linux-kernel - Re: [PATCH 01/10] mm/mmap/vma_merge: use only primary pointers for preparing merge

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Date:   Tue, 14 Mar 2023 22:52:17 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH 01/10] mm/mmap/vma_merge: use only primary pointers for
 preparing merge

On Thu, Mar 09, 2023 at 12:12:49PM +0100, Vlastimil Babka wrote:
> In the merging preparation part of vma_merge(), some vma pointer
> variables are assigned for later execution of the merge, but also read
> from in the block itself. The code is easier follow and check against
> the cases diagram in the comment if the code reads only from the
> "primary" vma variables prev, mid, next instead. No functional change.
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/mmap.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 740b54be3ed4..0a8b052e3022 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -950,16 +950,16 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
>  		remove = mid;				/* case 1 */
>  		vma_end = next->vm_end;
> -		err = dup_anon_vma(res, remove);
> +		err = dup_anon_vma(prev, mid);
>  		if (mid != next) {			/* case 6 */
>  			remove2 = next;
> -			if (!remove->anon_vma)
> -				err = dup_anon_vma(res, remove2);
> +			if (!mid->anon_vma)
> +				err = dup_anon_vma(prev, next);
>  		}
>  	} else if (merge_prev) {
>  		err = 0;				/* case 2 */
>  		if (mid && end > mid->vm_start) {
> -			err = dup_anon_vma(res, mid);
> +			err = dup_anon_vma(prev, mid);
>  			if (end == mid->vm_end) {	/* case 7 */
>  				remove = mid;
>  			} else {			/* case 5 */
> @@ -972,8 +972,8 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		if (prev && addr < prev->vm_end) {	/* case 4 */
>  			vma_end = addr;
>  			adjust = mid;
> -			adj_next = -(vma->vm_end - addr);
> -			err = dup_anon_vma(adjust, prev);
> +			adj_next = -(prev->vm_end - addr);
> +			err = dup_anon_vma(mid, prev);
>  		} else {
>  			vma = next;			/* case 3 */
>  			vma_start = addr;
> @@ -982,7 +982,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			err = 0;
>  			if (mid != next) {		/* case 8 */
>  				remove = mid;
> -				err = dup_anon_vma(res, remove);
> +				err = dup_anon_vma(next, mid);
>  			}
>  		}
>  	}
> --
> 2.39.2
>

Big improvement in readability already here.

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