[PATCH] mm/mremap: account mm->locked_vm correctly for MREMAP_DONTUNMAP

[Lorenzo Stoakes (ARM)]

When a VMA is mremap()'d with MREMAP_DONTUNMAP set, that results in the VMA
being copied, but the source VMA not being unmapped.

If the VMA is mlock()'d this is a legal operation, though the source VMA
has its VMA_LOCKED_BIT cleared.

However this is done in dontunmap_complete(), after mm->locked_vm was
incremented via vrm_stat_account(), resulting in double-counting.

Worse, this is not even corrected when source VMA is unmapped, due to
the VMA_LOCKED_BIT flag having been cleared.

This all works fine in the usual mremap() case (without MREMAP_DONTUNMAP),
as the source VMA is unmapped with VMA_LOCKED_BIT intact, at which time
mm->locked_vm is decremented accordingly.

Resolve the issue by invoking vrm_stat_account() only after
dontunmap_complete() has run.

Note that MREMAP_DONTUNMAP requires old_len == new_len, so no need to
account for a delta in size in this case.

The bug was introduced by commit b714ccb02a76 ("mm/mremap: complete
refactor of move_vma()") which incorrectly reordered the accounting and the
clearing of the VMA_LOCKED_BIT flag.

Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Closes: https://sashiko.dev/#/patchset/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@xxxxxxxxxx
Reported-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
Closes: https://lore.kernel.org/all/20260828094823.594279-1-kunwu.chan@xxxxxxxxx/
Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/mremap.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 2b4b523a86b8..7c368440fafe 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1355,12 +1355,11 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
 		if (vma_is_anonymous(vma) && !vma->vm_file)
 			vma_set_pgoff(vma, pgoff_unfaulted);
 	}
-
-	/* Because we won't unmap we don't need to touch locked_vm. */
 }
 
 static unsigned long move_vma(struct vma_remap_struct *vrm)
 {
+	const bool is_dontunmap = vrm->flags & MREMAP_DONTUNMAP;
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *new_vma;
 	unsigned long hiwater_vm;
@@ -1401,10 +1400,10 @@ static unsigned long move_vma(struct vma_remap_struct *vrm)
 	 */
 	hiwater_vm = mm->hiwater_vm;
 
-	vrm_stat_account(vrm, vrm->new_len);
-	if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP)))
+	if (unlikely(is_dontunmap && !err))
 		dontunmap_complete(vrm, new_vma);
-	else
+	vrm_stat_account(vrm, vrm->new_len);
+	if (!is_dontunmap || err)
 		unmap_source_vma(vrm);
 
 	mm->hiwater_vm = hiwater_vm;

---
base-commit: aeddb4d52acfcc5ce5e988acd48f2906fe966ca3
change-id: 20260828-mremap-fix-locked-vm-b8991ffc4181

Best regards,
-- 
Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>