Re: [PATCH v3 21/35] mm/mmap: write-lock adjacent VMAs if they can grow into unmapped area

[Liam R. Howlett]

* Suren Baghdasaryan <surenb@xxxxxxxxxx> [230216 14:36]:
> On Thu, Feb 16, 2023 at 7:34 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
> >
> >
> > First, sorry I didn't see this before v3..
> 
> Feedback at any time is highly appreciated!
> 
> >
> > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230216 00:18]:
> > > While unmapping VMAs, adjacent VMAs might be able to grow into the area
> > > being unmapped. In such cases write-lock adjacent VMAs to prevent this
> > > growth.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > ---
> > >  mm/mmap.c | 8 +++++---
> > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 118b2246bba9..00f8c5798936 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -2399,11 +2399,13 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >        * down_read(mmap_lock) and collide with the VMA we are about to unmap.
> > >        */
> > >       if (downgrade) {
> > > -             if (next && (next->vm_flags & VM_GROWSDOWN))
> > > +             if (next && (next->vm_flags & VM_GROWSDOWN)) {
> > > +                     vma_start_write(next);
> > >                       downgrade = false;
> >
> > If the mmap write lock is insufficient to protect us from next/prev
> > modifications then we need to move *most* of this block above the maple
> > tree write operation, otherwise we have a race here.  When I say most, I
> > mean everything besides the call to mmap_write_downgrade() needs to be
> > moved.
> 
> Which prior maple tree write operation are you referring to? I see
> __split_vma() and munmap_sidetree() which both already lock the VMAs
> they operate on, so page faults can't happen in those VMAs.

The write that removes the VMAs from the maple tree a few lines above..
/* Point of no return */

If the mmap lock is not sufficient, then we need to move the
vma_start_write() of prev/next to above the call to
vma_iter_clear_gfp() in do_vmi_align_munmap().

But I still think it IS enough.

> 
> >
> > If the mmap write lock is sufficient to protect us from next/prev
> > modifications then we don't need to write lock the vmas themselves.
> 
> mmap write lock is not sufficient because with per-VMA locks we do not
> take mmap lock at all.

Understood, but it also does not expand VMAs.

> 
> >
> > I believe this is for expand_stack() protection, so I believe it's okay
> > to not vma write lock these vmas.. I don't think there are other areas
> > where we can modify the vmas without holding the mmap lock, but others
> > on the CC list please chime in if I've forgotten something.
> >
> > So, if I am correct, then you shouldn't lock next/prev and allow the
> > vma locking fault method on these vmas.  This will work because
> > lock_vma_under_rcu() uses mas_walk() on the faulting address.  That is,
> > your lock_vma_under_rcu() will fail to find anything that needs to be
> > grown and go back to mmap lock protection.  As it is written today, the
> > vma locking fault handler will fail and we will wait for the mmap lock
> > to be released even when the vma isn't going to expand.
> 
> So, let's consider a case when the next VMA is not being removed (so
> it was neither removed nor locked by munmap_sidetree()) and it is
> found by lock_vma_under_rcu() in the page fault handling path.

By this point next VMA is either NULL or outside the munmap area, so
what you said here is always true.

>Page
> fault handler can now expand it and push into the area we are
> unmapping in unmap_region(). That is the race I'm trying to prevent
> here by locking the next/prev VMAs which can be expanded before
> unmap_region() unmaps them. Am I missing something?

Yes, I think the part you are missing (or I am missing..) is that
expand_stack() will never be called without the mmap lock.  We don't use
the vma locking to expand the stack.

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