Re: [PATCH v3 17/35] mm/mmap: write-lock VMA before shrinking or expanding it

[Suren Baghdasaryan]

On Thu, Feb 23, 2023 at 12:28 PM Liam R. Howlett
<Liam.Howlett@xxxxxxxxxx> wrote:
>
>
> Wait, I figured a better place to do this.
>
> init_multi_vma_prep() should vma_start_write() on any VMA that is passed
> in.. that we we catch any modifications here & in vma_merge(), which I
> think is missed in this patch set?

Hmm. That looks like a good idea but in that case, why not do the
locking inside vma_prepare() itself? From the description of that
function it sounds like it was designed to acquire locks before VMA
modifications, so would be the ideal location for doing that. WDYT?
The only concern is vma_adjust_trans_huge() being called before
vma_prepare() but I *think* that's safe because
vma_adjust_trans_huge() does its modifications after acquiring PTL
lock, which page fault handlers also have to take. Does that sound
right?

>
>
> * Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [230223 15:20]:
> > Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> >
> > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230216 00:18]:
> > > vma_expand and vma_shrink change VMA boundaries. Expansion might also
> > > result in freeing of an adjacent VMA. Write-lock affected VMAs to prevent
> > > concurrent page faults.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > ---
> > >  mm/mmap.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index ec2f8d0af280..f079e5bbcd57 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -674,6 +674,9 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >             ret = dup_anon_vma(vma, next);
> > >             if (ret)
> > >                     return ret;
> > > +
> > > +           /* Lock the VMA  before removing it */
> > > +           vma_start_write(next);
> > >     }
> > >
> > >     init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NULL);
> > > @@ -686,6 +689,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >     if (vma_iter_prealloc(vmi))
> > >             goto nomem;
> > >
> > > +   vma_start_write(vma);
> > >     vma_adjust_trans_huge(vma, start, end, 0);
> > >     /* VMA iterator points to previous, so set to start if necessary */
> > >     if (vma_iter_addr(vmi) != start)
> > > @@ -725,6 +729,7 @@ 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_adjust_trans_huge(vma, start, end, 0);
> > >     vma_prepare(&vp);
> > > --
> > > 2.39.1
> > >
>
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