On Tue, Mar 21, 2023 at 09:42:32PM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lstoakes@xxxxxxxxx> [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@xxxxxxxxx>
> > ---
> > 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
> >
