mm/mremap: account mm->locked_vm correctly for MREMAP_DONTUNMAP

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.

Link: https://lore.kernel.org/20260828-mremap-fix-locked-vm-v1-1-c80be7505d1e@kernel.org
Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@kernel.org
Reported-by: Kunwu Chan <kunwu.chan@gmail.com>
Closes: https://lore.kernel.org/all/20260828094823.594279-1-kunwu.chan@linux.dev/
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Tested-by: Kunwu Chan <kunwu.chan@gmail.com>
Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions