				return false;
+			thp_write_num("khugepaged/scan_sleep_millisecs",
+				      sleep_ms);
+			usleep(10 * 1000);
+			elapsed_ms += 10;
+		}
+	}
+	return true;
+}
+
 struct thp_settings *thp_current_settings(void)
 {
 	if (!settings_index) {
diff --git a/tools/testing/selftests/mm/hugepage_settings.h b/tools/testing/selftests/mm/hugepage_settings.h
index 726c73c43c05..ba7d38370d43 100644
--- a/tools/testing/selftests/mm/hugepage_settings.h
+++ b/tools/testing/selftests/mm/hugepage_settings.h
@@ -70,6 +70,7 @@ int thp_read_string(const char *name, const char * const strings[]);
 void thp_write_string(const char *name, const char *val);
 unsigned long thp_read_num(const char *name);
 void thp_write_num(const char *name, unsigned long num);
+void thp_update_num(const char *name, unsigned long num);
 
 void thp_write_settings(struct thp_settings *settings);
 void thp_read_settings(struct thp_settings *settings);
@@ -83,6 +84,8 @@ static inline void thp_save_settings(void)
 	hugepage_save_settings(/* thp = */ true, /* hugetlb = */ false);
 }
 
+bool khugepaged_full_pass(unsigned int timeout_s);
+
 void thp_set_read_ahead_path(char *path);
 unsigned long thp_supported_orders(void);
 unsigned long thp_shmem_supported_orders(void);
-- 
2.54.0

Re: [PATCH v4 10/19] selftests/mm: add khugepaged completion barrier helper

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:52AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> Race and functional tests need to drive khugepaged in step: set up a
> layout, let one full scan pass over it, check the result.  The khugepaged
> selftest already waits for full_scans to advance by two, but only makes
> progress if scan_sleep_millisecs happens to be short.
>
> Lift it into khugepaged_full_pass() and drive it through sysfs: any store
> to scan_sleep_millisecs wakes the daemon, so the barrier completes whatever
> the scan cadence.  A store can be lost when the daemon is between scans, so
> it keeps storing until the pass lands; a store to an awake daemon costs
> nothing and queues no extra pass.
>
> One wake completes one pass only if the whole mm list fits in a scan batch,
> so callers need a large pages_to_scan.
>
> Settings pushes must not start passes either.  A store to either sleep knob
> wakes the daemon, so thp_write_settings() now writes a khugepaged knob only
> when its value changes.  The other knobs do not wake, but writing them
> uniformly costs nothing.  thp_update_num() is exported for tests that want
> the same restraint.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  .../testing/selftests/mm/hugepage_settings.c  | 74 ++++++++++++++++---
>  .../testing/selftests/mm/hugepage_settings.h  |  3 +
>  2 files changed, 68 insertions(+), 9 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/hugepage_settings.c b/tools/testing/selftests/mm/hugepage_settings.c
> index d7917dce3aba..992efee17b71 100644
> --- a/tools/testing/selftests/mm/hugepage_settings.c
> +++ b/tools/testing/selftests/mm/hugepage_settings.c
> @@ -183,6 +183,19 @@ void thp_read_settings(struct thp_settings *settings)
>  	}
>  }
>
> +/*
> + * Write only on change: a store to either sleep knob wakes khugepaged --
> + * __sleep_millisecs_store() clears khugepaged_sleep_expire and wakes the
> + * queue -- and settings pushes/pops must not start scan passes nobody
> + * asked for; khugepaged_full_pass() is the only sanctioned wake.  The
> + * other knobs do not wake, but writing them the same way costs nothing.
> + */

This seems quite schloppy and way too much information in a single dense
comment.

You/claude don't need to give your life story in every comment :)

Smaller, clearer, human-written please.

	/*

> +void thp_update_num(const char *name, unsigned long num)
> +{
> +	if (thp_read_num(name) != num)
> +		thp_write_num(name, num);
> +}
> +
>  void thp_write_settings(struct thp_settings *settings)
>  {
>  	struct khugepaged_settings *khugepaged = &settings->khugepaged;
> @@ -198,15 +211,15 @@ void thp_write_settings(struct thp_settings *settings)
>  			shmem_enabled_strings[settings->shmem_enabled]);
>  	thp_write_num("use_zero_page", settings->use_zero_page);
>
> -	thp_write_num("khugepaged/defrag", khugepaged->defrag);
> -	thp_write_num("khugepaged/alloc_sleep_millisecs",
> -			khugepaged->alloc_sleep_millisecs);
> -	thp_write_num("khugepaged/scan_sleep_millisecs",
> -			khugepaged->scan_sleep_millisecs);
> -	thp_write_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
> -	thp_write_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
> -	thp_write_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
> -	thp_write_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
> +	thp_update_num("khugepaged/defrag", khugepaged->defrag);
> +	thp_update_num("khugepaged/alloc_sleep_millisecs",
> +		       khugepaged->alloc_sleep_millisecs);
> +	thp_update_num("khugepaged/scan_sleep_millisecs",
> +		       khugepaged->scan_sleep_millisecs);
> +	thp_update_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
> +	thp_update_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
> +	thp_update_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
> +	thp_update_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
>
>  	if (dev_queue_read_ahead_path[0])
>  		write_num(dev_queue_read_ahead_path, settings->read_ahead_kb);
> @@ -230,6 +243,49 @@ void thp_write_settings(struct thp_settings *settings)
>  	}
>  }
>
> +/*
> + * Completion barrier for khugepaged: wait until a full scan pass that
> + * started after this call has finished. full_scans must advance by two;
> + * a +1 step may complete a pass that examined this mm before the
> + * caller's setup was in place.
> + *
> + * Any store to scan_sleep_millisecs wakes the daemon, so the barrier works
> + * whatever the configured scan cadence -- but a store can be lost.
> + * __sleep_millisecs_store() clears khugepaged_sleep_expire and wakes the
> + * queue; if the daemon is between scans rather than sleeping, it sets
> + * khugepaged_sleep_expire itself on the way into khugepaged_wait_work() and
> + * then sleeps for the full interval, having never seen the store.  So keep
> + * storing until the pass lands; a store while the daemon is awake costs
> + * nothing and does not queue an extra pass.
> + *
> + * One wake completes one full pass only if the whole mm list fits in
> + * one scan batch, so callers must pair this with a large
> + * pages_to_scan.
> + */

Same comment about the comment. This is dense and schloppy. Clarity and
terseness please.

> +bool khugepaged_full_pass(unsigned int timeout_s)
> +{
> +	unsigned long deadline_ms = timeout_s * 1000UL;
> +	unsigned long sleep_ms =
> +		thp_read_num("khugepaged/scan_sleep_millisecs");
> +	unsigned long elapsed_ms = 0;
> +	int pass;
> +
> +	for (pass = 0; pass < 2; pass++) {
> +		unsigned long target =
> +			thp_read_num("khugepaged/full_scans") + 1;
> +
> +		while (thp_read_num("khugepaged/full_scans") < target) {
> +			if (elapsed_ms >= deadline_ms)
> +				return false;
> +			thp_write_num("khugepaged/scan_sleep_millisecs",
> +				      sleep_ms);
> +			usleep(10 * 1000);
> +			elapsed_ms += 10;
> +		}
> +	}
> +	return true;
> +}
> +
>  struct thp_settings *thp_current_settings(void)
>  {
>  	if (!settings_index) {
> diff --git a/tools/testing/selftests/mm/hugepage_settings.h b/tools/testing/selftests/mm/hugepage_settings.h
> index 726c73c43c05..ba7d38370d43 100644
> --- a/tools/testing/selftests/mm/hugepage_settings.h
> +++ b/tools/testing/selftests/mm/hugepage_settings.h
> @@ -70,6 +70,7 @@ int thp_read_string(const char *name, const char * const strings[]);
>  void thp_write_string(const char *name, const char *val);
>  unsigned long thp_read_num(const char *name);
>  void thp_write_num(const char *name, unsigned long num);
> +void thp_update_num(const char *name, unsigned long num);
>
>  void thp_write_settings(struct thp_settings *settings);
>  void thp_read_settings(struct thp_settings *settings);
> @@ -83,6 +84,8 @@ static inline void thp_save_settings(void)
>  	hugepage_save_settings(/* thp = */ true, /* hugetlb = */ false);
>  }
>
> +bool khugepaged_full_pass(unsigned int timeout_s);
> +
>  void thp_set_read_ahead_path(char *path);
>  unsigned long thp_supported_orders(void);
>  unsigned long thp_shmem_supported_orders(void);
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 11/19] selftests/mm: add order-parameterized khugepaged collapse cases

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The mthp_khugepaged context runs the generic cases at a sub-PMD order,
which answers how many folios of that order a range ends up with.  It
cannot say which window they are in, so "the populated window collapsed
and its neighbour did not" and "one window collapsed twice" look alike.

Add four cases that check each aligned window on its own, with the
folio-order helpers in vm_util:

- collapse_order_single_window(): only the populated window collapses;
- collapse_order_partial_window(): the default max_ptes_none lets a window
  with one present PTE collapse;
- collapse_order_max_ptes_none(): with max_ptes_none=0 a full window
  collapses and one missing a page does not;
- collapse_order_mixed_sources(): sources that are already large folios of
  a smaller order collapse to the target.

Each case faults its region before MADV_HUGEPAGE with only the target
order enabled, so the sources are order 0 and the result can only come
from khugepaged.  They wait for a full pass rather than for the result to
appear: without a completed pass, "not collapsed" and "not scanned yet"
are the same thing.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 230 ++++++++++++++++++++++++
 1 file changed, 230 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 0008862e7cbc..0489967d6ee0 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -31,6 +31,8 @@ static unsigned long page_size;
 static int hpage_pmd_nr;
 static int anon_order;
 static int collapse_order;
+static int pagemap_fd = -1;
+static int kpageflags_fd = -1;
 
 #define PID_SMAPS "/proc/self/smaps"
 #define TEST_FILE "collapse_test_file"
@@ -1227,6 +1229,216 @@ static void madvise_retracted_page_tables(struct collapse_context *c,
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
 
