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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 8/28/26 13:20, Lorenzo Stoakes (ARM) wrote:
> 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@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/
> Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  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,