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

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Message-ID: <467b6158-5c91-4c95-b05b-ea9331f7f563@lucifer.local>
Date:   Wed, 22 Mar 2023 06:23:43 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, 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

On Tue, Mar 21, 2023 at 09:42:32PM -0400, Liam R. Howlett wrote:
> * 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 can't believe I didn't see this :) wood for the trees + a great example as to
the benefit of code review.

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

You're right, will fix.

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

This is really turning out to be quite nicely succinct now I think! Will do.

>
> > +	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?
>

Ack will fix in respin.

> >
> >  	/* 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
> >