+/* Smallest order khugepaged will consider for mTHP collapse. */
+#define MIN_MTHP_ORDER 2
+
+/*
+ * Order-parameterized collapse cases for the mthp_khugepaged context.  What
+ * they add over the generic cases run under that context is per-window
+ * detection: which aligned window collapsed, and which of its neighbours did
+ * not.  check_huge() answers how many folios of the order the range holds,
+ * which cannot tell one window from another.
+ *
+ * The region is faulted before MADV_HUGEPAGE, and the target order is only
+ * enabled for madvise, so the sources are always order 0 and the collapse
+ * product can only have come from khugepaged.
+ */
+static size_t mthp_window_size(void)
+{
+	return page_size << collapse_order;
+}
+
+static void mthp_push_target_order(void)
+{
+	struct thp_settings settings = *thp_current_settings();
+	int i;
+
+	/*
+	 * The target order, for madvise only, and nothing else enabled: the
+	 * cases fault their region before MADV_HUGEPAGE, so the sources are
+	 * order 0 whatever -s asked the fault path for.  That matters for the
+	 * cases built around a hole -- a large source folio would fill it in
+	 * and the window would collapse after all.
+	 * collapse_order_mixed_sources enables the source order it wants on
+	 * top of this.
+	 */
+	settings.thp_enabled = THP_NEVER;
+	for (i = 0; i < NR_ORDERS; i++)
+		settings.hugepages[i].enabled = THP_NEVER;
+	settings.hugepages[collapse_order].enabled = THP_MADVISE;
+	thp_push_settings(&settings);
+}
+
+static bool window_collapsed(void *p, size_t len)
+{
+	return is_range_backed_by_folio_orders(p, len, collapse_order,
+					       pagemap_fd, kpageflags_fd);
+}
+
+/* No aligned window in [p, p + len) is backed at the target order. */
+static bool window_not_collapsed(void *p, size_t len)
+{
+	size_t window = mthp_window_size();
+	char *addr = p;
+
+	for (; len >= window; addr += window, len -= window) {
+		if (window_collapsed(addr, window))
+			return false;
+	}
+	return true;
+}
+
+static bool khugepaged_wait_full_pass(void)
+{
+	/* Wait up to 30 seconds for the pass to complete. */
+	return khugepaged_full_pass(30);
+}
+
+static void collapse_order_single_window(struct collapse_context *c,
+					 struct mem_ops *ops)
+{
+	size_t window = mthp_window_size();
+	void *p;
+
+	mthp_push_target_order();
+
+	p = ops->setup_area(1);
+	ops->fault(p, window, 2 * window);
+	if (!window_not_collapsed(p, hpage_pmd_size))
+		ksft_exit_fail_msg("Unexpected large folio after fault\n");
+
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	ksft_print_msg("Collapse one fully populated window...");
+	if (!khugepaged_wait_full_pass())
+		fail("Timeout");
+	else if (window_collapsed(p + window, window) &&
+		 window_not_collapsed(p, window) &&
+		 window_not_collapsed(p + 2 * window,
+				      hpage_pmd_size - 2 * window))
+		success("OK");
+	else
+		fail("Fail");
+
+	validate_memory(p, window, 2 * window);
+	ops->cleanup_area(p, hpage_pmd_size);
+	thp_pop_settings();
+	ksft_test_result_report(exit_status, "%s\n", __func__);
+}
+
+static void collapse_order_partial_window(struct collapse_context *c,
+					  struct mem_ops *ops)
+{
+	void *p;
+
+	mthp_push_target_order();
+
+	p = ops->setup_area(1);
+	ops->fault(p, 0, page_size);
+	if (!window_not_collapsed(p, hpage_pmd_size))
+		ksft_exit_fail_msg("Unexpected large folio after fault\n");
+
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	ksft_print_msg("Collapse window with single PTE entry present...");
+	if (!khugepaged_wait_full_pass())
+		fail("Timeout");
+	else if (window_collapsed(p, mthp_window_size()))
+		success("OK");
+	else
+		fail("Fail");
+
+	validate_memory(p, 0, page_size);
+	ops->cleanup_area(p, hpage_pmd_size);
+	thp_pop_settings();
+	ksft_test_result_report(exit_status, "%s\n", __func__);
+}
+
+static void collapse_order_max_ptes_none(struct collapse_context *c,
+					 struct mem_ops *ops)
+{
+	struct thp_settings settings;
+	size_t window = mthp_window_size();
+	void *p;
+
+	mthp_push_target_order();
+	settings = *thp_current_settings();
+	settings.khugepaged.max_ptes_none = 0;
+	thp_push_settings(&settings);
+
+	p = ops->setup_area(1);
+	ops->fault(p, 0, 2 * window - page_size);
+	if (!window_not_collapsed(p, hpage_pmd_size))
+		ksft_exit_fail_msg("Unexpected large folio after fault\n");
+
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	ksft_print_msg("Collapse full window, not the one missing a page...");
+	if (!khugepaged_wait_full_pass())
+		fail("Timeout");
+	else if (window_collapsed(p, window) &&
+		 window_not_collapsed(p + window, window))
+		success("OK");
+	else
+		fail("Fail");
+
+	validate_memory(p, 0, 2 * window - page_size);
+	ops->cleanup_area(p, hpage_pmd_size);
+	thp_pop_settings();
+	thp_pop_settings();
+	ksft_test_result_report(exit_status, "%s\n", __func__);
+}
+
+static void collapse_order_mixed_sources(struct collapse_context *c,
+					 struct mem_ops *ops)
+{
+	struct thp_settings settings;
+	void *p;
+
+	if (collapse_order <= MIN_MTHP_ORDER) {
+		ksft_test_result_skip("%s: no source order below target\n",
+				      __func__);
+		return;
+	}
+
+	mthp_push_target_order();
+
+	/* Fault the whole region as order-MIN_MTHP_ORDER folios. */
+	settings = *thp_current_settings();
+	settings.hugepages[MIN_MTHP_ORDER].enabled = THP_ALWAYS;
+	thp_push_settings(&settings);
+	p = ops->setup_area(1);
+	ops->fault(p, 0, hpage_pmd_size);
+	thp_pop_settings();
+
+	/*
+	 * The order is enabled, but the allocator can still fall back under
+	 * fragmentation.  That leaves nothing to collapse from, which is the
+	 * machine's answer rather than a reason to end the run.
+	 */
+	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, MIN_MTHP_ORDER,
+					     pagemap_fd, kpageflags_fd)) {
+		ksft_print_msg("No order-%d sources to collapse...",
+			       MIN_MTHP_ORDER);
+		skip("Skip");
+		ops->cleanup_area(p, hpage_pmd_size);
+		thp_pop_settings();
+		ksft_test_result_report(exit_status, "%s\n", __func__);
+		return;
+	}
+
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	ksft_print_msg("Collapse region backed by smaller large folios...");
+	if (!khugepaged_wait_full_pass())
+		fail("Timeout");
+	else if (window_collapsed(p, hpage_pmd_size))
+		success("OK");
+	else
+		fail("Fail");
+
+	validate_memory(p, 0, hpage_pmd_size);
+	ops->cleanup_area(p, hpage_pmd_size);
+	thp_pop_settings();
+	ksft_test_result_report(exit_status, "%s\n", __func__);
+}
+
 static void usage(void)
 {
 	fprintf(stderr, "\nUsage: ./khugepaged [OPTIONS] <test type> [dir]\n\n");
@@ -1395,6 +1607,20 @@ int main(int argc, char **argv)
 
 	parse_test_type(argc, argv);
 
+	if (mthp_khugepaged_context &&
+	    !(thp_supported_orders() & (1UL << collapse_order)))
+		ksft_exit_skip("Order %d is not a supported anon THP order\n",
+			       collapse_order);
+
+	if (mthp_khugepaged_context) {
+		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+		if (pagemap_fd < 0)
+			ksft_exit_fail_perror("open(/proc/self/pagemap)");
+		kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
+		if (kpageflags_fd < 0)
+			ksft_exit_fail_perror("open(/proc/kpageflags)");
+	}
+
 	setbuf(stdout, NULL);
 
 	/*
@@ -1450,6 +1676,10 @@ int main(int argc, char **argv)
 	TEST(collapse_empty, madvise_context, anon_ops);
 
 	TEST(collapse_single_mthp, mthp_khugepaged_context, anon_ops);
+	TEST(collapse_order_single_window, mthp_khugepaged_context, anon_ops);
+	TEST(collapse_order_partial_window, mthp_khugepaged_context, anon_ops);
+	TEST(collapse_order_max_ptes_none, mthp_khugepaged_context, anon_ops);
+	TEST(collapse_order_mixed_sources, mthp_khugepaged_context, anon_ops);
 
 	TEST(collapse_single_pte_entry, khugepaged_context, anon_ops);
 	TEST(collapse_single_pte_entry, khugepaged_context, read_only_file_ops);
-- 
2.54.0

Re: [PATCH v4 11/19] selftests/mm: add order-parameterized khugepaged collapse cases

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:53AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> The mthp_khugepaged context runs the generic cases at a sub-PMD order,
> which answers how many folios of that order a range ends up with.  It
> cannot say which window they are in, so "the populated window collapsed
> and its neighbour did not" and "one window collapsed twice" look alike.
>
> Add four cases that check each aligned window on its own, with the
> folio-order helpers in vm_util:
>
> - collapse_order_single_window(): only the populated window collapses;
> - collapse_order_partial_window(): the default max_ptes_none lets a window
>   with one present PTE collapse;
> - collapse_order_max_ptes_none(): with max_ptes_none=0 a full window
>   collapses and one missing a page does not;
> - collapse_order_mixed_sources(): sources that are already large folios of
>   a smaller order collapse to the target.
>
> Each case faults its region before MADV_HUGEPAGE with only the target
> order enabled, so the sources are order 0 and the result can only come
> from khugepaged.  They wait for a full pass rather than for the result to
> appear: without a completed pass, "not collapsed" and "not scanned yet"
> are the same thing.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Same comments on comments as other patches - far too dense, read like a
discussion and not a terse description of something the code doesn't make
clear.

Please write comments yourself, LLMs are terrible at it (I mean I feel the
same goes for code also).

> ---
>  tools/testing/selftests/mm/khugepaged.c | 230 ++++++++++++++++++++++++
>  1 file changed, 230 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 0008862e7cbc..0489967d6ee0 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -31,6 +31,8 @@ static unsigned long page_size;
>  static int hpage_pmd_nr;
>  static int anon_order;
>  static int collapse_order;
> +static int pagemap_fd = -1;
> +static int kpageflags_fd = -1;
>
>  #define PID_SMAPS "/proc/self/smaps"
>  #define TEST_FILE "collapse_test_file"
> @@ -1227,6 +1229,216 @@ static void madvise_retracted_page_tables(struct collapse_context *c,
>  	ksft_test_result_report(exit_status, "%s\n", __func__);
>  }
>
> +/* Smallest order khugepaged will consider for mTHP collapse. */
> +#define MIN_MTHP_ORDER 2
> +
> +/*
> + * Order-parameterized collapse cases for the mthp_khugepaged context.  What
> + * they add over the generic cases run under that context is per-window
> + * detection: which aligned window collapsed, and which of its neighbours did
> + * not.  check_huge() answers how many folios of the order the range holds,
> + * which cannot tell one window from another.
> + *
> + * The region is faulted before MADV_HUGEPAGE, and the target order is only
> + * enabled for madvise, so the sources are always order 0 and the collapse
> + * product can only have come from khugepaged.
> + */
> +static size_t mthp_window_size(void)
> +{
> +	return page_size << collapse_order;
> +}
> +
> +static void mthp_push_target_order(void)
> +{
> +	struct thp_settings settings = *thp_current_settings();
> +	int i;
> +
> +	/*
> +	 * The target order, for madvise only, and nothing else enabled: the
> +	 * cases fault their region before MADV_HUGEPAGE, so the sources are
> +	 * order 0 whatever -s asked the fault path for.  That matters for the
> +	 * cases built around a hole -- a large source folio would fill it in
> +	 * and the window would collapse after all.
> +	 * collapse_order_mixed_sources enables the source order it wants on
> +	 * top of this.
> +	 */
> +	settings.thp_enabled = THP_NEVER;
> +	for (i = 0; i < NR_ORDERS; i++)
> +		settings.hugepages[i].enabled = THP_NEVER;
> +	settings.hugepages[collapse_order].enabled = THP_MADVISE;
> +	thp_push_settings(&settings);
> +}
> +
> +static bool window_collapsed(void *p, size_t len)
> +{
> +	return is_range_backed_by_folio_orders(p, len, collapse_order,
> +					       pagemap_fd, kpageflags_fd);
> +}
> +
> +/* No aligned window in [p, p + len) is backed at the target order. */
> +static bool window_not_collapsed(void *p, size_t len)
> +{
> +	size_t window = mthp_window_size();
> +	char *addr = p;
> +
> +	for (; len >= window; addr += window, len -= window) {
> +		if (window_collapsed(addr, window))
> +			return false;
> +	}
> +	return true;
> +}
> +
> +static bool khugepaged_wait_full_pass(void)
> +{
> +	/* Wait up to 30 seconds for the pass to complete. */
> +	return khugepaged_full_pass(30);
> +}
> +
> +static void collapse_order_single_window(struct collapse_context *c,
> +					 struct mem_ops *ops)
> +{
> +	size_t window = mthp_window_size();
> +	void *p;
> +
> +	mthp_push_target_order();
> +
> +	p = ops->setup_area(1);
> +	ops->fault(p, window, 2 * window);
> +	if (!window_not_collapsed(p, hpage_pmd_size))
> +		ksft_exit_fail_msg("Unexpected large folio after fault\n");
> +
> +	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> +	ksft_print_msg("Collapse one fully populated window...");
> +	if (!khugepaged_wait_full_pass())
> +		fail("Timeout");
> +	else if (window_collapsed(p + window, window) &&
> +		 window_not_collapsed(p, window) &&
> +		 window_not_collapsed(p + 2 * window,
> +				      hpage_pmd_size - 2 * window))
> +		success("OK");
> +	else
> +		fail("Fail");
> +
> +	validate_memory(p, window, 2 * window);
> +	ops->cleanup_area(p, hpage_pmd_size);
> +	thp_pop_settings();
> +	ksft_test_result_report(exit_status, "%s\n", __func__);
> +}
> +
> +static void collapse_order_partial_window(struct collapse_context *c,
> +					  struct mem_ops *ops)
> +{
> +	void *p;
> +
> +	mthp_push_target_order();
> +
> +	p = ops->setup_area(1);
> +	ops->fault(p, 0, page_size);
> +	if (!window_not_collapsed(p, hpage_pmd_size))
> +		ksft_exit_fail_msg("Unexpected large folio after fault\n");
> +
> +	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> +	ksft_print_msg("Collapse window with single PTE entry present...");
> +	if (!khugepaged_wait_full_pass())
> +		fail("Timeout");
> +	else if (window_collapsed(p, mthp_window_size()))
> +		success("OK");
> +	else
> +		fail("Fail");
> +
> +	validate_memory(p, 0, page_size);
> +	ops->cleanup_area(p, hpage_pmd_size);
> +	thp_pop_settings();
> +	ksft_test_result_report(exit_status, "%s\n", __func__);
> +}
> +
> +static void collapse_order_max_ptes_none(struct collapse_context *c,
> +					 struct mem_ops *ops)
> +{
> +	struct thp_settings settings;
> +	size_t window = mthp_window_size();
> +	void *p;
> +
> +	mthp_push_target_order();
> +	settings = *thp_current_settings();
> +	settings.khugepaged.max_ptes_none = 0;
> +	thp_push_settings(&settings);
> +
> +	p = ops->setup_area(1);
> +	ops->fault(p, 0, 2 * window - page_size);
> +	if (!window_not_collapsed(p, hpage_pmd_size))
> +		ksft_exit_fail_msg("Unexpected large folio after fault\n");
> +
> +	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> +	ksft_print_msg("Collapse full window, not the one missing a page...");
> +	if (!khugepaged_wait_full_pass())
> +		fail("Timeout");
> +	else if (window_collapsed(p, window) &&
> +		 window_not_collapsed(p + window, window))
> +		success("OK");
> +	else
> +		fail("Fail");
> +
> +	validate_memory(p, 0, 2 * window - page_size);
> +	ops->cleanup_area(p, hpage_pmd_size);
> +	thp_pop_settings();
> +	thp_pop_settings();
> +	ksft_test_result_report(exit_status, "%s\n", __func__);
> +}
> +
> +static void collapse_order_mixed_sources(struct collapse_context *c,
> +					 struct mem_ops *ops)
> +{
> +	struct thp_settings settings;
> +	void *p;
> +
> +	if (collapse_order <= MIN_MTHP_ORDER) {
> +		ksft_test_result_skip("%s: no source order below target\n",
> +				      __func__);
> +		return;
> +	}
> +
> +	mthp_push_target_order();
> +
> +	/* Fault the whole region as order-MIN_MTHP_ORDER folios. */
> +	settings = *thp_current_settings();
> +	settings.hugepages[MIN_MTHP_ORDER].enabled = THP_ALWAYS;
> +	thp_push_settings(&settings);
> +	p = ops->setup_area(1);
> +	ops->fault(p, 0, hpage_pmd_size);
> +	thp_pop_settings();
> +
> +	/*
> +	 * The order is enabled, but the allocator can still fall back under
> +	 * fragmentation.  That leaves nothing to collapse from, which is the
> +	 * machine's answer rather than a reason to end the run.
> +	 */
> +	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, MIN_MTHP_ORDER,
> +					     pagemap_fd, kpageflags_fd)) {
> +		ksft_print_msg("No order-%d sources to collapse...",
> +			       MIN_MTHP_ORDER);
> +		skip("Skip");
> +		ops->cleanup_area(p, hpage_pmd_size);
> +		thp_pop_settings();
> +		ksft_test_result_report(exit_status, "%s\n", __func__);
> +		return;
> +	}
> +
> +	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> +	ksft_print_msg("Collapse region backed by smaller large folios...");
> +	if (!khugepaged_wait_full_pass())
> +		fail("Timeout");
> +	else if (window_collapsed(p, hpage_pmd_size))
> +		success("OK");
> +	else
> +		fail("Fail");
> +
> +	validate_memory(p, 0, hpage_pmd_size);
> +	ops->cleanup_area(p, hpage_pmd_size);
> +	thp_pop_settings();
> +	ksft_test_result_report(exit_status, "%s\n", __func__);
> +}
> +
>  static void usage(void)
>  {
>  	fprintf(stderr, "\nUsage: ./khugepaged [OPTIONS] <test type> [dir]\n\n");
> @@ -1395,6 +1607,20 @@ int main(int argc, char **argv)
>
>  	parse_test_type(argc, argv);
>
> +	if (mthp_khugepaged_context &&
> +	    !(thp_supported_orders() & (1UL << collapse_order)))
> +		ksft_exit_skip("Order %d is not a supported anon THP order\n",
> +			       collapse_order);
> +
> +	if (mthp_khugepaged_context) {
> +		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> +		if (pagemap_fd < 0)
> +			ksft_exit_fail_perror("open(/proc/self/pagemap)");
> +		kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
> +		if (kpageflags_fd < 0)
> +			ksft_exit_fail_perror("open(/proc/kpageflags)");
> +	}
> +
>  	setbuf(stdout, NULL);
>
>  	/*
> @@ -1450,6 +1676,10 @@ int main(int argc, char **argv)
>  	TEST(collapse_empty, madvise_context, anon_ops);
>
>  	TEST(collapse_single_mthp, mthp_khugepaged_context, anon_ops);
> +	TEST(collapse_order_single_window, mthp_khugepaged_context, anon_ops);
> +	TEST(collapse_order_partial_window, mthp_khugepaged_context, anon_ops);
> +	TEST(collapse_order_max_ptes_none, mthp_khugepaged_context, anon_ops);
> +	TEST(collapse_order_mixed_sources, mthp_khugepaged_context, anon_ops);
>
>  	TEST(collapse_single_pte_entry, khugepaged_context, anon_ops);
>  	TEST(collapse_single_pte_entry, khugepaged_context, read_only_file_ops);
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 12/19] selftests/mm: parameterize the mixed-source collapse case by source order

