+ selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it.patch added to mm-unstable branch

+ selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it.patch added to mm-unstable branch

[Andrew Morton]

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The patch titled
     Subject: selftests/mm: mremap_test: Validate the preamble region before mapping it
has been added to the -mm mm-unstable branch.  Its filename is
     selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it.patch

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

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------------------------------------------------------
From: Ricardo B. Marlière (SUSE) <ricardo@marliere.net>
Subject: selftests/mm: mremap_test: Validate the preamble region before mapping it
Date: Wed, 14 Oct 2026 15:46:46 -0300

remap_region() checks that the destination region is free before mapping
it, but not the preamble region placed right before it, so mapping the
preamble can intermittently fail with EEXIST:

  # Failed to map dest preamble region: File exists
  not ok 15 5MB mremap - Source 1MB-aligned, Dest 1MB-aligned with 40MB Preamble

This happens when the kernel's heap ASLR (arch_randomize_brk()) lands the
process heap inside that unchecked range.  Confirmed by correlating
brk(NULL) at startup with pass/fail across 60 runs: every failure had brk
land inside that range.

Fix it by including the preamble in the free-region check.

Link: https://lore.kernel.org/20261014-selftests-mm-mremap_preamble-v1-1-2e97f501cb56@marliere.net
Signed-off-by: Ricardo B. Marlière (SUSE) <ricardo@marliere.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
---

 tools/testing/selftests/mm/mremap_test.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/tools/testing/selftests/mm/mremap_test.c~selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it
+++ a/tools/testing/selftests/mm/mremap_test.c
@@ -1006,8 +1006,10 @@ static long long remap_region(struct con
 	if (!((unsigned long long) addr & c.dest_alignment))
 		addr = (void *) ((unsigned long long) addr | c.dest_alignment);
 
-	/* Don't destroy existing mappings unless expected to overlap */
-	while (!is_remap_region_valid(addr, c.region_size) && !c.overlapping) {
+	/* Don't destroy existing mappings unless expected to overlap. */
+	while (!is_remap_region_valid(addr - c.dest_preamble_size,
+				      c.dest_preamble_size + c.region_size) &&
+	       !c.overlapping) {
 		/* Check for unsigned overflow */
 		tmp_addr = addr + c.dest_alignment;
 		if (tmp_addr < addr) {
_

Patches currently in -mm which might be from ricardo@marliere.net are

selftests-mm-mremap_test-validate-the-preamble-region-before-mapping-it.patch