+ selftests-mm-race-the-collapse-of-windows-with-holes.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: selftests/mm: race the collapse of windows with holes
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-race-the-collapse-of-windows-with-holes.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-race-the-collapse-of-windows-with-holes.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
Subject: selftests/mm: race the collapse of windows with holes
Date: Sat, 19 Sep 2026 01:24:47 +0100

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 in the new folio
rather than copied.  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: LLM
Link: https://lore.kernel.org/20260919002451.496763-18-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "David Hildenbrand (arm)" <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: kernel-team@meta.com
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Nico Pache (Red Hat) <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
---

 tools/testing/selftests/mm/khugepaged_race.c |   32 ++++++++++-------
 1 file changed, 20 insertions(+), 12 deletions(-)

--- a/tools/testing/selftests/mm/khugepaged_race.c~selftests-mm-race-the-collapse-of-windows-with-holes
+++ a/tools/testing/selftests/mm/khugepaged_race.c
@@ -236,6 +236,8 @@ 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);
@@ -303,7 +305,7 @@ 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);
@@ -311,8 +313,9 @@ int main(int argc, char **argv)
 	/* Base of the settings stack; the bottom entry is never popped */
 	thp_push_settings(&settings);
 
-	for (int m = 0; m < nr_modes; m++) {
-		const char *mode = modes[m];
+	for (int run = 0; run < nr_modes * nr_occupancies; run++) {
+		const char *mode = modes[run / nr_occupancies];
+		bool holes = occupancies[run % nr_occupancies];
 
 		thp_read_settings(&settings);
 		settings.thp_enabled = THP_MADVISE;
@@ -322,12 +325,14 @@ int main(int argc, char **argv)
 		settings.khugepaged.scan_sleep_millisecs =
 			strcmp(mode, "free") ? 1000 : 0;
 		settings.khugepaged.alloc_sleep_millisecs = 10;
+
 		/*
-		 * mTHP collapse honours only 0 or HPAGE_PMD_NR - 1 here, and 0
-		 * keeps a step from being spent on PMD allocations that racing
-		 * MADV_DONTNEED will not let succeed.
+		 * mTHP collapse honours only 0 or HPAGE_PMD_NR - 1 here.  The two
+		 * ends race different paths: a strict window has every PTE
+		 * present, a hole-heavy one is mostly zero-filled.
 		 */
-		settings.khugepaged.max_ptes_none = 0;
+		settings.khugepaged.max_ptes_none = holes ?
+			(hpage_pmd_size / page_size) - 1 : 0;
 		/* One wake, one pass: the playground plus the forked children's copies */
 		settings.khugepaged.pages_to_scan =
 			nr_areas * (hpage_pmd_size / page_size) * 8;
@@ -393,8 +398,9 @@ 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);
 
 		/* The next mode maps the same fixed address with its own settings */
 		munmap(region, nr_areas * hpage_pmd_size);
@@ -404,9 +410,11 @@ 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 (++run < nr_modes * nr_occupancies)
+				ksft_test_result_skip("%s/%s: skipped after corruption\n",
+						      modes[run / nr_occupancies],
+						      occupancies[run % nr_occupancies] ?
+						      "holes" : "strict");
 			break;
 		}
 	}
_

Patches currently in -mm which might be from kas@kernel.org are

mm-huge_memory-do-not-touch-frozen-folios-in-deferred_split_isolate.patch
mm-huge_memory-dequeue-the-deferred-split-after-the-split-freeze.patch
mm-huge_memory-add-folio_reset_partially_mapped.patch
mm-khugepaged-drop-redundant-mm_struct-pin-in-madvise_collapse.patch
mm-khugepaged-count-collapses-where-khugepaged-makes-them.patch
mm-khugepaged-rename-mthp_present_ptes-bitmap-to-eligible_ptes.patch
mm-collapse-add-collapseh-for-the-collapse-interface.patch
mm-collapse-state-what-a-collapse-may-do-in-the-policy.patch
mm-collapse-drop-the-collapse_possible-wrapper.patch
mm-collapse-name-the-per-table-scan-reset-for-what-it-resets.patch
mm-collapse-separate-scanning-a-pte-table-from-collapsing-it.patch
mm-collapse-open-code-collapse_single_pmd-in-its-two-callers.patch
mm-collapse-work-out-the-orders-a-vma-allows-once-per-vma.patch
mm-collapse-declare-the-collapse-interface-in-collapseh.patch
mm-collapse-implement-madv_collapse-in-madvisec.patch
selftests-mm-raise-the-khugepaged-test-case-cap.patch
selftests-mm-skip-collapse_compound_extreme-where-the-pmd-is-too-large.patch
selftests-mm-scale-khugepageds-collapse-wait-with-the-pmd-size.patch
selftests-mm-skip-khugepaged-page-cache-cases-without-a-pmd-folio.patch
selftests-mm-make-the-swap-cases-swapout-reliable.patch
selftests-mm-stop-khugepaged-during-the-madv_collapse-cases.patch
selftests-mm-move-is_backed_by_folio-into-vm_util.patch
selftests-mm-add-folio-order-check-for-address-ranges.patch
selftests-mm-add-folio-order-detection-self-check.patch
selftests-mm-add-khugepaged-completion-barrier-helper.patch
selftests-mm-add-order-parameterized-khugepaged-collapse-cases.patch
selftests-mm-parameterize-the-mixed-source-collapse-case-by-source-order.patch
selftests-mm-cover-a-shared-source-collapse-write-race.patch
selftests-mm-run-every-supported-collapse-order-by-default.patch
selftests-mm-check-that-one-khugepaged-pass-collapses-one-window.patch
selftests-mm-add-khugepaged-race-harness.patch
selftests-mm-race-the-collapse-of-windows-with-holes.patch
selftests-mm-add-memory-pressure-threads-to-the-khugepaged-race-harness.patch
selftests-mm-zap-whole-pte-tables-in-the-khugepaged-race-harness.patch