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

collapse_order_mixed_sources() faults its region as order-2 folios and
collapses them to the -c target.  Order 2 sits below the contpte threshold
on both arm64 page-size configurations, so nothing in this suite unfolds a
contpte source on purpose.

Let -s name the source order alongside -c.  The case then faults at that
order, keeping order 2 when -s is absent, and the source order has to be a
supported mTHP order below the target.  The other mTHP cases are
unaffected: mthp_push_target_order() enables only the target order.

"-s 5 -c 7" on arm64/64K then collapses contpte-mapped sources into a
larger mTHP.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 0489967d6ee0..1844ddd77b59 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1389,10 +1389,13 @@ static void collapse_order_max_ptes_none(struct collapse_context *c,
 static void collapse_order_mixed_sources(struct collapse_context *c,
 					 struct mem_ops *ops)
 {
+	int source_order = anon_order ? anon_order : MIN_MTHP_ORDER;
 	struct thp_settings settings;
 	void *p;
 
-	if (collapse_order <= MIN_MTHP_ORDER) {
+	/* Sources must be a supported mTHP order strictly below the target. */
+	if (source_order >= collapse_order ||
+	    !(thp_supported_orders() & (1UL << source_order))) {
 		ksft_test_result_skip("%s: no source order below target\n",
 				      __func__);
 		return;
@@ -1400,23 +1403,22 @@ static void collapse_order_mixed_sources(struct collapse_context *c,
 
 	mthp_push_target_order();
 
-	/* Fault the whole region as order-MIN_MTHP_ORDER folios. */
+	/* Fault the whole region as order-@source_order folios. */
 	settings = *thp_current_settings();
-	settings.hugepages[MIN_MTHP_ORDER].enabled = THP_ALWAYS;
+	settings.hugepages[source_order].enabled = THP_ALWAYS;
 	thp_push_settings(&settings);
 	p = ops->setup_area(1);
 	ops->fault(p, 0, hpage_pmd_size);
 	thp_pop_settings();
 
 	/*
-	 * The order is enabled, but the allocator can still fall back under
-	 * fragmentation.  That leaves nothing to collapse from, which is the
-	 * machine's answer rather than a reason to end the run.
+	 * The order is enabled and supported, but the allocator can still fall
+	 * back under fragmentation.  That leaves nothing to collapse from,
+	 * which is the machine's answer rather than a reason to end the run.
 	 */
-	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, MIN_MTHP_ORDER,
+	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, source_order,
 					     pagemap_fd, kpageflags_fd)) {
-		ksft_print_msg("No order-%d sources to collapse...",
-			       MIN_MTHP_ORDER);
+		ksft_print_msg("No order-%d sources to collapse...", source_order);
 		skip("Skip");
 		ops->cleanup_area(p, hpage_pmd_size);
 		thp_pop_settings();
@@ -1425,7 +1427,8 @@ static void collapse_order_mixed_sources(struct collapse_context *c,
 	}
 
 	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
-	ksft_print_msg("Collapse region backed by smaller large folios...");
+	ksft_print_msg("Collapse region backed by order-%d sources...",
+		       source_order);
 	if (!khugepaged_wait_full_pass())
 		fail("Timeout");
 	else if (window_collapsed(p, hpage_pmd_size))
@@ -1456,6 +1459,8 @@ static void usage(void)
 	fprintf(stderr,	"\t\t-s: mTHP size, expressed as page order.\n");
 	fprintf(stderr,	"\t\t    Defaults to 0. Use this size for anon or shmem allocations.\n");
 	fprintf(stderr,	"\t\t-c: collapse order for mTHP collapse, expressed as page order.\n");
+	fprintf(stderr,	"\t\t    With -s, -s names the mTHP source order for the\n");
+	fprintf(stderr,	"\t\t    mixed-source case (source order below the target).\n");
 	exit(1);
 }
 
-- 
2.54.0

Re: [PATCH v4 12/19] selftests/mm: parameterize the mixed-source collapse case by source order

[Lorenzo Stoakes (ARM)]

On Sat, Aug 15, 2026 at 02:58:54AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> collapse_order_mixed_sources() faults its region as order-2 folios and
> collapses them to the -c target.  Order 2 sits below the contpte threshold
> on both arm64 page-size configurations, so nothing in this suite unfolds a
> contpte source on purpose.
>
> Let -s name the source order alongside -c.  The case then faults at that
> order, keeping order 2 when -s is absent, and the source order has to be a
> supported mTHP order below the target.  The other mTHP cases are
> unaffected: mthp_push_target_order() enables only the target order.
>
> "-s 5 -c 7" on arm64/64K then collapses contpte-mapped sources into a
> larger mTHP.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Looks reasonable to me so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 0489967d6ee0..1844ddd77b59 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -1389,10 +1389,13 @@ static void collapse_order_max_ptes_none(struct collapse_context *c,
>  static void collapse_order_mixed_sources(struct collapse_context *c,
>  					 struct mem_ops *ops)
>  {
> +	int source_order = anon_order ? anon_order : MIN_MTHP_ORDER;
>  	struct thp_settings settings;
>  	void *p;
>
> -	if (collapse_order <= MIN_MTHP_ORDER) {
> +	/* Sources must be a supported mTHP order strictly below the target. */
> +	if (source_order >= collapse_order ||
> +	    !(thp_supported_orders() & (1UL << source_order))) {
>  		ksft_test_result_skip("%s: no source order below target\n",
>  				      __func__);
>  		return;
> @@ -1400,23 +1403,22 @@ static void collapse_order_mixed_sources(struct collapse_context *c,
>
>  	mthp_push_target_order();
>
> -	/* Fault the whole region as order-MIN_MTHP_ORDER folios. */
> +	/* Fault the whole region as order-@source_order folios. */
>  	settings = *thp_current_settings();
> -	settings.hugepages[MIN_MTHP_ORDER].enabled = THP_ALWAYS;
> +	settings.hugepages[source_order].enabled = THP_ALWAYS;
>  	thp_push_settings(&settings);
>  	p = ops->setup_area(1);
>  	ops->fault(p, 0, hpage_pmd_size);
>  	thp_pop_settings();
>
>  	/*
> -	 * The order is enabled, but the allocator can still fall back under
> -	 * fragmentation.  That leaves nothing to collapse from, which is the
> -	 * machine's answer rather than a reason to end the run.
> +	 * The order is enabled and supported, but the allocator can still fall
> +	 * back under fragmentation.  That leaves nothing to collapse from,
> +	 * which is the machine's answer rather than a reason to end the run.
>  	 */
> -	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, MIN_MTHP_ORDER,
> +	if (!is_range_backed_by_folio_orders(p, hpage_pmd_size, source_order,
>  					     pagemap_fd, kpageflags_fd)) {
> -		ksft_print_msg("No order-%d sources to collapse...",
> -			       MIN_MTHP_ORDER);
> +		ksft_print_msg("No order-%d sources to collapse...", source_order);
>  		skip("Skip");
>  		ops->cleanup_area(p, hpage_pmd_size);
>  		thp_pop_settings();
> @@ -1425,7 +1427,8 @@ static void collapse_order_mixed_sources(struct collapse_context *c,
>  	}
>
>  	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> -	ksft_print_msg("Collapse region backed by smaller large folios...");
> +	ksft_print_msg("Collapse region backed by order-%d sources...",
> +		       source_order);
>  	if (!khugepaged_wait_full_pass())
>  		fail("Timeout");
>  	else if (window_collapsed(p, hpage_pmd_size))
> @@ -1456,6 +1459,8 @@ static void usage(void)
>  	fprintf(stderr,	"\t\t-s: mTHP size, expressed as page order.\n");
>  	fprintf(stderr,	"\t\t    Defaults to 0. Use this size for anon or shmem allocations.\n");
>  	fprintf(stderr,	"\t\t-c: collapse order for mTHP collapse, expressed as page order.\n");
> +	fprintf(stderr,	"\t\t    With -s, -s names the mTHP source order for the\n");
> +	fprintf(stderr,	"\t\t    mixed-source case (source order below the target).\n");
>  	exit(1);
>  }
>
> --
> 2.54.0
>

--
Cheers, Lorenzo

[PATCH v4 13/19] selftests/mm: cover a shared-source collapse write race

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

collapse_fork() checks that a fork-shared range collapses in the process
that asks for it while the co-sharer keeps its own page, but the co-sharer
sits still while that happens.

Add a case where the co-sharer writes to the shared range throughout the
collapse.  CoW has to keep the two sides apart under those writes: the
collapsing child must see the content from before the fork, and the
writing parent must see only its own writes.

