+ selftests-mm-test-a-partial-mremap-of-hugetlb-memory.patch added to mm-new branch

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

The patch titled
     Subject: selftests/mm: test a partial mremap() of hugetlb memory
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-test-a-partial-mremap-of-hugetlb-memory.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-test-a-partial-mremap-of-hugetlb-memory.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

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Josef Bacik <josef@toxicpanda.com>
Subject: selftests/mm: test a partial mremap() of hugetlb memory
Date: Thu, 05 Nov 2026 14:02:04 +0000

hugetlb-mremap only moves whole mappings, so it did not catch the
reservation leak of a partial move.

Map four private huge pages without touching them, move the second one,
unmap everything, and check that the number of reserved huge pages is back
where it was.

Link: https://lore.kernel.org/20261105-b4-mremap-pre-split-v3-2-f8789de76849@toxicpanda.com
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Sarthak Sharma <sarthak.sharma@arm.com>
---

 tools/testing/selftests/mm/hugetlb-mremap.c |   52 +++++++++++++++++-
 1 file changed, 50 insertions(+), 2 deletions(-)

--- a/tools/testing/selftests/mm/hugetlb-mremap.c~selftests-mm-test-a-partial-mremap-of-hugetlb-memory
+++ a/tools/testing/selftests/mm/hugetlb-mremap.c
@@ -100,6 +100,51 @@ static void register_region_with_uffd(ch
 	ksft_print_msg("Registered memory at address %p with userfaultfd\n", addr);
 }
 
+/*
+ * Move one page out of the middle of a private mapping that has not been
+ * touched, so that every page of it still holds its reservation.  Once all of
+ * it is unmapped, all of the reservations have to be back in the pool.
+ */
+static void test_partial_move(unsigned long hugepage_size)
+{
+	const int flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB;
+	const int prot = PROT_READ | PROT_WRITE;
+	const unsigned long nr = 4;
+	unsigned long resv = hugetlb_nr_resv_pages(hugepage_size);
+	unsigned long resv_after;
+	char *src, *dst, *addr;
+
+	/* The pages of the test before this one can still be in use. */
+	if (!hugetlb_setup_default(resv + nr + 1)) {
+		ksft_test_result_skip("Partial move: not enough free huge pages\n");
+		return;
+	}
+
+	src = mmap(NULL, nr * hugepage_size, prot, flags, -1, 0);
+	if (src == MAP_FAILED)
+		ksft_exit_fail_msg("mmap src: %s\n", strerror(errno));
+
+	/* The destination has to be hugetlb-backed, mremap() replaces it. */
+	dst = mmap(NULL, hugepage_size, prot, flags, -1, 0);
+	if (dst == MAP_FAILED)
+		ksft_exit_fail_msg("mmap dst: %s\n", strerror(errno));
+
+	addr = mremap(src + hugepage_size, hugepage_size, hugepage_size,
+		      MREMAP_MAYMOVE | MREMAP_FIXED, dst);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("partial mremap: %s\n", strerror(errno));
+
+	munmap(addr, hugepage_size);
+	munmap(src, nr * hugepage_size);
+
+	resv_after = hugetlb_nr_resv_pages(hugepage_size);
+	if (resv_after != resv)
+		ksft_print_msg("Reserved huge pages: %lu before, %lu after\n",
+			       resv, resv_after);
+	ksft_test_result(resv_after == resv,
+			 "Partial move returns all reservations\n");
+}
+
 int main(int argc, char *argv[])
 {
 	unsigned long hugepage_size;
@@ -107,7 +152,7 @@ int main(int argc, char *argv[])
 	size_t length = 0;
 
 	ksft_print_header();
-	ksft_set_plan(1);
+	ksft_set_plan(2);
 
 	if (argc >= 2 && !strcmp(argv[1], "-h"))
 		ksft_exit_fail_msg("Usage: %s [length_in_MB]\n", argv[0]);
@@ -175,5 +220,8 @@ int main(int argc, char *argv[])
 	close(fd);
 
 	ksft_test_result(!ret, "Read same data\n");
-	ksft_exit(!ret);
+
+	test_partial_move(hugepage_size);
+
+	ksft_finished();
 }
_

Patches currently in -mm which might be from josef@toxicpanda.com are

mm-zswap-drop-the-stale-zswap-entry-of-a-zero-folio.patch
mm-swap-account-synchronous-swap-reads-as-memory-stall.patch
mm-mremap-fix-hugetlb-reservation-leak-on-partial-move.patch
selftests-mm-test-a-partial-mremap-of-hugetlb-memory.patch
mm-mremap-reaccount-on-unmap-failure-only-if-accounted.patch
mm-mremap-restore-vma_account_bit-if-the-unmap-fails.patch
selftests-mm-test-mremap-with-an-old-size-of-0.patch
zram-lz4-lz4hc-accept-dictionaries-of-more-than-64-kib.patch