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In-Reply-To: <20230802165900.otlirhbjr54wymvl@revolver>
From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 2 Aug 2023 10:24:31 -0700
Message-ID: <CAJuCfpF-E65wJcprA80zLWfbfHdaDJMafNe087aUs_-SkwUTdA@mail.gmail.com>
Subject: Re: [PATCH v2 6/6] mm: move vma locking out of vma_prepare
To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Suren Baghdasaryan <surenb@google.com>,
        akpm@linux-foundation.org, torvalds@linux-foundation.org,
        jannh@google.com, willy@infradead.org, david@redhat.com,
        peterx@redhat.com, ldufour@linux.ibm.com, vbabka@suse.cz,
        michel@lespinasse.org, jglisse@google.com, mhocko@suse.com,
        hannes@cmpxchg.org, dave@stgolabs.net, hughd@google.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org,
        Linus Torvalds <torvalds@linuxfoundation.org>
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On Wed, Aug 2, 2023 at 9:59=E2=80=AFAM Liam R. Howlett <Liam.Howlett@oracle=
.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [230801 18:07]:
> > vma_prepare() is currently the central place where vmas are being locke=
d
> > before vma_complete() applies changes to them. While this is convenient=
,
> > it also obscures vma locking and makes it hard to follow the locking ru=
les.
> > Move vma locking out of vma_prepare() and take vma locks explicitly at =
the
> > locations where vmas are being modified.
> >
> > Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.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_prepar=
e *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 =3D false;
> >       struct vma_prepare vp;
> >
> > +     vma_start_write(vma);
>
> This lock made me think about the lock in dup_anon_vma().. the only
> reason we need to dup the anon vma is because the VMA _will_ be
> modified..  So if we are to remove next the dup_anon_vma() call may lock
> vma again.  This happens multiple times through this patch set.
>
> If we lock both dst and src VMAs before calling dup_anon_vma, then I
> think it will become more clear in the code below which VMAs are locked.
> We could remove the lock in the dup_anon_vma() (and replace with an
> assert?), and drop the vma locking of "vma".
>
> I think this would address the confusion of the locking below that I
> experienced and avoid calling the vma_start_write() multiple times for
> the same vma.  I mean, having the vma_start_write(vma) obscures which
> VMA is locked as well.

Agree, it would result in an easier to understand locking code. I'll
move the lock outside of dup_anon_vma() and will replace it with an
assert. Thanks!

>
>
> >       if (next && (vma !=3D next) && (end =3D=3D next->vm_end)) {
> >               int ret;
> >
> > @@ -657,6 +648,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_=
area_struct *vma,
> >               ret =3D dup_anon_vma(vma, next);
> >               if (ret)
> >                       return ret;
> > +             vma_start_write(next);
> >       }
> >
> >       init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NU=
LL);
> > @@ -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_itera=
tor *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 =3D next;                          /* case 1 */
> >               vma_end =3D next->vm_end;
> >               err =3D dup_anon_vma(prev, next);
> >               if (curr) {                             /* case 6 */
> > +                     vma_start_write(curr);
> >                       remove =3D curr;
> >                       remove2 =3D next;
> >                       if (!next->anon_vma)
> > @@ -958,6 +954,7 @@ struct vm_area_struct *vma_merge(struct vma_iterato=
r *vmi, struct mm_struct *mm,
> >       } else if (merge_prev) {                        /* case 2 */
> >               if (curr) {
> >                       err =3D dup_anon_vma(prev, curr);
> > +                     vma_start_write(curr);
> >                       if (end =3D=3D curr->vm_end) {      /* case 7 */
> >                               remove =3D curr;
> >                       } else {                        /* case 5 */
> > @@ -969,6 +966,7 @@ struct vm_area_struct *vma_merge(struct vma_iterato=
r *vmi, struct mm_struct *mm,
> >               res =3D next;
> >               if (prev && addr < prev->vm_end) {      /* case 4 */
> >                       vma_end =3D addr;
> > +                     vma_start_write(next);
> >                       adjust =3D next;
> >                       adj_start =3D -(prev->vm_end - addr);
> >                       err =3D dup_anon_vma(next, prev);
> > @@ -983,6 +981,7 @@ struct vm_area_struct *vma_merge(struct vma_iterato=
r *vmi, struct mm_struct *mm,
> >                       vma_pgoff =3D next->vm_pgoff - pglen;
> >                       if (curr) {                     /* case 8 */
> >                               vma_pgoff =3D curr->vm_pgoff;
> > +                             vma_start_write(curr);
> >                               remove =3D curr;
> >                               err =3D dup_anon_vma(next, curr);
> >                       }
> > @@ -996,6 +995,8 @@ struct vm_area_struct *vma_merge(struct vma_iterato=
r *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 !=3D 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 =3D 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);
> > --
> > 2.41.0.585.gd2178a4bd4-goog
> >