[PATCH] mm/vma: fix expansion of VMA_MIXEDMAP_BIT VMAs

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Commit ca3d76b0aa80 ("mm: add merging after mremap resize") combined
merging with expansion of mremap()'d VMAs, however it also unintentionally
disallowed the expansion of unmergeable VMAs.

Previously it was possible to expand a mixed mapping which mremap() had
already ensured was a legal expansion.

This is clearly seen in the mremap() logic, as it checks
vma_is_fixed_mapping() to determine legality of an expansion rather than
vma_can_merge().

Additionally, copy_vma() allows for a failed merge to result in a new VMA
being instantiated if MREMAP_MAYMOVE is specified.

However, this is not attempted, as a failure in vma_merge_extend() is
assumed to be an -ENOMEM failure.

Since vma_merge_extend() is used exclusively by mremap() to perform a merge
and/or expansion, resolve the issue by invoking vma_expand() for
unmergeable VMAs there.

Explicitly assert that the VMA is both non-fixed and a mixed mapping.
This provides double insurance against broken changes in future.

Note that khugepaged_enter_vma() (which vma_merge_new_range() runs) is
redundant in this case, as a mixed mapping is not subject to khugepaged.

Also update the VMA userland tests to add a required predicate.

Reported-by: Luiz Capitulino <luizcap@redhat.com>
Closes: https://lore.kernel.org/all/20261023023106.515365-1-luizcap@redhat.com/
Fixes: ca3d76b0aa80 ("mm: add merging after mremap resize")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
Confirmed that this fixes the repro and passes mm selftests and the VMA
userland tests.
---
 mm/vma.c                        | 19 ++++++++++++++++---
 tools/testing/vma/include/dup.h |  5 +++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index 1be613044b4a..88ef92ab24ec 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1851,12 +1851,25 @@ struct vm_area_struct *vma_merge_extend(struct vma_iterator *vmi,
 					struct vm_area_struct *vma,
 					unsigned long delta)
 {
-	VMG_VMA_STATE(vmg, vmi, vma, vma, vma->vm_end, vma->vm_end + delta);
+	const unsigned long start =
+		vma_can_merge(vma) ? vma->vm_end : vma->vm_start;
+	VMG_VMA_STATE(vmg, vmi, vma, vma, start, vma->vm_end + delta);
 
-	vmg.next = vma_iter_next_rewind(vmi, NULL);
 	vmg.middle = NULL; /* We use the VMA to populate VMG fields only. */
 
-	return vma_merge_new_range(&vmg);
+	if (vma_can_merge(vma)) {
+		vmg.next = vma_iter_next_rewind(vmi, NULL);
+		return vma_merge_new_range(&vmg);
+	}
+
+	/* Mixed mappings are permitted to be expanded, not merged. */
+	VM_WARN_ON_ONCE(vma_is_fixed_mapping(vma));
+	VM_WARN_ON_ONCE(!vma_test(vma, VMA_MIXEDMAP_BIT));
+	vma_prev(vmi); /* Reposition to start of VMA. */
+	vmg.target = vma;
+	if (vma_expand(&vmg))
+		return NULL;
+	return vma;
 }
 
 void unlink_file_vma_batch_init(struct unlink_vma_file_batch *vb)
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index f449129ea97a..e0eab4eca998 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1727,3 +1727,8 @@ static inline bool vma_flags_can_merge(const vma_flags_t *flags)
 	 */
 	return !vma_flags_is_hugetlb(flags);
 }
+
+static inline bool vma_can_merge(const struct vm_area_struct *vma)
+{
+	return vma_flags_can_merge(&vma->flags);
+}

---
base-commit: 2bba28e98578de622ef307d00875aa8f8bca3d10
change-id: 20261023-vma-expansion-fix-6843391177c4

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>