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

[Suren Baghdasaryan]

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.

>
> 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.

>
> 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. 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?

>
>
> > -             else if (prev && (prev->vm_flags & VM_GROWSUP))
> > +             } else if (prev && (prev->vm_flags & VM_GROWSUP)) {
> > +                     vma_start_write(prev);
> >                       downgrade = false;
> > -             else
> > +             } else
> >                       mmap_write_downgrade(mm);
> >       }
> >
> > --
> > 2.39.1
>
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