On Mon, Mar 20, 2023 at 12:27:08PM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lstoakes@xxxxxxxxx> [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@xxxxxxxxx>
> > ---
> > 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
> >
Thanks, I will investigate all of these and will try to apply everything
that is workable from here + respin.