The co-sharer unshares one page every 10ms, and only once the collapsing
side says it is about to start.  Writing the range in a burst breaks CoW
on all of it before the collapse begins, leaving the child to collapse
pages that are already exclusive to it.

Nothing here is new behaviour: the case passes on mainline, and locks in
isolation that collapse already provides.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c | 94 +++++++++++++++++++++++++
 1 file changed, 94 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 1844ddd77b59..856decd2950a 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1184,6 +1184,97 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
 
+/*
+ * Content stays isolated while a co-sharer writes concurrently. A shared
+ * source is copied live (not frozen), relying on it being CoW - immutable
+ * for the duration of the copy; a co-sharer's write goes to a CoW copy. The
+ * collapsing child must see the pre-fork content, the writing parent only
+ * its own writes.
+ */
+static void collapse_fork_cow_race(struct collapse_context *c, struct mem_ops *ops)
+{
+	const unsigned long shared = 64 * page_size;
+	const int stride = page_size / sizeof(int);
+	int wstatus, child_status, i, n = shared / page_size;
+	/* volatile: the loop below must really store, on every iteration */
+	volatile int *ip;
+	pid_t child;
+	int sync[2];
+	char go = 1;
+	void *p;
+
+	p = ops->setup_area(1);
+	ip = p;
+	ops->fault(p, 0, shared);		/* shared prefix, pre-fork pattern */
+	if (pipe(sync))
+		ksft_exit_fail_perror("pipe()");
+
+	ksft_print_msg("Fork, collapse in the child while the parent rewrites...");
+	child = fork();
+	if (!child) {
+		int collapse_status;
+
+		close(sync[0]);
+		ops->fault(p, shared, hpage_pmd_size);	/* private remainder */
+		/* Start the parent unsharing, and give it a head start. */
+		if (write(sync[1], &go, 1) != 1)
+			_exit(KSFT_FAIL);
+		usleep(5000);
+		c->collapse("Collapse a range shared with a writing co-sharer",
+			    p, 1, ops, true);
+		collapse_status = exit_status;
+		for (i = 0; i < n; i++)
+			if (ip[i * stride] != i + 0xdead0000)
+				break;
+		if (i == n)
+			success("OK");
+		else
+			fail("Fail: child content");
+		/* The content check must not bury a failed collapse. */
+		if (exit_status != KSFT_FAIL)
+			exit_status = collapse_status;
+		ops->cleanup_area(p, hpage_pmd_size);
+		_exit(exit_status);
+	}
+
+	close(sync[1]);
+	if (read(sync[0], &go, 1) != 1)
+		ksft_exit_fail_msg("child never reached the collapse\n");
+
+	/*
+	 * Unshare one page at a time.  A burst would break CoW on all of them
+	 * in microseconds -- wait_for_scan() does not even poll for TICK --
+	 * and the child would collapse pages already exclusive to it.
+	 */
+	i = 0;
+	do {
+		if (i < n)
+			ip[i * stride] = i + 0xbeef0000;
+		i++;
+		usleep(10 * 1000);
+	} while (waitpid(child, &wstatus, WNOHANG) == 0);
+
+	/* Whatever the paced sweep did not reach, so the check below is exact. */
+	for (; i < n; i++)
+		ip[i * stride] = i + 0xbeef0000;
+	/* A child that died reading the racing pages is a failure, not a zero. */
+	child_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL;
+
+	ksft_print_msg("Check the parent sees only its own writes...");
+	for (i = 0; i < n; i++)
+		if (ip[i * stride] != i + 0xbeef0000)
+			break;
+	if (i == n)
+		success("OK");
+	else
+		fail("Fail: parent content");
+	ops->cleanup_area(p, hpage_pmd_size);
+	/* Same again: our own check must not bury the child's verdict. */
+	if (exit_status != KSFT_FAIL)
+		exit_status = child_status;
+	ksft_test_result_report(exit_status, "%s\n", __func__);
+}
+
 static void madvise_collapse_existing_thps(struct collapse_context *c,
 					   struct mem_ops *ops)
 {
@@ -1740,6 +1831,9 @@ int main(int argc, char **argv)
 	TEST(collapse_max_ptes_shared, khugepaged_context, anon_ops);
 	TEST(collapse_max_ptes_shared, madvise_context, anon_ops);
 
+	TEST(collapse_fork_cow_race, khugepaged_context, anon_ops);
+	TEST(collapse_fork_cow_race, madvise_context, anon_ops);
+
 	TEST(madvise_collapse_existing_thps, madvise_context, anon_ops);
 	TEST(madvise_collapse_existing_thps, madvise_context, read_only_file_ops);
 	TEST(madvise_collapse_existing_thps, madvise_context, read_write_file_read_ops);
-- 
2.54.0

[PATCH v4 14/19] selftests/mm: run every supported collapse order by default

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The mTHP collapse cases only run when the caller names both the context
and an order, so a plain ./khugepaged covers the PMD contexts on anon and
nothing else.  run_vmtests.sh pinned order 4 and covered no other.

Run the mTHP cases once per supported anon THP order below the PMD when
-c is absent, and pull that context into both the no-argument invocation
and "all".  Around that:

  - -c still pins one order, and now says what is wrong instead of
    printing the usage text.  An order at or below the -s source order is
    skipped: the sources would already be the size being asked for.

  - Both orders end up as array indices and shift counts, so -s and -c
    are range-checked before they get there.

  - The mTHP context has only anon cases, so a run that names a different
    mem_type -- "all:shmem", say -- drops it again rather than refusing
    to start.  Naming both explicitly still refuses.

  - A case carries the order it was registered at, so a result names it:

      # Run test: collapse_single_mthp (mthp_khugepaged:anon, order 6)

On x86-64 with 4K pages that is orders 2 through 8, and ./khugepaged goes
from 28 results to 77 in 21 seconds, so run_vmtests.sh can drop its
pinned order-4 line.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged.c   | 105 +++++++++++++++++-----
 tools/testing/selftests/mm/run_vmtests.sh |   2 -
 2 files changed, 85 insertions(+), 22 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 856decd2950a..172e7307eeee 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -31,6 +31,9 @@ static unsigned long page_size;
 static int hpage_pmd_nr;
 static int anon_order;
 static int collapse_order;
+static bool collapse_order_given;
+static int collapse_orders[NR_ORDERS];
+static int nr_collapse_orders;
 static int pagemap_fd = -1;
 static int kpageflags_fd = -1;
 
@@ -1550,6 +1553,7 @@ static void usage(void)
 	fprintf(stderr,	"\t\t-s: mTHP size, expressed as page order.\n");
 	fprintf(stderr,	"\t\t    Defaults to 0. Use this size for anon or shmem allocations.\n");
 	fprintf(stderr,	"\t\t-c: collapse order for mTHP collapse, expressed as page order.\n");
+	fprintf(stderr,	"\t\t    Defaults to every supported order below the PMD.\n");
 	fprintf(stderr,	"\t\t    With -s, -s names the mTHP source order for the\n");
 	fprintf(stderr,	"\t\t    mixed-source case (source order below the target).\n");
 	exit(1);
@@ -1557,6 +1561,7 @@ static void usage(void)
 
 static void parse_test_type(int argc, char **argv)
 {
+	bool mthp_context_implied = false;
 	int opt;
 	char *buf;
 	const char *token;
@@ -1568,6 +1573,7 @@ static void parse_test_type(int argc, char **argv)
 			break;
 		case 'c':
 			collapse_order = atoi(optarg);
+			collapse_order_given = true;
 			break;
 		case 'h':
 		default:
@@ -1575,12 +1581,25 @@ static void parse_test_type(int argc, char **argv)
 		}
 	}
 
+	/*
+	 * Both orders end up as array indices and shift counts, so neither
+	 * can be negative, and a zero collapse order asks for base pages.
+	 */
+	if (anon_order < 0 || anon_order > hpage_pmd_order)
+		ksft_exit_fail_msg("-s takes an order in 0..%d, not %d\n",
+				   hpage_pmd_order, anon_order);
+	if (collapse_order_given &&
+	    (collapse_order <= 0 || collapse_order >= hpage_pmd_order))
+		ksft_exit_fail_msg("-c takes an order in 1..%d, not %d\n",
+				   hpage_pmd_order - 1, collapse_order);
+
 	argv += optind;
 	argc -= optind;
 
 	if (argc == 0) {
-		/* Backwards compatibility */
+		/* Everything that needs no argument of its own: anon, every context */
 		khugepaged_context =  &__khugepaged_context;
+		mthp_khugepaged_context =  &__mthp_khugepaged_context;
 		madvise_context =  &__madvise_context;
 		anon_ops = &__anon_ops;
 		return;
@@ -1591,13 +1610,19 @@ static void parse_test_type(int argc, char **argv)
 
 	if (!strcmp(token, "all")) {
 		khugepaged_context =  &__khugepaged_context;
+		mthp_khugepaged_context =  &__mthp_khugepaged_context;
 		madvise_context =  &__madvise_context;
+
+		/*
+		 * "all" sweeps the mTHP context in, but it only has anon
+		 * cases: step it aside for the other mem_types rather than
+		 * refusing the whole run.
+		 */
+		mthp_context_implied = true;
 	} else if (!strcmp(token, "khugepaged")) {
 		khugepaged_context =  &__khugepaged_context;
 	} else if (!strcmp(token, "mthp_khugepaged")) {
 		mthp_khugepaged_context =  &__mthp_khugepaged_context;
-		if (collapse_order <= 0 || collapse_order >= hpage_pmd_order)
-			usage();
 	} else if (!strcmp(token, "madvise")) {
 		madvise_context =  &__madvise_context;
 	} else {
@@ -1613,20 +1638,20 @@ static void parse_test_type(int argc, char **argv)
 		read_write_file_write_ops =  &__read_write_file_write_ops;
 		anon_ops = &__anon_ops;
 		shmem_ops = &__shmem_ops;
-		if (mthp_khugepaged_context)
-			usage();
 	} else if (!strcmp(buf, "anon")) {
 		anon_ops = &__anon_ops;
 	} else if (!strcmp(buf, "file")) {
 		read_only_file_ops =  &__read_only_file_ops;
 		read_write_file_read_ops =  &__read_write_file_read_ops;
 		read_write_file_write_ops =  &__read_write_file_write_ops;
-		if (mthp_khugepaged_context)
+		if (mthp_khugepaged_context && !mthp_context_implied)
 			usage();
+		mthp_khugepaged_context = NULL;
 	} else if (!strcmp(buf, "shmem")) {
 		shmem_ops = &__shmem_ops;
-		if (mthp_khugepaged_context)
+		if (mthp_khugepaged_context && !mthp_context_implied)
 			usage();
+		mthp_khugepaged_context = NULL;
 	} else {
 		usage();
 	}
@@ -1648,6 +1673,7 @@ struct test_case {
 	struct mem_ops *ops;
 	const char *desc;
 	test_fn fn;
+	int order;		/* mTHP contexts: the collapse order */
 };
 
 #define MAX_TEST_CASES 256
@@ -1663,6 +1689,7 @@ static int nr_test_cases;
 			.ops	= o,					\
 			.desc	= #t,					\
 			.fn	= t,					\
+			.order	= collapse_order,			\
 		};							\
 	}								\
 	} while (0)
@@ -1703,10 +1730,37 @@ int main(int argc, char **argv)
 
 	parse_test_type(argc, argv);
 
-	if (mthp_khugepaged_context &&
-	    !(thp_supported_orders() & (1UL << collapse_order)))
-		ksft_exit_skip("Order %d is not a supported anon THP order\n",
-			       collapse_order);
+	if (mthp_khugepaged_context) {
+		unsigned long orders = thp_supported_orders();
+
+		if (collapse_order_given) {
+			/* -c pins one order; it has to be one we can build */
+			if (!(orders & (1UL << collapse_order)))
+				ksft_exit_skip("Order %d is not a supported anon THP order\n",
+					       collapse_order);
+			if (collapse_order <= anon_order)
+				ksft_exit_skip("-c %d needs a source order below it, -s says %d\n",
+					       collapse_order, anon_order);
+			collapse_orders[nr_collapse_orders++] = collapse_order;
+		} else {
+			/*
+			 * Otherwise every order a collapse could produce.  -s
+			 * makes the fault path hand out folios of that order,
+			 * so a target at or below it has nothing to collapse:
+			 * the sources are already the size being asked for.
+			 */
+			int first = anon_order + 1;
+
+			if (first < MIN_MTHP_ORDER)
+				first = MIN_MTHP_ORDER;
+			for (int i = first; i < hpage_pmd_order; i++) {
+				if (orders & (1UL << i))
+					collapse_orders[nr_collapse_orders++] = i;
+			}
+			if (!nr_collapse_orders)
+				ksft_print_msg("mTHP cases skipped: no order above the source\n");
+		}
+	}
 
 	if (mthp_khugepaged_context) {
 		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
@@ -1760,7 +1814,17 @@ int main(int argc, char **argv)
 	TEST(collapse_full, khugepaged_context, read_write_file_read_ops);
 	TEST(collapse_full, khugepaged_context, read_write_file_write_ops);
 	TEST(collapse_full, khugepaged_context, shmem_ops);
-	TEST(collapse_full, mthp_khugepaged_context, anon_ops);
+	for (int i = 0; i < nr_collapse_orders; i++) {
+		collapse_order = collapse_orders[i];
+		TEST(collapse_full, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_empty, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_single_mthp, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_order_single_window, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_order_partial_window, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_order_max_ptes_none, mthp_khugepaged_context, anon_ops);
+		TEST(collapse_order_mixed_sources, mthp_khugepaged_context, anon_ops);
+	}
+
 	TEST(collapse_full, madvise_context, anon_ops);
 	TEST(collapse_full, madvise_context, read_only_file_ops);
 	TEST(collapse_full, madvise_context, read_write_file_read_ops);
@@ -1768,15 +1832,8 @@ int main(int argc, char **argv)
 	TEST(collapse_full, madvise_context, shmem_ops);
 
