linux-kernel - Re: [PATCH v3 2/4] mm/mmap/vma_merge: fold curr, next assignment logic

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Message-ID: <bdbb5362-a4bf-a87c-95e6-e94ab28665bd@suse.cz>
Date:   Wed, 22 Mar 2023 10:06:36 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        maple-tree@...ts.infradead.org, Vernon Yang <vernon2gm@...il.com>
Subject: Re: [PATCH v3 2/4] mm/mmap/vma_merge: fold curr, next assignment
 logic

On 3/22/23 08:13, Lorenzo Stoakes wrote:
> Use find_vma_intersection() and vma_lookup() to both simplify the logic and
> to fold the end == next->vm_start condition into one block.
> 
> This groups all of the simple range checks together and establishes the
> invariant that, if prev, curr or next are non-NULL then their positions are
> as expected.
> 
> This has no functional impact.

I'm not so sure...

> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  mm/mmap.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c9834364ac98..dbdbb92493b2 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -930,15 +930,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	if (vm_flags & VM_SPECIAL)
>  		return NULL;
> 
> -	curr = find_vma(mm, prev ? prev->vm_end : 0);
> -	if (curr && curr->vm_end == end)		/* cases 6, 7, 8 */
> -		next = find_vma(mm, curr->vm_end);
> -	else
> -		next = curr;
> +	/* Does the input range span an existing VMA? (cases 5 - 8) */
> +	curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
> 
> -	/* In cases 1 - 4 there's no CCCC vma */
> -	if (curr && end <= curr->vm_start)
> -		curr = NULL;
> +	if (!curr ||			/* cases 1 - 4 */
> +	    end == curr->vm_end)	/* cases 6 - 8, adjacent VMA */
> +		next = vma_lookup(mm, end);

AFAICS if the next vma is not adjacent to CCCC or ****, but there's a gap,
this will still give you a non-NULL result?

> +	else
> +		next = NULL;		/* case 5 */
> 
>  	/* verify some invariant that must be enforced by the caller */
>  	VM_WARN_ON(prev && addr <= prev->vm_start);
> @@ -959,11 +958,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		}
>  	}
>  	/* Can we merge the successor? */
> -	if (next && end == next->vm_start &&

And then without this end == next->vm_start check, merge will be done
despite the gap.

> -			mpol_equal(policy, vma_policy(next)) &&
> -			can_vma_merge_before(next, vm_flags,
> -					     anon_vma, file, pgoff+pglen,
> -					     vm_userfaultfd_ctx, anon_name)) {
> +	if (next && mpol_equal(policy, vma_policy(next)) &&
> +	    can_vma_merge_before(next, vm_flags,
> +				 anon_vma, file, pgoff+pglen,
> +				 vm_userfaultfd_ctx, anon_name)) {
>  		merge_next = true;
>  	}
> 
> --
> 2.39.2