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

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Jul 31, 2023 at 1:30 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> Adding Lorenzo since this also touches vma_merge() again..
>
> * Suren Baghdasaryan <surenb@google.com> [230731 13:12]:
> > 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 hard 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.
>
> I get the idea of locking closer to the edits, but vma_merge() is very
> hard to follow.  It was worse when it was two functions and much larger,
> but adding this into vma_merge() is difficult to validate.
>
> We still set vma = <another vma> in places, so that adds to the
> difficulty of ensuring the end result is all VMAs that will be
> modified/removed have been locked...and the 'locking rules' are also
> obscured.
>
> It's also annoying that this doesn't fully allow you to follow the
> locking anyways.  dup_anon_vma() still locks internally, with good
> reason, but it's still not clear that the VMA is locked when looking at
> this.
>
> That being said, I did go through each case and it looks like it locks
> the correct VMA(s) to me.

Thanks!
Yeah, it took me some time to ensure the locking there is correct. If
you see a better alternative to make the locking more obvious I'm open
to suggestions. I accept that locking in vma_merge() is not easy to
follow.

>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
>
> >
> > Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/mmap.c | 26 ++++++++++++++++----------
> >  1 file changed, 16 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 850a39dee075..e59d83cb1d7a 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);
> >
> > @@ -650,6 +640,7 @@ 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;
> >
> > @@ -657,6 +648,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >               ret = dup_anon_vma(vma, next);
> >               if (ret)
> >                       return ret;
> > +             vma_start_write(next);
> >       }
> >
> >       init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NULL);
> > @@ -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);
> > @@ -946,10 +940,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >       /* 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)
> > @@ -958,6 +954,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >       } else if (merge_prev) {                        /* case 2 */
> >               if (curr) {
> >                       err = dup_anon_vma(prev, curr);
> > +                     vma_start_write(curr);
> >                       if (end == curr->vm_end) {      /* case 7 */
> >                               remove = curr;
> >                       } else {                        /* case 5 */
> > @@ -969,6 +966,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >               res = next;
> >               if (prev && addr < prev->vm_end) {      /* case 4 */
> >                       vma_end = addr;
> > +                     vma_start_write(next);
> >                       adjust = next;
> >                       adj_start = -(prev->vm_end - addr);
> >                       err = dup_anon_vma(next, prev);
> > @@ -983,6 +981,7 @@ 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);
> >                       }
> > @@ -996,6 +995,8 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >       if (vma_iter_prealloc(vmi))
> >               return NULL;
> >
> > +     vma_start_write(vma);
> > +
> >       init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
> >       VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
> >                  vp.anon_vma != adjust->anon_vma);
> > @@ -2373,6 +2374,9 @@ 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 +3082,8 @@ 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);
> > --
> > 2.41.0.487.g6d72f3e995-goog
> >