[PATCH] mm/mremap: fix locked_vm accounting for MREMAP_DONTUNMAP

[Kunwu Chan <kunwu.chan@gmail.com>]

From: Kunwu Chan <kunwu.chan@gmail.com>

When mremap() is called with MREMAP_DONTUNMAP on a locked VMA,
vrm_stat_account() increments mm->locked_vm for the new VMA while the
source VMA is still marked VM_LOCKED.

For a normal mremap(), the source VMA is subsequently unmapped and the
munmap path decrements mm->locked_vm, balancing this accounting.

With MREMAP_DONTUNMAP, the source VMA is left in place and
dontunmap_complete() clears VMA_LOCKED_MASK.  When the source VMA is
subsequently unmapped, VMA_LOCKED_BIT is already clear, so the munmap
path does not undo the increment from vrm_stat_account().  This leaves
mm->locked_vm over-accounted.

Undo the locked_vm increment in dontunmap_complete() before clearing
VMA_LOCKED_MASK.  MREMAP_DONTUNMAP requires old_len == new_len, and
only VMA_LOCKED_BIT is checked to match the accounting performed by
vrm_stat_account().

Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()")
Cc: stable@vger.kernel.org
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 mm/mremap.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e8df5cdb0ac..37ea5ee1986 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1334,6 +1334,14 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
 	unsigned long old_start = vrm->vma->vm_start;
 	unsigned long old_end = vrm->vma->vm_end;
 
+	/*
+	 * vrm_stat_account() accounted the new VMA while the source VMA
+	 * was still locked.  Since DONTUNMAP leaves the source VMA in
+	 * place after clearing VMA_LOCKED_MASK, undo that accounting here.
+	 */
+	if (vma_test(vrm->vma, VMA_LOCKED_BIT))
+		vrm->vma->vm_mm->locked_vm -= vrm->old_len >> PAGE_SHIFT;
+
 	/* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */
 	vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK);
 
@@ -1343,8 +1351,6 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
 	 */
 	if (new_vma != vrm->vma && start == old_start && end == old_end)
 		unlink_anon_vmas(vrm->vma);
-
-	/* Because we won't unmap we don't need to touch locked_vm. */
 }
 
 static unsigned long move_vma(struct vma_remap_struct *vrm)
-- 
2.43.0