linux-kernel - Re: [PATCH 05/10] mm/mmap/vma_merge: initialize mid and next in natural order

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Message-ID: <b2675efc-e019-43f6-b322-c0217fef5ed6@lucifer.local>
Date:   Wed, 15 Mar 2023 20:10:04 +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 05/10] mm/mmap/vma_merge: initialize mid and next in
 natural order

On Thu, Mar 09, 2023 at 12:12:53PM +0100, Vlastimil Babka wrote:
> It is more intuitive to go from prev to mid and then next. No functional
> change.
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/mmap.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 420d6847c94c..be60b344e4b1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -912,10 +912,11 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	if (vm_flags & VM_SPECIAL)
>  		return NULL;
>
> -	next = find_vma(mm, prev ? prev->vm_end : 0);
> -	mid = next;
> -	if (next && next->vm_end == end)		/* cases 6, 7, 8 */
> -		next = find_vma(mm, next->vm_end);
> +	mid = find_vma(mm, prev ? prev->vm_end : 0);
> +	if (mid && mid->vm_end == end)			/* cases 6, 7, 8 */
> +		next = find_vma(mm, mid->vm_end);
> +	else
> +		next = mid;

It feels like the original implementation may have been backwards like this just
to avoid this else branch. Which is silly.

>
>  	/* verify some invariant that must be enforced by the caller */
>  	VM_WARN_ON(prev && addr <= prev->vm_start);
> --
> 2.39.2
>

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