* [PATCH v2 1/2] selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results
2025-09-03 7:02 [PATCH v2 0/2] selftests/mm: split_huge_page_test: split_pte_mapped_thp improvements David Hildenbrand
@ 2025-09-03 7:02 ` David Hildenbrand
2025-09-03 7:02 ` [PATCH v2 2/2] selftests/mm: split_huge_page_test: cleanups for split_pte_mapped_thp test David Hildenbrand
1 sibling, 0 replies; 4+ messages in thread
From: David Hildenbrand @ 2025-09-03 7:02 UTC (permalink / raw)
To: linux-kernel
Cc: linux-mm, linux-kselftest, David Hildenbrand, Andrew Morton,
Lorenzo Stoakes, Zi Yan, Baolin Wang, Liam R. Howlett, Nico Pache,
Ryan Roberts, Dev Jain, Barry Song, Wei Yang
When checking for actual tail or head pages of a folio, we must make
sure that the KPF_COMPOUND_HEAD/KPF_COMPOUND_TAIL flag is paired with
KPF_THP.
For example, if we have another large folio after our large folio in
physical memory, our "pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)" would
trigger even though it's actually a head page of the next folio.
If is_backed_by_folio() returns a wrong result, split_pte_mapped_thp()
can fail with "Some THPs are missing during mremap".
Fix it by checking for head/tail pages of folios properly. Add
folio_tail_flags/folio_head_flags to improve readability and use these
masks also when just testing for any compound page.
Fixes: 169b456b0162 ("selftests/mm: reimplement is_backed_by_thp() with more precise check")
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
tools/testing/selftests/mm/split_huge_page_test.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)
diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 10ae65ea032f6..72d6d8bb329ed 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -44,6 +44,8 @@ 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;
@@ -61,7 +63,7 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
/* check for order-0 pages */
if (!order) {
- if (pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD | KPF_COMPOUND_TAIL))
+ if (pfn_flags & (folio_head_flags | folio_tail_flags))
return false;
return true;
}
@@ -76,14 +78,14 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
goto fail;
/* head PFN has no compound_head flag set */
- if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD)))
+ 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 & (KPF_THP | KPF_COMPOUND_TAIL)))
+ if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
return false;
}
@@ -94,11 +96,8 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
return true;
- /* this folio is bigger than the given order */
- if (pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL))
- return false;
-
- 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;
--
2.50.1
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v2 2/2] selftests/mm: split_huge_page_test: cleanups for split_pte_mapped_thp test
2025-09-03 7:02 [PATCH v2 0/2] selftests/mm: split_huge_page_test: split_pte_mapped_thp improvements David Hildenbrand
2025-09-03 7:02 ` [PATCH v2 1/2] selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results David Hildenbrand
@ 2025-09-03 7:02 ` David Hildenbrand
1 sibling, 0 replies; 4+ messages in thread
From: David Hildenbrand @ 2025-09-03 7:02 UTC (permalink / raw)
To: linux-kernel
Cc: linux-mm, linux-kselftest, David Hildenbrand, Andrew Morton,
Lorenzo Stoakes, Zi Yan, Baolin Wang, Liam R. Howlett, Nico Pache,
Ryan Roberts, Dev Jain, Barry Song, Wei Yang
There is room for improvement, so let's clean up a bit:
(1) Define "4" as a constant.
(2) SKIP if we fail to allocate all THPs (e.g., fragmented) and add
recovery code for all other failure cases: no need to exit the test.
(3) Rename "len" to thp_area_size, and "one_page" to "thp_area".
(4) Allocate a new area "page_area" into which we will mremap the
pages; add "page_area_size". Now we can easily merge the two
mremap instances into a single one.
(5) Iterate THPs instead of bytes when checking for missed THPs after
mremap.
(6) Rename "pte_mapped2" to "tmp", used to verify mremap(MAP_FIXED)
result.
(7) Split the corruption test from the failed-split test, so we can just
iterate bytes vs. thps naturally.
(8) Extend comments and clarify why we are using mremap in the first
place.