 	TEST(collapse_empty, khugepaged_context, anon_ops);
-	TEST(collapse_empty, mthp_khugepaged_context, anon_ops);
 	TEST(collapse_empty, madvise_context, anon_ops);
 
-	TEST(collapse_single_mthp, mthp_khugepaged_context, anon_ops);
-	TEST(collapse_order_single_window, mthp_khugepaged_context, anon_ops);
-	TEST(collapse_order_partial_window, mthp_khugepaged_context, anon_ops);
-	TEST(collapse_order_max_ptes_none, mthp_khugepaged_context, anon_ops);
-	TEST(collapse_order_mixed_sources, mthp_khugepaged_context, anon_ops);
-
 	TEST(collapse_single_pte_entry, khugepaged_context, anon_ops);
 	TEST(collapse_single_pte_entry, khugepaged_context, read_only_file_ops);
 	TEST(collapse_single_pte_entry, khugepaged_context, read_write_file_read_ops);
@@ -1849,7 +1906,15 @@ int main(int argc, char **argv)
 	for (int i = 0; i < nr_test_cases; i++) {
 		struct test_case *t = &test_cases[i];
 
-		ksft_print_msg("\n# Run test: %s (%s:%s)\n", t->desc, t->ctx->name, t->ops->name);
+		if (t->ctx == &__mthp_khugepaged_context) {
+			collapse_order = t->order;
+			ksft_print_msg("\n# Run test: %s (%s:%s, order %d)\n",
+				       t->desc, t->ctx->name, t->ops->name,
+				       t->order);
+		} else {
+			ksft_print_msg("\n# Run test: %s (%s:%s)\n", t->desc,
+				       t->ctx->name, t->ops->name);
+		}
 		t->fn(t->ctx, t->ops);
 	}
 
diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
index 2652a7920b80..8bf898b71350 100755
--- a/tools/testing/selftests/mm/run_vmtests.sh
+++ b/tools/testing/selftests/mm/run_vmtests.sh
@@ -412,8 +412,6 @@ CATEGORY="thp" run_test ./khugepaged all:shmem
 
 CATEGORY="thp" run_test ./khugepaged -s 4 all:shmem
 
-CATEGORY="thp" run_test ./khugepaged -c 4 mthp_khugepaged:anon
-
 # Try to create XFS if not provided
 if [ -z "${SPLIT_HUGE_PAGE_TEST_XFS_PATH}" ]; then
     if test_selected "thp"; then
-- 
2.54.0

[PATCH v4 15/19] selftests/mm: check that one khugepaged pass collapses one window

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The khugepaged cases drive the daemon through sysfs: a store to
scan_sleep_millisecs wakes it, and full_scans advancing by two marks one
pass that started after setup.  Every khugepaged result in the suite
rests on that pair, and nothing checks it.

Add khugepaged_sync_check.  Each step:

  - prepare one aligned window
  - record its source PFNs from pagemap
  - run one khugepaged_full_pass() barrier
  - require the window came out collapsed, with exactly one collapse
    attempt attributed to it

The anon events carry no virtual address, so an attempt is matched by the
source folio PFN and order that mm_collapse_huge_page_isolate() reports.

Reading the trace buffer takes four small helpers in vm_util: open an
event subsystem's enable file, flip it, clear the buffer, and open it for
reading.

scan_sleep_millisecs is set to a minute, so a step that took a sleep
instead of a wake would blow the budget.

Passes 5/5 on x86-64 4K and arm64 64K.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/Makefile           |   1 +
 .../selftests/mm/khugepaged_sync_check.c      | 196 ++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh     |   2 +
 tools/testing/selftests/mm/vm_util.c          |  41 ++++
 tools/testing/selftests/mm/vm_util.h          |   4 +
 5 files changed, 244 insertions(+)
 create mode 100644 tools/testing/selftests/mm/khugepaged_sync_check.c

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index 2093fcf6e915..b2d6e5c12934 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -105,6 +105,7 @@ TEST_GEN_FILES += merge
 TEST_GEN_FILES += rmap
 TEST_GEN_FILES += folio_split_race_test
 TEST_GEN_FILES += folio_order_check
+TEST_GEN_FILES += khugepaged_sync_check
 
 ifneq ($(ARCH),arm64)
 TEST_GEN_FILES += soft-dirty
diff --git a/tools/testing/selftests/mm/khugepaged_sync_check.c b/tools/testing/selftests/mm/khugepaged_sync_check.c
new file mode 100644
index 000000000000..45001996b57a
--- /dev/null
+++ b/tools/testing/selftests/mm/khugepaged_sync_check.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Synchronous khugepaged driving check.
+ *
+ * Race tests drive khugepaged through the existing sysfs controls: a
+ * store to scan_sleep_millisecs wakes the daemon, and full_scans
+ * advancing by two is a completion barrier for one full pass that
+ * started after setup (khugepaged_full_pass()). Verify the pair gives
+ * deterministic, attributable results: one barrier step over one
+ * prepared window produces exactly one collapse attempt on that
+ * window's source pages (mm_collapse_huge_page_isolate events filtered
+ * by source PFN and order) and the window is collapsed
+ * afterwards, repeatably.
+ *
+ * scan_sleep_millisecs is set to 60s to prove the wake path: without
+ * the wake, one barrier step would sleep multiples of that and blow
+ * the timeout. It also keeps the daemon from free-running between
+ * steps, per the khugepaged_full_pass() discipline.
+ */
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+#include "vm_util.h"
+#include "hugepage_settings.h"
+
+#define BASE_ADDR ((void *)(1UL << 30))
+#define TARGET_ORDER 2	/* smallest order khugepaged considers */
+#define NR_ITERATIONS 5
+
+static int pagemap_fd;
+static int kpageflags_fd;
+static int trace_events_fd = -1;
+static unsigned long hpage_pmd_size;
+
+/*
+ * Each step switches the events off again, but a helper can still give up
+ * on us in between (a failing sysfs write ends the test from inside
+ * thp_write_num()), and huge_memory events left on are the whole machine's
+ * problem, not this test's.
+ */
+static void trace_events_off(void)
+{
+	if (trace_events_fd >= 0)
+		tracing_events_enable(trace_events_fd, false);
+}
+
+/*
+ * Count collapse attempts attributable to our window: isolate events whose
+ * scan_pfn is one of the window's source PFNs, reported once per attempt.
+ */
+static int count_attributed(unsigned long *pfns, int nr_pfns,
+			    unsigned int order)
+{
+	char line[1024];
+	int count = 0;
+	FILE *fp;
+
+	fp = tracing_open_trace();
+	if (!fp)
+		ksft_exit_fail_msg("Cannot open trace buffer\n");
+
+	while (fgets(line, sizeof(line), fp)) {
+		unsigned long val;
+		unsigned int ord;
+		char *s, *o;
+		int i;
+
+		s = strstr(line, "mm_collapse_huge_page_isolate:");
+		if (!s)
+			continue;
+		if (sscanf(s, "mm_collapse_huge_page_isolate: scan_pfn=0x%lx",
+			   &val) != 1)
+			continue;
+		o = strstr(s, "order=");
+		if (!o || sscanf(o, "order=%u", &ord) != 1 || ord != order)
+			continue;
+		for (i = 0; i < nr_pfns; i++) {
+			if (val == pfns[i]) {
+				count++;
+				break;
+			}
+		}
+	}
+	fclose(fp);
+	return count;
+}
+
+static void one_step(int iteration)
+{
+	const size_t window = getpagesize() << TARGET_ORDER;
+	const int nr_pages = 1 << TARGET_ORDER;
+	unsigned long pfns[1 << TARGET_ORDER];
+	bool collapsed, passed;
+	int attributed;
+	char *p;
+	int i;
+
+	p = mmap(BASE_ADDR, hpage_pmd_size, PROT_READ | PROT_WRITE,
+		 MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE, -1, 0);
+	if (p != BASE_ADDR)
+		ksft_exit_fail_perror("mmap() window");
+
+	/* Prepare one window; record its source PFNs. */
+	for (i = 0; i < nr_pages; i++) {
+		p[i * getpagesize()] = i + 1;
+		pfns[i] = pagemap_get_pfn(pagemap_fd, p + i * getpagesize());
+		if (pfns[i] == -1UL)
+			ksft_exit_fail_msg("Source page not present\n");
+	}
+
+	/* Clear first: with the events still off there is nothing to undo. */
+	if (tracing_clear_trace())
+		ksft_exit_fail_msg("Cannot clear the trace buffer\n");
+	if (tracing_events_enable(trace_events_fd, true))
+		ksft_exit_fail_msg("Cannot enable huge_memory events\n");
+
+	if (madvise(p, hpage_pmd_size, MADV_HUGEPAGE))
+		ksft_exit_fail_perror("madvise(MADV_HUGEPAGE)");
+	/* Wait up to 120 seconds for the pass to complete. */
+	passed = khugepaged_full_pass(120);
+
+	/* Off before anything that can give up: the events are system-wide. */
+	if (tracing_events_enable(trace_events_fd, false))
+		ksft_exit_fail_msg("Cannot disable huge_memory events\n");
+	if (!passed)
+		ksft_exit_fail_msg("khugepaged did not complete a full pass\n");
+
+	collapsed = is_range_backed_by_folio_orders(p, window, TARGET_ORDER,
+						    pagemap_fd, kpageflags_fd);
+	attributed = count_attributed(pfns, nr_pages, TARGET_ORDER);
+
+	ksft_test_result(collapsed && attributed == 1,
+			 "step %d: window collapsed, %d attributed result(s)\n",
+			 iteration, attributed);
+
+	munmap(p, hpage_pmd_size);
+}
+
+int main(void)
+{
+	struct thp_settings settings;
+	int i;
+
+	ksft_print_header();
+
+	if (!thp_available())
+		ksft_exit_skip("Transparent Hugepages not available\n");
+	if (!(thp_supported_orders() & (1UL << TARGET_ORDER)))
+		ksft_exit_skip("Order %d is not a supported anon THP order\n",
+			       TARGET_ORDER);
+
+	hpage_pmd_size = read_pmd_pagesize();
+	if (!hpage_pmd_size)
+		ksft_exit_fail_msg("Reading PMD pagesize failed\n");
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		ksft_exit_fail_perror("open(/proc/self/pagemap)");
+	kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
+	if (kpageflags_fd < 0)
+		ksft_exit_skip("open(\"/proc/kpageflags\") requires root\n");
+	trace_events_fd = tracing_events_open("huge_memory");
+	if (trace_events_fd < 0)
+		ksft_exit_skip("huge_memory events require tracefs and root\n");
+	atexit(trace_events_off);
+
+	ksft_set_plan(NR_ITERATIONS);
+
+	thp_save_settings();
+	thp_read_settings(&settings);
+	settings.thp_enabled = THP_MADVISE;
+	settings.thp_defrag = THP_DEFRAG_ALWAYS;
+	settings.khugepaged.defrag = 1;
+	settings.khugepaged.scan_sleep_millisecs = 60000;
+	settings.khugepaged.alloc_sleep_millisecs = 60000;
+	settings.khugepaged.max_ptes_none = (hpage_pmd_size / getpagesize()) - 1;
+	/* One wake must complete one full pass; see khugepaged_full_pass(). */
+	settings.khugepaged.pages_to_scan = 1UL << 24;
+	for (i = 0; i < NR_ORDERS; i++)
+		settings.hugepages[i].enabled = THP_NEVER;
+	settings.hugepages[TARGET_ORDER].enabled = THP_INHERIT;
+	/* Base of the settings stack; the bottom entry is never popped. */
+	thp_push_settings(&settings);
+
+	for (i = 0; i < NR_ITERATIONS; i++)
+		one_step(i);
+
+	thp_restore_settings();
+
+	ksft_finished();
+}
diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
index 8bf898b71350..c0f69da3fd3b 100755
--- a/tools/testing/selftests/mm/run_vmtests.sh
+++ b/tools/testing/selftests/mm/run_vmtests.sh
@@ -404,6 +404,8 @@ CATEGORY="cow" run_test ./cow
 
