linux-kernel - Re: [PATCH v2 2/4] mm/mmap/vma_merge: set next to NULL if not applicable

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Date:   Tue, 21 Mar 2023 21:42:32 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        maple-tree@...ts.infradead.org, Vernon Yang <vernon2gm@...il.com>
Subject: Re: [PATCH v2 2/4] mm/mmap/vma_merge: set next to NULL if not
 applicable

* Lorenzo Stoakes <lstoakes@...il.com> [230321 16:51]:
> We are only interested in next if end == next->vm_start (in which case we
> check to see if we can set merge_next), so perform this check alongside
> checking whether curr should be set.
> 
> 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.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  mm/mmap.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c9834364ac98..6361baf75601 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -930,15 +930,15 @@ 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 && end == curr->vm_end)
> +		/* Is there is a VMA immediately adjacent (cases 6 - 8)? */
> +		next = vma_lookup(mm, curr->vm_end);

Since end == curr->vm_end, this lookup is the same as below so these two
statements can be combined.

I also think you may need to initialize next to null since it may not be
set for the 'cannot merge' case.

Something like:
	if ((!curr) ||		    /* case 1-4 */
	     (end == curr->vm_end)) /* Case 6-8, adjacent vma */
		next = vma_lookup(mm, end);
	else
		next = NULL


> +	else if (!curr)
> +		/* Is there a VMA next to a hole (case 1 - 3) or prev (4)? */
> +		next = vma_lookup(mm, end);

Nit, can we have braces for comments that make the if/else look like
it's unguarded?

>  
>  	/* verify some invariant that must be enforced by the caller */
>  	VM_WARN_ON(prev && addr <= prev->vm_start);
> @@ -959,11 +959,10 @@ 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 &&
> -			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
>