+ selftests-mm-test-mremap-with-an-old-size-of-0.patch added to mm-new branch

+ selftests-mm-test-mremap-with-an-old-size-of-0.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: selftests/mm: test mremap() with an old size of 0
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-test-mremap-with-an-old-size-of-0.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-mremap-with-an-old-size-of-0.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: Josef Bacik <josef@toxicpanda.com>
Subject: selftests/mm: test mremap() with an old size of 0
Date: Thu, 05 Nov 2026 14:02:07 +0000

mremap() with an old size of 0 duplicates a shared mapping, and
mremap_test has no case for it.

Duplicate a shared anonymous mapping, once from its start and once from
the middle.  Check that the source and the duplicate are mapped by one VMA
each and that they show the same pages.

Link: https://lore.kernel.org/20261105-b4-mremap-pre-split-v3-5-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/mremap_test.c |   51 ++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

--- a/tools/testing/selftests/mm/mremap_test.c~selftests-mm-test-mremap-with-an-old-size-of-0
+++ a/tools/testing/selftests/mm/mremap_test.c
@@ -604,6 +604,53 @@ out:
 				      inplace ? " [inplace]" : "");
 }
 
+/*
+ * An old size of 0 duplicates a shared mapping: the pages show up at a second
+ * address, and the mapping they were taken from stays as it is.
+ */
+static void mremap_duplicate_shared(FILE *maps_fp, unsigned long page_size,
+				    unsigned long offset)
+{
+	char *test_name = "mremap duplicate shared mapping";
+	const size_t size = 3 * page_size;
+	const size_t dup_size = size - offset;
+	char *dup = MAP_FAILED;
+	bool success = false;
+	char *ptr;
+
+	ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
+		   MAP_SHARED | MAP_ANON, -1, 0);
+	if (ptr == MAP_FAILED) {
+		ksft_perror("mmap");
+		goto out;
+	}
+	memset(ptr, 'a', size);
+
+	dup = mremap(ptr + offset, 0, dup_size, MREMAP_MAYMOVE);
+	if (dup == MAP_FAILED) {
+		ksft_perror("mremap");
+		goto out_unmap;
+	}
+
+	/* Both are mapped, each by a single VMA. */
+	if (!is_ptr_mapped(maps_fp, ptr, size) ||
+	    !is_ptr_mapped(maps_fp, dup, dup_size))
+		goto out_unmap;
+
+	/* They show the same pages. */
+	ptr[offset] = 'b';
+	dup[dup_size - 1] = 'c';
+	success = dup[0] == 'b' && ptr[size - 1] == 'c' && ptr[0] != 'c';
+
+out_unmap:
+	if (dup != MAP_FAILED)
+		munmap(dup, dup_size);
+	munmap(ptr, size);
+out:
+	ksft_test_result(success, "%s%s\n", test_name,
+			 offset ? " [from the middle]" : "");
+}
+
 static void mremap_move_multiple_vmas_split(unsigned int pattern_seed,
 					    unsigned long page_size,
 					    bool dont_unmap)
@@ -1249,7 +1296,7 @@ int main(int argc, char **argv)
 	char *rand_addr;
 	size_t rand_size;
 	int num_expand_tests = 2;
-	int num_misc_tests = 9;
+	int num_misc_tests = 11;
 	struct test test_cases[MAX_TEST] = {};
 	struct test perf_test_cases[MAX_PERF_TEST];
 	int page_size;
@@ -1386,6 +1433,8 @@ int main(int argc, char **argv)
 	mremap_move_multiple_vmas_split(pattern_seed, page_size, /* dontunmap= */ false);
 	mremap_move_multiple_vmas_split(pattern_seed, page_size, /* dontunmap= */ true);
 	mremap_move_multi_invalid_vmas(maps_fp, page_size);
+	mremap_duplicate_shared(maps_fp, page_size, 0);
+	mremap_duplicate_shared(maps_fp, page_size, page_size);
 
 	fclose(maps_fp);
 
_

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