Re: [PATCH v3 6/6] mm: move vma locking out of vma_prepare and dup_anon_vma

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* Suren Baghdasaryan <surenb@google.com> [230803 15:16]:
> On Thu, Aug 3, 2023 at 11:32 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Suren Baghdasaryan <surenb@google.com> [230803 13:27]:
> > > vma_prepare() is currently the central place where vmas are being locked
> > > before vma_complete() applies changes to them. While this is convenient,
> > > it also obscures vma locking and makes it harder to follow the locking
> > > rules. Move vma locking out of vma_prepare() and take vma locks
> > > explicitly at the locations where vmas are being modified. Move vma
> > > locking and replace it with an assertion inside dup_anon_vma() to further
> > > clarify the locking pattern inside vma_merge().
> > >
> > > Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
> > > Suggested-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  mm/mmap.c | 29 ++++++++++++++++++-----------
> > >  1 file changed, 18 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 850a39dee075..ae28d6f94c34 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -476,16 +476,6 @@ static inline void init_vma_prep(struct vma_prepare *vp,
> > >   */
> > >  static inline void vma_prepare(struct vma_prepare *vp)
> > >  {
> > > -     vma_start_write(vp->vma);
> > > -     if (vp->adj_next)
> > > -             vma_start_write(vp->adj_next);
> > > -     if (vp->insert)
> > > -             vma_start_write(vp->insert);
> > > -     if (vp->remove)
> > > -             vma_start_write(vp->remove);
> > > -     if (vp->remove2)
> > > -             vma_start_write(vp->remove2);
> > > -
> > >       if (vp->file) {
> > >               uprobe_munmap(vp->vma, vp->vma->vm_start, vp->vma->vm_end);
> > >
> > > @@ -618,7 +608,7 @@ static inline int dup_anon_vma(struct vm_area_struct *dst,
> > >        * anon pages imported.
> > >        */
> > >       if (src->anon_vma && !dst->anon_vma) {
> > > -             vma_start_write(dst);
> > > +             vma_assert_write_locked(dst);
> > >               dst->anon_vma = src->anon_vma;
> > >               return anon_vma_clone(dst, src);
> > >       }
> > > @@ -650,10 +640,12 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       bool remove_next = false;
> > >       struct vma_prepare vp;
> > >
> > > +     vma_start_write(vma);
> > >       if (next && (vma != next) && (end == next->vm_end)) {
> > >               int ret;
> > >
> > >               remove_next = true;
> > > +             vma_start_write(next);
> > >               ret = dup_anon_vma(vma, next);
> > >               if (ret)
> > >                       return ret;
> > > @@ -708,6 +700,8 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       if (vma_iter_prealloc(vmi))
> > >               return -ENOMEM;
> > >
> > > +     vma_start_write(vma);
> > > +
> > >       init_vma_prep(&vp, vma);
> > >       vma_prepare(&vp);
> > >       vma_adjust_trans_huge(vma, start, end, 0);
> > > @@ -940,16 +934,21 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > >       if (!merge_prev && !merge_next)
> > >               return NULL; /* Not mergeable. */
> > >
> > > +     if (prev)
> >
> > Maybe if (merge_prev) instead of prev?  We will write lock prev if it
> > exists and won't change with the current check (case 3 and 8,
> > specifically), with this change case 4 will need to lock prev as it
> > shifts prev->vm_end lower.
> 
> Ah, I see. I was trying to make sure we don't miss any locks and
> over-locked it for case 3 and 8.
> Ok, I'll change the check to if (merge_prev) and will add a separate
> locking for case 4. I think that's what you meant?

Yes, I think that will cover all the cases.  Thanks.

> 
> >
> > > +             vma_start_write(prev);
> > > +
> > >       res = vma = prev;
> > >       remove = remove2 = adjust = NULL;
> > >
> > >       /* Can we merge both the predecessor and the successor? */
> > >       if (merge_prev && merge_next &&
> > >           is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
> > > +             vma_start_write(next);
> > >               remove = next;                          /* case 1 */
> > >               vma_end = next->vm_end;
> > >               err = dup_anon_vma(prev, next);
> > >               if (curr) {                             /* case 6 */
> > > +                     vma_start_write(curr);
> > >                       remove = curr;
> > >                       remove2 = next;
> > >                       if (!next->anon_vma)
> > > @@ -957,6 +956,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > >               }
> > >       } else if (merge_prev) {                        /* case 2 */
> > >               if (curr) {
> > > +                     vma_start_write(curr);
> > >                       err = dup_anon_vma(prev, curr);
> > >                       if (end == curr->vm_end) {      /* case 7 */
> > >                               remove = curr;
> > > @@ -966,6 +966,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > >                       }
> > >               }
> > >       } else { /* merge_next */
> > > +             vma_start_write(next);
> > >               res = next;
> > >               if (prev && addr < prev->vm_end) {      /* case 4 */
> > >                       vma_end = addr;
> > > @@ -983,6 +984,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > >                       vma_pgoff = next->vm_pgoff - pglen;
> > >                       if (curr) {                     /* case 8 */
> > >                               vma_pgoff = curr->vm_pgoff;
> > > +                             vma_start_write(curr);
> > >                               remove = curr;
> > >                               err = dup_anon_vma(next, curr);
> > >                       }
> > > @@ -2373,6 +2375,9 @@ int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       if (new->vm_ops && new->vm_ops->open)
> > >               new->vm_ops->open(new);
> > >
> > > +     vma_start_write(vma);
> > > +     vma_start_write(new);
> > > +
> > >       init_vma_prep(&vp, vma);
> > >       vp.insert = new;
> > >       vma_prepare(&vp);
> > > @@ -3078,6 +3083,8 @@ static int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >               if (vma_iter_prealloc(vmi))
> > >                       goto unacct_fail;
> > >
> > > +             vma_start_write(vma);
> > > +
> > >               init_vma_prep(&vp, vma);
> > >               vma_prepare(&vp);
> > >               vma_adjust_trans_huge(vma, vma->vm_start, addr + len, 0);
> > > --
> > > 2.41.0.585.gd2178a4bd4-goog
> > >