 CATEGORY="thp" run_test ./folio_order_check
 
+CATEGORY="thp" run_test ./khugepaged_sync_check
+
 CATEGORY="thp" run_test ./khugepaged
 
 CATEGORY="thp" run_test ./khugepaged -s 2
diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index c9bd6c92fa41..ee1334778391 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -598,6 +598,47 @@ bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
 	return true;
 }
 
+#define TRACEFS_ROOT "/sys/kernel/tracing"
+
+/*
+ * Open the enable file of one ftrace event subsystem (e.g. "huge_memory").
+ * Returns a descriptor for tracing_events_enable(), or -1 if tracefs or the
+ * subsystem is not there.  The events are system-wide state: whoever
+ * switches them on owns them until it switches them off, including on the
+ * paths where the test gives up.
+ */
+int tracing_events_open(const char *subsys)
+{
+	char path[256];
+
+	snprintf(path, sizeof(path), TRACEFS_ROOT "/events/%s/enable",
+		 subsys);
+	return open(path, O_WRONLY);
+}
+
+int tracing_events_enable(int fd, bool enable)
+{
+	if (pwrite(fd, enable ? "1" : "0", 1, 0) != 1)
+		return -1;
+	return 0;
+}
+
+/* Drop what the trace buffer holds so far. */
+int tracing_clear_trace(void)
+{
+	int fd = open(TRACEFS_ROOT "/trace", O_WRONLY | O_TRUNC);
+
+	if (fd < 0)
+		return -1;
+	close(fd);
+	return 0;
+}
+
+FILE *tracing_open_trace(void)
+{
+	return fopen(TRACEFS_ROOT "/trace", "r");
+}
+
 /* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */
 int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len,
 			      bool miss, bool wp, bool minor, uint64_t *ioctls)
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index ce05bce4670d..10c7be46e44c 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -119,6 +119,10 @@ int close_procmap(struct procmap_fd *procmap);
 int write_sysfs(const char *file_path, unsigned long val);
 int read_sysfs(const char *file_path, unsigned long *val);
 bool softdirty_supported(void);
+int tracing_events_open(const char *subsys);
+int tracing_events_enable(int fd, bool enable);
+int tracing_clear_trace(void);
+FILE *tracing_open_trace(void);
 
 static inline int open_self_procmap(struct procmap_fd *procmap_out)
 {
-- 
2.54.0

[PATCH v4 16/19] selftests/mm: add khugepaged race harness

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

Collapse serialises against faults, GUP, fork, mremap and zapping through
a protocol of locks, TLB flushes and refcount checks.  No khugepaged
selftest exercises any of it under contention.

Add khugepaged_race.  Six racing threads work the same address space:

  - two faulters
  - an MADV_DONTNEED thread
  - a transient FOLL_PIN thread (gup_test)
  - a forker
  - an mremap thread

One of three drivers collapses under them:

  stepped   khugepaged, one full pass at a time via
            khugepaged_full_pass(), so each step covers a known extent;
  free      khugepaged left to run (scan_sleep_millisecs=0), for soak;
  madvise   an MADV_COLLAPSE and MADV_DONTNEED loop.

Every mode runs in turn unless -m names one, five seconds each.  Every
supported anon THP order is set to inherit and max_ptes_none is 0, so a
window collapses only once fully populated and the racing MADV_DONTNEED
steers selection across orders.

The rule is that a racing page reads as its pattern or as zero, never
anything else.  The faulters and fork children check it throughout, and a
final sweep checks it again.  The other half of the check is the kernel's
own assertions -- DEBUG_VM, page_table_check, KASAN, lockdep -- so read
dmesg too.

The pin thread goes through gup_test, so the harness skips without
CONFIG_GUP_TEST or root.  The threads share three PMD-sized areas plus
one for the mremap thread; -a sets the count, since at a 512M PMD that is
over two gigabytes.  -d sets how long each mode runs.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/Makefile          |   1 +
 tools/testing/selftests/mm/khugepaged_race.c | 442 +++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh    |   2 +
 3 files changed, 445 insertions(+)
 create mode 100644 tools/testing/selftests/mm/khugepaged_race.c

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index b2d6e5c12934..308bbad73c11 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -106,6 +106,7 @@ TEST_GEN_FILES += rmap
 TEST_GEN_FILES += folio_split_race_test
 TEST_GEN_FILES += folio_order_check
 TEST_GEN_FILES += khugepaged_sync_check
+TEST_GEN_FILES += khugepaged_race
 
 ifneq ($(ARCH),arm64)
 TEST_GEN_FILES += soft-dirty
diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
new file mode 100644
index 000000000000..3128031e28bc
--- /dev/null
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -0,0 +1,442 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * khugepaged race harness.
+ *
+ * Runs collapse against concurrent faults, transient GUP pins
+ * (gup_test), fork, mremap and MADV_DONTNEED over the same ranges, in
+ * one of three driver modes:
+ *
+ *   stepped	khugepaged, one full pass at a time through
+ *		khugepaged_full_pass(), so a step covers a known extent;
+ *   free	khugepaged left to run (scan_sleep_millisecs=0), for soak;
+ *   madvise	MADV_COLLAPSE and MADV_DONTNEED in a loop.
+ *
+ * All anon THP orders are enabled (inherit) and max_ptes_none is 0, so a
+ * window has to be fully populated before khugepaged will collapse it, and
+ * the racing MADV_DONTNEED decides which orders it can still use.
+ *
+ * Correctness signals: every racing page must read as its pattern or
+ * zero (MADV_DONTNEED), never anything else.  The faulters and the fork
+ * children check that continuously, a final sweep checks it once more, plus
+ * whatever DEBUG_VM / page_table_check / KASAN / lockdep report in
+ * dmesg, which the caller is expected to inspect.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/time.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+#include "vm_util.h"
+#include "hugepage_settings.h"
+#include "../../../../mm/gup_test.h"
+
+#define BASE_ADDR	((void *)(1UL << 30))
+
+/*
+ * Shared playground for faults/pins/fork/dontneed: several PMD-sized
+ * areas the racing threads spread across, plus one area owned by the
+ * mremap thread. More areas means more independent regions collapsing
+ * at once; the default suits a normal machine. On a memory-constrained
+ * host -- or under emulation, where a 512M PMD (arm64/64K) makes the
+ * default playground multi-gigabyte -- pass -a to shrink it.
+ */
+#define DEFAULT_SHARED_AREAS	3
+static int nr_shared_areas;
+static int nr_areas;
+
+static unsigned long hpage_pmd_size;
+static unsigned long page_size;
+static char *region;		/* NR_AREAS * hpage_pmd_size */
+static char *mremap_area;	/* region + NR_SHARED_AREAS areas */
+static char *mremap_scratch;	/* well above the region */
+static int gup_fd = -1;
+static volatile int stop;
+static volatile int corrupted;
+
+static unsigned int pattern(unsigned long page_idx)
+{
+	unsigned int val = (unsigned int)page_idx * 2654435761U;
+
+	return val ? val : 1;	/* never collides with the zero-fill */
+}
+
+/* Zero means never written; anything else must be this page's pattern */
+static bool page_is_corrupt(unsigned long page_idx, unsigned int *val)
+{
+	*val = *(unsigned int *)(region + page_idx * page_size);
+
+	return *val && *val != pattern(page_idx);
+}
+
+static void check_page(unsigned long page_idx)
+{
+	unsigned int val;
+
+	if (page_is_corrupt(page_idx, &val)) {
+		corrupted = 1;
+		ksft_print_msg("Corruption at page %lu: %#x != %#x\n",
+			       page_idx, val, pattern(page_idx));
+	}
+}
+
+static unsigned long shared_pages(void)
+{
+	return nr_shared_areas * hpage_pmd_size / page_size;
+}
+
+static unsigned long rand_page(unsigned int *seed)
+{
+	return (unsigned long)rand_r(seed) % shared_pages();
+}
+
+/* Pages left from @page_idx, so a range never reaches the mremap thread's area */
+static unsigned long room_from(unsigned long page_idx, unsigned long want)
+{
+	unsigned long left = shared_pages() - page_idx;
+
+	return want < left ? want : left;
+}
+
+static void *faulter_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+
+	while (!stop) {
+		unsigned long page_idx = rand_page(&seed);
+
+		if (rand_r(&seed) & 1)
+			*(unsigned int *)(region + page_idx * page_size) =
+				pattern(page_idx);
+		else
+			check_page(page_idx);
+	}
+	return NULL;
+}
+
+static void *dontneed_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+
+	while (!stop) {
+		unsigned long page_idx = rand_page(&seed);
+		unsigned long nr = 1UL << (rand_r(&seed) % 6);	/* 1..32 pages */
+
+		madvise(region + page_idx * page_size,
+			room_from(page_idx, nr) * page_size, MADV_DONTNEED);
+		usleep(rand_r(&seed) % 500);
+	}
+	return NULL;
+}
+
+static void *pinner_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+
+	while (!stop) {
+		struct gup_test gup = {};
+		unsigned long page_idx = rand_page(&seed);
+
+		unsigned long nr = room_from(page_idx, 16);
+
+		gup.addr = (unsigned long)(region + page_idx * page_size);
+		gup.size = nr * page_size;
+		gup.nr_pages_per_call = nr;
+		gup.gup_flags = 1;	/* FOLL_WRITE */
+		/* Racing MADV_DONTNEED makes transient failures expected. */
+		ioctl(gup_fd, PIN_FAST_BENCHMARK, &gup);
+		usleep(rand_r(&seed) % 200);
+	}
+	return NULL;
+}
+
+static void *forker_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+
+	while (!stop) {
+		pid_t pid = fork();
+
+		if (pid == 0) {
+			unsigned int val;
+			int bad = 0;
+
+			/*
+			 * No stdio in the child: a thread may have held
+			 * stdout's lock when we forked, and printing under an
+			 * inherited lock hangs.  The parent reports what the
+			 * exit status says.
+			 */
+			for (int i = 0; i < 16; i++)
+				bad |= page_is_corrupt(rand_page(&seed), &val);
+			_exit(bad);
+		}
+		if (pid > 0) {
+			int wstatus;
+
+			if (waitpid(pid, &wstatus, 0) < 0)
+				ksft_exit_fail_perror("waitpid()");
+			/* A child killed on the read counts too, not just its exit code. */
+			if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus))
+				corrupted = 1;
+		}
+		usleep(rand_r(&seed) % 2000);
+	}
+	return NULL;
+}
+
+static void *mremapper_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+
+	while (!stop) {
+		void *p;
+
+		p = mremap(mremap_area, hpage_pmd_size, hpage_pmd_size,
+			   MREMAP_MAYMOVE | MREMAP_FIXED, mremap_scratch);
+		if (p == MAP_FAILED)
+			ksft_exit_fail_perror("mremap() away");
+		for (int i = 0; i < 8; i++)
+			mremap_scratch[(rand_r(&seed) %
+				(hpage_pmd_size / page_size)) * page_size] = 1;
+		p = mremap(mremap_scratch, hpage_pmd_size, hpage_pmd_size,
+			   MREMAP_MAYMOVE | MREMAP_FIXED, mremap_area);
+		if (p == MAP_FAILED)
+			ksft_exit_fail_perror("mremap() back");
+		usleep(rand_r(&seed) % 2000);
+	}
+	return NULL;
+}
+
+static unsigned long now_ms(void)
+{
+	struct timeval tv;
+
+	gettimeofday(&tv, NULL);
+	return tv.tv_sec * 1000UL + tv.tv_usec / 1000;
+}
+
+static void usage(void)
+{
+	fprintf(stderr,
+		"Usage: khugepaged_race [-d seconds] [-m stepped|free|madvise] [-a areas]\n"
+		"\tWithout -m, every mode runs in turn.\n"
+		"\t-d: seconds per mode (default 5)\n"
+		"\t-a: number of shared PMD-sized playground areas (default 3)\n");
+	exit(1);
+}
+
+int main(int argc, char **argv)
+{
+	static const char * const thread_names[] = {
+		"faulter", "faulter2", "dontneed", "pinner", "forker",
+		"mremapper",
+	};
+	void *(*const thread_fns[])(void *) = {
+		faulter_fn, faulter_fn, dontneed_fn, pinner_fn, forker_fn,
+		mremapper_fn,
+	};
+	const int nr_threads = ARRAY_SIZE(thread_names);
+	pthread_t threads[ARRAY_SIZE(thread_names)];
+	static const char * const all_modes[] = { "stepped", "free", "madvise" };
+	const char *one_mode[1];
+	const char * const *modes = all_modes;
+	int nr_modes = ARRAY_SIZE(all_modes);
+	const char *mode_arg = NULL;
+	struct thp_settings settings;
+	unsigned long end_ms;
+	int duration_s = 5;
+	unsigned long thread_mask = ~0UL;
+	int nr_areas_arg = 0;
+	unsigned long i;
+	int steps = 0;
+	int opt;
+
+	while ((opt = getopt(argc, argv, "a:d:m:t:h")) != -1) {
+		switch (opt) {
+		case 'a':
+			nr_areas_arg = atoi(optarg);
+			break;
+		case 'd':
+			duration_s = atoi(optarg);
+			break;
+		case 'm':
+			mode_arg = optarg;
+			break;
+		case 't':
+			/* debug: bitmask of racing threads to start */
+			thread_mask = strtoul(optarg, NULL, 0);
+			break;
+		default:
+			usage();
+		}
+	}
+	if (mode_arg) {
+		if (strcmp(mode_arg, "stepped") && strcmp(mode_arg, "free") &&
+		    strcmp(mode_arg, "madvise"))
+			usage();
+		one_mode[0] = mode_arg;
+		modes = one_mode;
+		nr_modes = 1;
+	}
+
+	ksft_print_header();
+	if (!thp_available())
+		ksft_exit_skip("Transparent Hugepages not available\n");
+
+	page_size = getpagesize();
+	hpage_pmd_size = read_pmd_pagesize();
+	if (!hpage_pmd_size)
+		ksft_exit_fail_msg("Reading PMD pagesize failed\n");
+
+	gup_fd = open("/sys/kernel/debug/gup_test", O_RDWR);
+	if (gup_fd < 0)
+		ksft_exit_skip("/sys/kernel/debug/gup_test requires CONFIG_GUP_TEST and root\n");
+
+	nr_shared_areas = nr_areas_arg > 0 ? nr_areas_arg : DEFAULT_SHARED_AREAS;
+	nr_areas = nr_shared_areas + 1;
+
+	/*
+	 * The mremap thread moves its area to this address and back, and
+	 * MREMAP_FIXED unmaps whatever is in the way without saying so.  Claim
+	 * the address here, so a layout that does not match this assumption
+	 * fails now instead of losing a mapping later.  Nothing else in the
+	 * process maps this low: thread stacks and malloc arenas come from the
+	 * top-down mmap area, well above.
+	 */
+	mremap_scratch = (char *)BASE_ADDR + 2 * nr_areas * hpage_pmd_size;
+	if (mmap(mremap_scratch, hpage_pmd_size, PROT_NONE,
+		 MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE,
+		 -1, 0) != (void *)mremap_scratch)
+		ksft_exit_fail_perror("mmap() mremap scratch");
+
+	ksft_set_plan(nr_modes);
+
+	thp_save_settings();
+	thp_read_settings(&settings);
+
+	/*
+	 * A base entry for the stack, so that the pop at the end of a mode
+	 * always has something to write back: thp_pop_settings() on an empty
+	 * stack has no settings to apply and gives up.
+	 */
+	thp_push_settings(&settings);
+
+	for (int m = 0; m < nr_modes; m++) {
+		const char *mode = modes[m];
+
+		thp_read_settings(&settings);
+		settings.thp_enabled = THP_MADVISE;
+		settings.thp_defrag = THP_DEFRAG_ALWAYS;
+		settings.shmem_enabled = SHMEM_NEVER;
+		settings.khugepaged.defrag = 1;
+		settings.khugepaged.scan_sleep_millisecs =
+			strcmp(mode, "free") ? 1000 : 0;
+		settings.khugepaged.alloc_sleep_millisecs = 10;
+		/*
+		 * Strict occupancy: mTHP collapse only supports 0 or
+		 * HPAGE_PMD_NR - 1 and coerces anything else to 0 anyway, and 0
+		 * also keeps khugepaged from burning the whole step in doomed
+		 * PMD-sized allocations on 512M-PMD configs: under racing
+		 * MADV_DONTNEED a fully populated PMD area is rare.
+		 */
+		settings.khugepaged.max_ptes_none = 0;
+		settings.khugepaged.pages_to_scan =
+			nr_areas * (hpage_pmd_size / page_size) * 8;
+		for (i = 0; i < NR_ORDERS; i++) {
+			if (thp_supported_orders() & (1UL << i))
+				settings.hugepages[i].enabled = THP_INHERIT;
+		}
+		/* Popped at the end of this mode, before the next one. */
+		thp_push_settings(&settings);
+
+		region = mmap(BASE_ADDR, nr_areas * hpage_pmd_size,
+			      PROT_READ | PROT_WRITE, MAP_ANONYMOUS |
+			      MAP_PRIVATE | MAP_FIXED_NOREPLACE, -1, 0);
+		if (region != BASE_ADDR)
+			ksft_exit_fail_perror("mmap() playground");
+		mremap_area = region + nr_shared_areas * hpage_pmd_size;
+
+		/* Populate so the first pass has something to collapse. */
+		for (i = 0; i < nr_shared_areas * hpage_pmd_size / page_size; i++)
+			*(unsigned int *)(region + i * page_size) = pattern(i);
+		memset(mremap_area, 1, hpage_pmd_size);
+		if (madvise(region, nr_areas * hpage_pmd_size, MADV_HUGEPAGE))
+			ksft_exit_fail_perror("madvise(MADV_HUGEPAGE)");
+
+		for (i = 0; i < nr_threads; i++) {
+			if (!(thread_mask & (1UL << i))) {
+				threads[i] = 0;
+				continue;
+			}
+			if (pthread_create(&threads[i], NULL, thread_fns[i],
+					   (void *)(i + 1)))
+				ksft_exit_fail_perror(thread_names[i]);
+		}
+
+		end_ms = now_ms() + duration_s * 1000UL;
+		if (!strcmp(mode, "stepped")) {
+			while (now_ms() < end_ms && !corrupted) {
+				if (!khugepaged_full_pass(600))
+					ksft_exit_fail_msg("khugepaged pass timed out\n");
+				steps++;
+			}
+		} else if (!strcmp(mode, "free")) {
+			while (now_ms() < end_ms && !corrupted)
+				usleep(100 * 1000);
+		} else {	/* madvise */
+			while (now_ms() < end_ms && !corrupted) {
+				for (i = 0; i < nr_shared_areas; i++) {
+					madvise(region + i * hpage_pmd_size,
+						hpage_pmd_size, MADV_COLLAPSE);
+				}
+				madvise(region, nr_shared_areas * hpage_pmd_size,
+					MADV_DONTNEED);
+				steps++;
+			}
+		}
+
+		stop = 1;
+		for (i = 0; i < nr_threads; i++) {
+			if (threads[i])
+				pthread_join(threads[i], NULL);
+		}
+
+		/* Final integrity sweep. */
+		for (i = 0; i < nr_shared_areas * hpage_pmd_size / page_size; i++)
+			check_page(i);
+
+		ksft_test_result(!corrupted,
+				 "%s: %ds, %d steps, no corruption\n",
+				 mode, duration_s, steps);
+
+		/*
+		 * Hand the address space and the settings back before the
+		 * next mode: it maps the region at the same fixed address,
+		 * and its scan cadence differs.
+		 */
+		munmap(region, nr_areas * hpage_pmd_size);
+		thp_pop_settings();
+		stop = 0;
+		steps = 0;
+
+		if (corrupted) {
+			/* Memory is suspect; the rest would prove nothing. */
+			while (++m < nr_modes)
+				ksft_test_result_skip("%s: skipped after corruption\n",
+						      modes[m]);
+			break;
+		}
+	}
+
+	thp_restore_settings();
+	ksft_finished();
+}
diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
index c0f69da3fd3b..fc61907aa3b2 100755
--- a/tools/testing/selftests/mm/run_vmtests.sh
+++ b/tools/testing/selftests/mm/run_vmtests.sh
@@ -406,6 +406,8 @@ CATEGORY="thp" run_test ./folio_order_check
 
 CATEGORY="thp" run_test ./khugepaged_sync_check
 
+CATEGORY="thp" run_test ./khugepaged_race
+
 CATEGORY="thp" run_test ./khugepaged
 
 CATEGORY="thp" run_test ./khugepaged -s 2
-- 
2.54.0

[PATCH v4 17/19] selftests/mm: race collapse of windows with holes

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The harness pins max_ptes_none to 0, so khugepaged only collapses a window
once every PTE in it is present.  A window with holes takes a different
route, and never gets raced.  A hole is zero-filled into the new folio by
clear_user_highpage() rather than copied from anywhere.  Which slots count
as holes keeps moving under the racing MADV_DONTNEED, right up to the
moment the PMD is detached.

Run both ends of the occupancy scale for every driver mode, one after the
other.  mTHP collapse supports only those two, 0 and HPAGE_PMD_NR - 1, and
coerces anything between them to 0.  Each result says which end it ran:

  ok 1 stepped/strict: 5s, 231 steps, no corruption
  ok 2 stepped/holes: 5s, 194 steps, no corruption

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged_race.c | 45 ++++++++++++--------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index 3128031e28bc..3b369046edf5 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -11,9 +11,11 @@
  *   free	khugepaged left to run (scan_sleep_millisecs=0), for soak;
  *   madvise	MADV_COLLAPSE and MADV_DONTNEED in a loop.
  *
- * All anon THP orders are enabled (inherit) and max_ptes_none is 0, so a
- * window has to be fully populated before khugepaged will collapse it, and
- * the racing MADV_DONTNEED decides which orders it can still use.
+ * All anon THP orders are enabled (inherit).  Occupancy runs at both ends
+ * of what mTHP collapse supports: max_ptes_none 0, where a window must be
+ * fully populated, and HPAGE_PMD_NR - 1, where a window full of holes
+ * collapses too.  A hole is zero-filled into the new folio rather than
+ * copied, and the racing MADV_DONTNEED keeps moving which slots are holes.
  *
  * Correctness signals: every racing page must read as its pattern or
  * zero (MADV_DONTNEED), never anything else.  The faulters and the fork
@@ -247,6 +249,8 @@ int main(int argc, char **argv)
 	const int nr_threads = ARRAY_SIZE(thread_names);
 	pthread_t threads[ARRAY_SIZE(thread_names)];
 	static const char * const all_modes[] = { "stepped", "free", "madvise" };
+	static const bool occupancies[] = { false, true };	/* strict, holes */
+	const int nr_occupancies = ARRAY_SIZE(occupancies);
 	const char *one_mode[1];
 	const char * const *modes = all_modes;
 	int nr_modes = ARRAY_SIZE(all_modes);
@@ -279,6 +283,7 @@ int main(int argc, char **argv)
 			usage();
 		}
 	}
+
 	if (mode_arg) {
 		if (strcmp(mode_arg, "stepped") && strcmp(mode_arg, "free") &&
 		    strcmp(mode_arg, "madvise"))
@@ -318,7 +323,7 @@ int main(int argc, char **argv)
 		 -1, 0) != (void *)mremap_scratch)
 		ksft_exit_fail_perror("mmap() mremap scratch");
 
-	ksft_set_plan(nr_modes);
+	ksft_set_plan(nr_modes * nr_occupancies);
 
 	thp_save_settings();
 	thp_read_settings(&settings);
@@ -330,8 +335,9 @@ int main(int argc, char **argv)
 	 */
 	thp_push_settings(&settings);
 
-	for (int m = 0; m < nr_modes; m++) {
-		const char *mode = modes[m];
+	for (int mn = 0; mn < nr_modes * nr_occupancies; mn++) {
+		const char *mode = modes[mn / nr_occupancies];
+		bool holes = occupancies[mn % nr_occupancies];
 
 		thp_read_settings(&settings);
 		settings.thp_enabled = THP_MADVISE;
@@ -341,14 +347,16 @@ int main(int argc, char **argv)
 		settings.khugepaged.scan_sleep_millisecs =
 			strcmp(mode, "free") ? 1000 : 0;
 		settings.khugepaged.alloc_sleep_millisecs = 10;
+
 		/*
-		 * Strict occupancy: mTHP collapse only supports 0 or
-		 * HPAGE_PMD_NR - 1 and coerces anything else to 0 anyway, and 0
-		 * also keeps khugepaged from burning the whole step in doomed
-		 * PMD-sized allocations on 512M-PMD configs: under racing
-		 * MADV_DONTNEED a fully populated PMD area is rare.
+		 * mTHP collapse only supports the two ends of the occupancy
+		 * scale: 0 or HPAGE_PMD_NR - 1 (anything else coerces to 0).
+		 * Strict needs a fully populated window, which is rare under
+		 * racing MADV_DONTNEED; hole-heavy windows collapse instead,
+		 * so the two ends race different paths.
 		 */
-		settings.khugepaged.max_ptes_none = 0;
+		settings.khugepaged.max_ptes_none = holes ?
+			(hpage_pmd_size / page_size) - 1 : 0;
 		settings.khugepaged.pages_to_scan =
 			nr_areas * (hpage_pmd_size / page_size) * 8;
 		for (i = 0; i < NR_ORDERS; i++) {
@@ -415,8 +423,9 @@ int main(int argc, char **argv)
 			check_page(i);
 
 		ksft_test_result(!corrupted,
-				 "%s: %ds, %d steps, no corruption\n",
-				 mode, duration_s, steps);
+				 "%s/%s: %ds, %d steps, no corruption\n",
+				 mode, holes ? "holes" : "strict",
+				 duration_s, steps);
 
 		/*
 		 * Hand the address space and the settings back before the
@@ -430,9 +439,11 @@ int main(int argc, char **argv)
 
 		if (corrupted) {
 			/* Memory is suspect; the rest would prove nothing. */
-			while (++m < nr_modes)
-				ksft_test_result_skip("%s: skipped after corruption\n",
-						      modes[m]);
+			while (++mn < nr_modes * nr_occupancies)
+				ksft_test_result_skip("%s/%s: skipped after corruption\n",
+						      modes[mn / nr_occupancies],
+						      occupancies[mn % nr_occupancies] ?
+						      "holes" : "strict");
 			break;
 		}
 	}
-- 
2.54.0

[PATCH v4 18/19] selftests/mm: add memory-pressure threads to the khugepaged race harness

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The harness races collapse against faults, pins, fork, mremap and
MADV_DONTNEED, but nothing in it elevates a source folio's refcount from
the reclaim or compaction side.