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
.../selftests/mm/split_huge_page_test.c | 123 +++++++++++-------
1 file changed, 74 insertions(+), 49 deletions(-)
diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 72d6d8bb329ed..7731191cc8e9b 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -389,67 +389,92 @@ static void split_pmd_thp_to_order(int order)
static void split_pte_mapped_thp(void)
{
- char *one_page, *pte_mapped, *pte_mapped2;
- size_t len = 4 * pmd_pagesize;
- uint64_t thp_size;
+ const size_t nr_thps = 4;
+ const size_t thp_area_size = nr_thps * pmd_pagesize;
+ const size_t page_area_size = nr_thps * pagesize;
+ char *thp_area, *tmp, *page_area = MAP_FAILED;
size_t i;
- one_page = mmap((void *)(1UL << 30), len, PROT_READ | PROT_WRITE,
+ thp_area = mmap((void *)(1UL << 30), thp_area_size, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
- if (one_page == MAP_FAILED)
- ksft_exit_fail_msg("Fail to allocate memory: %s\n", strerror(errno));
+ if (thp_area == MAP_FAILED) {
+ ksft_test_result_fail("Fail to allocate memory: %s\n", strerror(errno));
+ return;
+ }
- madvise(one_page, len, MADV_HUGEPAGE);
+ madvise(thp_area, thp_area_size, MADV_HUGEPAGE);
- for (i = 0; i < len; i++)
- one_page[i] = (char)i;
+ for (i = 0; i < thp_area_size; i++)
+ thp_area[i] = (char)i;
- if (!check_huge_anon(one_page, 4, pmd_pagesize))
- ksft_exit_fail_msg("No THP is allocated\n");
+ if (!check_huge_anon(thp_area, nr_thps, pmd_pagesize)) {
+ ksft_test_result_skip("Not all THPs allocated\n");
+ goto out;
+ }
- /* remap the first pagesize of first THP */
- pte_mapped = mremap(one_page, pagesize, pagesize, MREMAP_MAYMOVE);
-
- /* remap the Nth pagesize of Nth THP */
- for (i = 1; i < 4; i++) {
- pte_mapped2 = mremap(one_page + pmd_pagesize * i + pagesize * i,
- pagesize, pagesize,
- MREMAP_MAYMOVE|MREMAP_FIXED,
- pte_mapped + pagesize * i);
- if (pte_mapped2 == MAP_FAILED)
- ksft_exit_fail_msg("mremap failed: %s\n", strerror(errno));
- }
-
- /* smap does not show THPs after mremap, use kpageflags instead */
- thp_size = 0;
- for (i = 0; i < pagesize * 4; i++)
- if (i % pagesize == 0 &&
- is_backed_by_folio(&pte_mapped[i], pmd_order, pagemap_fd, kpageflags_fd))
- thp_size++;
-
- if (thp_size != 4)
- ksft_exit_fail_msg("Some THPs are missing during mremap\n");
-
- /* split all remapped THPs */
- write_debugfs(PID_FMT, getpid(), (uint64_t)pte_mapped,
- (uint64_t)pte_mapped + pagesize * 4, 0);
-
- /* smap does not show THPs after mremap, use kpageflags instead */
- thp_size = 0;
- for (i = 0; i < pagesize * 4; i++) {
- if (pte_mapped[i] != (char)i)
- ksft_exit_fail_msg("%ld byte corrupted\n", i);
+ /*
+ * To challenge spitting code, we will mremap a single page of each
+ * THP (page[i] of thp[i]) in the thp_area into page_area. This will
+ * replace the PMD mappings in the thp_area by PTE mappings first,
+ * but leaving the THP unsplit, to then create a page-sized hole in
+ * the thp_area.
+ * We will then manually trigger splitting of all THPs through the
+ * single mremap'ed pages of each THP in the page_area.
+ */
+ page_area = mmap(NULL, page_area_size, PROT_READ | PROT_WRITE,
+ MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+ if (page_area == MAP_FAILED) {
+ ksft_test_result_fail("Fail to allocate memory: %s\n", strerror(errno));
+ goto out;
+ }
- if (i % pagesize == 0 &&
- !is_backed_by_folio(&pte_mapped[i], 0, pagemap_fd, kpageflags_fd))
- thp_size++;
+ for (i = 0; i < nr_thps; i++) {
+ tmp = mremap(thp_area + pmd_pagesize * i + pagesize * i,
+ pagesize, pagesize, MREMAP_MAYMOVE|MREMAP_FIXED,
+ page_area + pagesize * i);
+ if (tmp != MAP_FAILED)
+ continue;
+ ksft_test_result_fail("mremap failed: %s\n", strerror(errno));
+ goto out;
+ }
+
+ /*
+ * Verify that our THPs were not split yet. Note that
+ * check_huge_anon() cannot be used as it checks for PMD mappings.
+ */
+ for (i = 0; i < nr_thps; i++) {
+ if (is_backed_by_folio(page_area + i * pagesize, pmd_order,
+ pagemap_fd, kpageflags_fd))
+ continue;
+ ksft_test_result_fail("THP %zu missing after mremap\n", i);
+ goto out;
}
- if (thp_size)
- ksft_exit_fail_msg("Still %ld THPs not split\n", thp_size);
+ /* Split all THPs through the remapped pages. */
+ write_debugfs(PID_FMT, getpid(), (uint64_t)page_area,
+ (uint64_t)page_area + page_area_size, 0);
+
+ /* Corruption during mremap or split? */
+ for (i = 0; i < page_area_size; i++) {
+ if (page_area[i] == (char)i)
+ continue;
+ ksft_test_result_fail("%zu byte corrupted\n", i);
+ goto out;
+ }
+
+ /* Split failed? */
+ for (i = 0; i < nr_thps; i++) {
+ if (is_backed_by_folio(page_area + i * pagesize, 0,
+ pagemap_fd, kpageflags_fd))
+ continue;
+ ksft_test_result_fail("THP %zu not split\n", i);
+ }
ksft_test_result_pass("Split PTE-mapped huge pages successful\n");
- munmap(one_page, len);
+out:
+ munmap(thp_area, thp_area_size);
+ if (page_area != MAP_FAILED)
+ munmap(page_area, page_area_size);
}
static void split_file_backed_thp(int order)
--
2.50.1
^ permalink raw reply related [flat|nested] 4+ messages in thread