Re: [PATCH 03/10] mm/mmap/vma_merge: use the proper vma pointers in cases 1 and 6

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Thu, Mar 09, 2023 at 12:12:51PM +0100, Vlastimil Babka wrote:
> Case 1 is now shown in the comment as next vma being merged with prev,
> so use 'next' instead of 'mid'. In case 1 they both point to the same
> vma.
>
> As a consequence, in case 6, the dup_anon_vma() is now tried first on
> 'next' and then on 'mid', before it was the opposite order. This is not
> a functional change, as those two vma's cannnot have a different
> anon_vma, as that would have prevented the merging in the first place.
>

This makes me wonder whether there might be further simplifications based upon
known conditions of mergeability to be had (as e.g. is_mergeable_anon_vma()
would have prevented otherwise). But perhaps I will discover these later in the
series :)

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/mmap.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 1af4c9bc2c87..c33237b283c9 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -605,7 +605,7 @@ static inline void vma_complete(struct vma_prepare *vp,
>
>  		/*
>  		 * In mprotect's case 6 (see comments on vma_merge),
> -		 * we must remove the one after next as well.
> +		 * we are removing both mid and next vmas
>  		 */
>  		if (vp->remove2) {
>  			vp->remove = vp->remove2;
> @@ -948,13 +948,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	/* Can we merge both the predecessor and the successor? */
>  	if (merge_prev && merge_next &&
>  	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
> -		remove = mid;				/* case 1 */
> +		remove = next;				/* case 1 */
>  		vma_end = next->vm_end;
> -		err = dup_anon_vma(prev, mid);
> +		err = dup_anon_vma(prev, next);
>  		if (mid != next) {			/* case 6 */
> +			remove = mid;
>  			remove2 = next;
> -			if (!mid->anon_vma)
> -				err = dup_anon_vma(prev, next);
> +			if (!next->anon_vma)
> +				err = dup_anon_vma(prev, mid);
>  		}
>  	} else if (merge_prev) {
>  		err = 0;				/* case 2 */
> --
> 2.39.2
>

Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>