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

[Suren Baghdasaryan]

On Thu, Feb 23, 2023 at 5:46 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230223 16:16]:
> > 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?
>
> That might be even better.  I think it will result in even less code.

Yes.

>
> There is also a vma_complete() which might work to call
> vma_end_write_all() as well?

If there are other VMAs already locked before vma_prepare() then we
would unlock them too. Safer to just let mmap_unlock do
vma_end_write_all().

>
> > 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?
>
> I am not sure.  We are certainly safe the way it is, and the PTL has to
> be safe for concurrent faults.. but this could alter the walk to a page
> table while that walk is occurring and I don't think that happens today.
>
> It might be best to leave the locking order the way you have it, unless
> someone can tell us it's safe?

Yes, I have the same feelings about changing this.

>
> We could pass through the three extra variables that are needed to move
> the vma_adjust_trans_huge() call within that function as well?  This
> would have the added benefit of having all locking grouped in the one
> location, but the argument list would be getting long, however we could
> use the struct.

Any issues if I change the order to have vma_prepare() called always
before vma_adjust_trans_huge()? That way the VMA will always be locked
before vma_adjust_trans_huge() executes and we don't need any
additional arguments.

>
> remove & remove2 should be be detached in vma_prepare() or
> vma_complete() as well?

They are marked detached in vma_complete() (see
https://lore.kernel.org/all/20230216051750.3125598-25-surenb@xxxxxxxxxx/)
and that should be enough. We should be safe as long as we mark them
detached before unlocking the VMA.

>
> >
> > >
> > >
> > > * 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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> > >
>
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