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

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Message-ID: <20230320162708.xq64y4ig6f3sf747@revolver>
Date:   Mon, 20 Mar 2023 12:27:08 -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
Subject: Re: [PATCH 2/4] mm/mmap/vma_merge: set next to NULL if not applicable

* Lorenzo Stoakes <lstoakes@...il.com> [230318 07:15]:
> 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.
> 
> Additionally, use the abstract 'vma' object to look up the possible curr or
> next VMA in order to avoid any confusion as to what these variables
> represent - now curr and next are assigned once and only once.
> 
> This has no functional impact.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  mm/mmap.c | 61 ++++++++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 49 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c9834364ac98..66893fc72e03 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -930,15 +930,53 @@ 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;
> +	/*
> +	 * If there is a previous VMA specified, find the next, otherwise find
> +	 * the first.
> +	 */
> +	vma = find_vma(mm, prev ? prev->vm_end : 0);
> +
> +	/*
> +	 * Does the input range span an existing VMA? If so, we designate this
> +	 * VMA 'curr'. The caller will have ensured that curr->vm_start == addr.
> +	 *
> +	 * Cases 5 - 8.
> +	 */
> +	if (vma && end > vma->vm_start) {
> +		curr = vma;

It might be better to set:
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)
> +		/*
> +		 * If the addr - end range spans this VMA entirely, then we
> +		 * check to see if another VMA follows it.
> +		 *
> +		 * If it is _immediately_ adjacent (checked below), then we
> +		 * designate it 'next' (cases 6 - 8).
> +		 */
> +		if (curr->vm_end == end)
> +			vma = find_vma(mm, curr->vm_end);

You can change this to:
next = vma_lookup(mm, curr->vm_end);
Then you don't need to validate below, in this case.

> +		else
> +			/* Case 5. */
> +			vma = NULL;


> +	} else {
> +		/*
> +		 * The addr - end range either spans the end of prev or spans no
> +		 * VMA at all - in either case we dispense with 'curr' and
> +		 * maintain only 'prev' and (possibly) 'next'.

Possibly next here would be:
next = vma_lookup(mm, end);
I think?

> +		 *
> +		 * Cases 1 - 4.
> +		 */
>  		curr = NULL;
> +	}
> +
> +	/*
> +	 * We only actually examine the next VMA if it is immediately adjacent
> +	 * to end which sits either at the end of a hole (cases 1 - 3), PPPP
> +	 * (case 4) or CCCC (cases 6 - 8).
> +	 */
> +	if (vma && end == vma->vm_start)
> +		next = vma;
> +	else
> +		next = NULL;

If I'm correct above, then we can drop this next checking.

>  
>  	/* verify some invariant that must be enforced by the caller */
>  	VM_WARN_ON(prev && addr <= prev->vm_start);
> @@ -959,11 +997,10 @@ 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)) {

I think we can keep this chunk with the next = vma_lookup() changes as
well.

>  		merge_next = true;
>  	}
>  
> -- 
> 2.39.2
>