Add two more threads, and run every mode and occupancy limit both with and
without them:

 - pageout: cycles MADV_PAGEOUT over a dedicated neighbour region, faults
   it back in and checks the content each round, since a page's pattern
   must survive the trip through swap.  Left out when the host has no
   swap, because then there is no anon reclaim to drive.

 - compactor: writes /proc/sys/vm/compact_memory in a loop.  Compaction
   isolates and migrates folios, so it competes with a collapse for the
   pages it is gathering, with refcount elevations and migration entries
   of its own.

Each result says whether it ran under pressure:

  ok 2 stepped/strict/pressure: 5s, 88 steps, no corruption

A full run is now twelve combinations; -m and -d narrow it.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged_race.c | 144 +++++++++++++++++--
 1 file changed, 132 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index 3b369046edf5..a23e5bfe78af 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -17,6 +17,12 @@
  * collapses too.  A hole is zero-filled into the new folio rather than
  * copied, and the racing MADV_DONTNEED keeps moving which slots are holes.
  *
+ * Every combination also runs under memory pressure: MADV_PAGEOUT cycling
+ * on a dedicated neighbour region (swap traffic and LRU churn; left out
+ * with a note when the host has no swap) and a compact_memory trigger
+ * loop (compaction migrates source folios, racing collapse's freeze
+ * with refcount elevation and migration entries of its own).
+ *
  * Correctness signals: every racing page must read as its pattern or
  * zero (MADV_DONTNEED), never anything else.  The faulters and the fork
  * children check that continuously, a final sweep checks it once more, plus
@@ -60,6 +66,8 @@ static unsigned long page_size;
 static char *region;		/* NR_AREAS * hpage_pmd_size */
 static char *mremap_area;	/* region + NR_SHARED_AREAS areas */
 static char *mremap_scratch;	/* well above the region */
+static char *pageout_area;	/* dedicated pressure region */
+static size_t pageout_size;
 static int gup_fd = -1;
 static volatile int stop;
 static volatile int corrupted;
@@ -218,6 +226,70 @@ static void *mremapper_fn(void *arg)
 	return NULL;
 }
 
+/*
+ * Swap traffic and LRU churn on a region of our own. The content
+ * check is exact: a page out and back through swap must preserve the
+ * pattern, and nothing else ever writes here.
+ */
+static void *pageout_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+	unsigned long nr = pageout_size / page_size;
+	unsigned long i;
+
+	for (i = 0; i < nr; i++)
+		*(unsigned int *)(pageout_area + i * page_size) = pattern(i);
+
+	while (!stop) {
+		madvise(pageout_area, pageout_size, MADV_PAGEOUT);
+		for (i = 0; i < nr && !stop; i++) {
+			unsigned int val = *(unsigned int *)(pageout_area +
+							     i * page_size);
+
+			if (val != pattern(i)) {
+				corrupted = 1;
+				ksft_print_msg("Pageout corruption at page %lu: %#x != %#x\n",
+					       i, val, pattern(i));
+			}
+		}
+		usleep(rand_r(&seed) % 2000);
+	}
+	return NULL;
+}
+
+/* Compaction migrates the collapse sources out from under us. */
+static void *compactor_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+	int fd = open("/proc/sys/vm/compact_memory", O_WRONLY);
+
+	if (fd < 0) {
+		ksft_print_msg("No compact_memory; compactor idle\n");
+		return NULL;
+	}
+	while (!stop) {
+		if (write(fd, "1", 1) < 0)
+			break;
+		usleep(10000 + rand_r(&seed) % 100000);
+	}
+	close(fd);
+	return NULL;
+}
+
+static bool swap_available(void)
+{
+	char line[256];
+	int lines = 0;
+	FILE *fp = fopen("/proc/swaps", "r");
+
+	if (!fp)
+		return false;
+	while (fgets(line, sizeof(line), fp))
+		lines++;
+	fclose(fp);
+	return lines > 1;
+}
+
 static unsigned long now_ms(void)
 {
 	struct timeval tv;
@@ -240,17 +312,23 @@ int main(int argc, char **argv)
 {
 	static const char * const thread_names[] = {
 		"faulter", "faulter2", "dontneed", "pinner", "forker",
-		"mremapper",
+		"mremapper", "pageout", "compactor",
 	};
 	void *(*const thread_fns[])(void *) = {
 		faulter_fn, faulter_fn, dontneed_fn, pinner_fn, forker_fn,
-		mremapper_fn,
+		mremapper_fn, pageout_fn, compactor_fn,
 	};
+	enum { T_FAULTER, T_FAULTER2, T_DONTNEED, T_PINNER, T_FORKER,
+	       T_MREMAPPER, T_PAGEOUT, T_COMPACTOR };
+	const unsigned long pageout_bit = 1UL << T_PAGEOUT;
+	const unsigned long compactor_bit = 1UL << T_COMPACTOR;
 	const int nr_threads = ARRAY_SIZE(thread_names);
 	pthread_t threads[ARRAY_SIZE(thread_names)];
 	static const char * const all_modes[] = { "stepped", "free", "madvise" };
 	static const bool occupancies[] = { false, true };	/* strict, holes */
+	static const bool pressures[] = { false, true };	/* quiet, under pressure */
 	const int nr_occupancies = ARRAY_SIZE(occupancies);
+	const int nr_pressures = ARRAY_SIZE(pressures);
 	const char *one_mode[1];
 	const char * const *modes = all_modes;
 	int nr_modes = ARRAY_SIZE(all_modes);
@@ -259,6 +337,7 @@ int main(int argc, char **argv)
 	unsigned long end_ms;
 	int duration_s = 5;
 	unsigned long thread_mask = ~0UL;
+	unsigned long base_mask;
 	int nr_areas_arg = 0;
 	unsigned long i;
 	int steps = 0;
@@ -323,7 +402,12 @@ int main(int argc, char **argv)
 		 -1, 0) != (void *)mremap_scratch)
 		ksft_exit_fail_perror("mmap() mremap scratch");
 
-	ksft_set_plan(nr_modes * nr_occupancies);
+	base_mask = thread_mask;
+	if (!swap_available())
+		/* No swap, no anon reclaim: compaction-only pressure. */
+		ksft_print_msg("no swap: the pageout thread stays idle\n");
+
+	ksft_set_plan(nr_modes * nr_occupancies * nr_pressures);
 
 	thp_save_settings();
 	thp_read_settings(&settings);
@@ -335,9 +419,17 @@ int main(int argc, char **argv)
 	 */
 	thp_push_settings(&settings);
 
-	for (int mn = 0; mn < nr_modes * nr_occupancies; mn++) {
-		const char *mode = modes[mn / nr_occupancies];
-		bool holes = occupancies[mn % nr_occupancies];
+	for (int run = 0; run < nr_modes * nr_occupancies * nr_pressures; run++) {
+		int rem = run % (nr_occupancies * nr_pressures);
+		const char *mode = modes[run / (nr_occupancies * nr_pressures)];
+		bool holes = occupancies[rem / nr_pressures];
+		bool pressure = pressures[rem % nr_pressures];
+
+		thread_mask = base_mask;
+		if (!pressure)
+			thread_mask &= ~(pageout_bit | compactor_bit);
+		else if (!swap_available())
+			thread_mask &= ~pageout_bit;
 
 		thp_read_settings(&settings);
 		settings.thp_enabled = THP_MADVISE;
@@ -373,6 +465,24 @@ int main(int argc, char **argv)
 			ksft_exit_fail_perror("mmap() playground");
 		mremap_area = region + nr_shared_areas * hpage_pmd_size;
 
+		if (thread_mask & pageout_bit) {
+			/*
+			 * Big enough to cycle real reclaim, small enough not
+			 * to dominate a TCG guest: 4 PMD areas, clamped to
+			 * [16M, 64M].
+			 */
+			pageout_size = 4 * hpage_pmd_size;
+			if (pageout_size < 16UL << 20)
+				pageout_size = 16UL << 20;
+			if (pageout_size > 64UL << 20)
+				pageout_size = 64UL << 20;
+			pageout_area = mmap(NULL, pageout_size,
+					    PROT_READ | PROT_WRITE,
+					    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+			if (pageout_area == MAP_FAILED)
+				ksft_exit_fail_perror("mmap() pageout area");
+		}
+
 		/* Populate so the first pass has something to collapse. */
 		for (i = 0; i < nr_shared_areas * hpage_pmd_size / page_size; i++)
 			*(unsigned int *)(region + i * page_size) = pattern(i);
@@ -423,8 +533,9 @@ int main(int argc, char **argv)
 			check_page(i);
 
 		ksft_test_result(!corrupted,
-				 "%s/%s: %ds, %d steps, no corruption\n",
+				 "%s/%s%s: %ds, %d steps, no corruption\n",
 				 mode, holes ? "holes" : "strict",
+				 pressure ? "/pressure" : "",
 				 duration_s, steps);
 
 		/*
@@ -433,17 +544,26 @@ int main(int argc, char **argv)
 		 * and its scan cadence differs.
 		 */
 		munmap(region, nr_areas * hpage_pmd_size);
+		if (pageout_area) {
+			munmap(pageout_area, pageout_size);
+			pageout_area = NULL;
+		}
 		thp_pop_settings();
 		stop = 0;
 		steps = 0;
 
 		if (corrupted) {
 			/* Memory is suspect; the rest would prove nothing. */
-			while (++mn < nr_modes * nr_occupancies)
-				ksft_test_result_skip("%s/%s: skipped after corruption\n",
-						      modes[mn / nr_occupancies],
-						      occupancies[mn % nr_occupancies] ?
-						      "holes" : "strict");
+			while (++run < nr_modes * nr_occupancies * nr_pressures) {
+				rem = run % (nr_occupancies * nr_pressures);
+
+				ksft_test_result_skip("%s/%s%s: skipped after corruption\n",
+						      modes[run / (nr_occupancies * nr_pressures)],
+						      occupancies[rem / nr_pressures] ?
+						      "holes" : "strict",
+						      pressures[rem % nr_pressures] ?
+						      "/pressure" : "");
+			}
 			break;
 		}
 	}
-- 
2.54.0

[PATCH v4 19/19] selftests/mm: zap whole PTE tables in the khugepaged race harness

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The harness's MADV_DONTNEED thread zaps 1 to 32 pages at a time, never a
whole PMD-aligned area, and only a zap that covers a full table frees the
table itself (CONFIG_PT_RECLAIM).

Make the thread zap a whole PMD-aligned area once every 64 iterations, and
keep the fine-grained zaps as the common case.  The new case frees page
tables, racing that against a collapse walking the same table.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 tools/testing/selftests/mm/khugepaged_race.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index a23e5bfe78af..6682bbae0a8f 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -140,8 +140,24 @@ static void *dontneed_fn(void *arg)
 		unsigned long page_idx = rand_page(&seed);
 		unsigned long nr = 1UL << (rand_r(&seed) % 6);	/* 1..32 pages */
 
-		madvise(region + page_idx * page_size,
-			room_from(page_idx, nr) * page_size, MADV_DONTNEED);
+		/*
+		 * Once in a while zap a whole PMD-aligned area: only a zap
+		 * spanning the full table triggers the empty-table reclaim
+		 * (CONFIG_PT_RECLAIM), which can free the table under a
+		 * collapse that is midway through it.  Sub-table zaps never
+		 * reach that path.
+		 */
+		if (!(rand_r(&seed) % 64)) {
+			unsigned long area = page_idx /
+					(hpage_pmd_size / page_size);
+
+			madvise(region + area * hpage_pmd_size,
+				hpage_pmd_size, MADV_DONTNEED);
+		} else {
+			madvise(region + page_idx * page_size,
+				room_from(page_idx, nr) * page_size,
+				MADV_DONTNEED);
+		}
 		usleep(rand_r(&seed) % 500);
 	}
 	return NULL;
-- 
2.54.0