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Message-ID: <20230322152924.mqffqk2wvn6qmqqv@revolver>
Date: Wed, 22 Mar 2023 11:29:24 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Lorenzo Stoakes <lstoakes@...il.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>,
maple-tree@...ts.infradead.org, Vernon Yang <vernon2gm@...il.com>
Subject: Re: [PATCH v3 4/4] mm/mmap/vma_merge: init cleanup, be explicit
about the non-mergeable case
* Vlastimil Babka <vbabka@...e.cz> [230322 05:28]:
> On 3/22/23 08:13, Lorenzo Stoakes wrote:
> > Rather than setting err = -1 and only resetting if we hit merge cases,
> > explicitly check the non-mergeable case to make it abundantly clear that we
> > only proceed with the rest if something is mergeable, default err to 0 and
> > only update if an error might occur.
> >
> > Move the merge_prev, merge_next cases closer to the logic determining curr,
> > next and reorder initial variables so they are more logically grouped.
> >
> > This has no functional impact.
> >
> > Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> Some nits:
>
> > ---
> > mm/mmap.c | 57 ++++++++++++++++++++++++++++---------------------------
> > 1 file changed, 29 insertions(+), 28 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 2a4f63716231..642f3d063be1 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -909,18 +909,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
> > struct anon_vma_name *anon_name)
> > {
> > - pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> > - pgoff_t vma_pgoff;
> > struct vm_area_struct *curr, *next, *res;
> > struct vm_area_struct *vma, *adjust, *remove, *remove2;
> > - int err = -1;
> > + struct vma_prepare vp;
> > + pgoff_t vma_pgoff;
> > + int err = 0;
> > bool merge_prev = false;
> > bool merge_next = false;
> > bool vma_expanded = false;
> > - struct vma_prepare vp;
> > + unsigned long vma_start = addr;
> > unsigned long vma_end = end;
> > + pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> > long adj_start = 0;
> > - unsigned long vma_start = addr;
> >
> > validate_mm(mm);
> > /*
> > @@ -939,36 +939,38 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > else
> > next = NULL; /* case 5 */
> >
> > - /*
> > - * By default, we return prev. Cases 3, 4, 8 will instead return next
> > - * and cases 3, 8 will also update vma to point at next.
> > - */
> > - res = vma = prev;
> > -
> > - /* Verify some invariant that must be enforced by the caller. */
> > - VM_WARN_ON(prev && addr <= prev->vm_start);
> > - VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > - VM_WARN_ON(addr >= end);
> > -
> > if (prev) {
> > vma_start = prev->vm_start;
> > vma_pgoff = prev->vm_pgoff;
> > +
> > /* Can we merge the predecessor? */
> > - if (prev->vm_end == addr && mpol_equal(vma_policy(prev), policy)
> > + if (addr == prev->vm_end && mpol_equal(vma_policy(prev), policy)
> > && can_vma_merge_after(prev, vm_flags, anon_vma, file,
> > - pgoff, vm_userfaultfd_ctx, anon_name)) {
> > + pgoff, vm_userfaultfd_ctx, anon_name)) {
> > merge_prev = true;
> > vma_prev(vmi);
> > }
> > }
> >
> > /* Can we merge the successor? */
> > - 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;
> > - }
> > + merge_next = next && mpol_equal(policy, vma_policy(next)) &&
> > + can_vma_merge_before(next, vm_flags,
> > + anon_vma, file, pgoff+pglen,
> > + vm_userfaultfd_ctx, anon_name);
>
> Not a great fan of this, I think the if() is more readable, but if you and
> Liam agree, I won't mind much. Either way could consolidate the parameters
> on less lines maybe.
I think it's more readable with the if() as well, fwiw. If you revert
this to an if(), then you should keep the braces as it looks very
awkward without them.
>
> > +
> > + if (!merge_prev && !merge_next)
> > + return NULL; /* Not mergeable. */
> > +
> > + /*
> > + * By default, we return prev. Cases 3, 4, 8 will instead return next
> > + * and cases 3, 8 will also update vma to point at next.
> > + */
> > + res = vma = prev;
> > +
> > + /* Verify some invariant that must be enforced by the caller. */
> > + VM_WARN_ON(prev && addr <= prev->vm_start);
> > + VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > + VM_WARN_ON(addr >= end);
> >
> > remove = remove2 = adjust = NULL;
> > /* Can we merge both the predecessor and the successor? */
> > @@ -984,7 +986,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > err = dup_anon_vma(prev, curr);
> > }
> > } else if (merge_prev) {
> > - err = 0; /* case 2 */
> > + /* case 2 */
>
> Move the comment from this now weirdly empty line to the "else if" one above?
Yeah, this also makes sense to me, and brings it in line with case 8/4,
etc.
>
> > if (curr) {
> > err = dup_anon_vma(prev, curr);
> > if (end == curr->vm_end) { /* case 7 */
> > @@ -994,7 +996,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > adj_start = (end - curr->vm_start);
> > }
> > }
> > - } else if (merge_next) {
> > + } else { /* merge_next */
> > res = next;
> > if (prev && addr < prev->vm_end) { /* case 4 */
> > vma_end = addr;
> > @@ -1010,7 +1012,6 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > vma_start = addr;
> > vma_end = next->vm_end;
> > vma_pgoff = next->vm_pgoff;
> > - err = 0;
> > if (curr) { /* case 8 */
> > vma_pgoff = curr->vm_pgoff;
> > remove = curr;
> > @@ -1019,7 +1020,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > }
> > }
> >
> > - /* Cannot merge or error in anon_vma clone */
> > + /* Error in anon_vma clone. */
> > if (err)
> > return NULL;
> >
>
>
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