Re: [PATCH v2 3/3] userfaultfd: use per-vma locks in userfaultfd operations

[Liam R. Howlett]

* Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240205 16:55]:
..

> > > > We can take care of anon_vma as well here right? I can take a bool
> > > > parameter ('prepare_anon' or something) and then:
> > > >
> > > >            if (vma) {
> > > >                     if (prepare_anon && vma_is_anonymous(vma)) &&
> > > > !anon_vma_prepare(vma)) {
> > > >                                       vma = ERR_PTR(-ENOMEM);
> > > >                                       goto out_unlock;
> > > >                    }
> > > > >                 vma_aquire_read_lock(vma);
> > > >            }
> > > > out_unlock:
> > > > >         mmap_read_unlock(mm);
> > > > >         return vma;
> > > > > }
> > >
> > > Do you need this?  I didn't think this was happening in the code as
> > > written?  If you need it I would suggest making it happen always and
> > > ditch the flag until a user needs this variant, but document what's
> > > going on in here or even have a better name.
> >
> > I think yes, you do need this. I can see calls to anon_vma_prepare()
> > under mmap_read_lock() protection in both mfill_atomic_hugetlb() and
> > in mfill_atomic(). This means, just like in the pagefault path, we
> > modify vma->anon_vma under mmap_read_lock protection which guarantees
> > that adjacent VMAs won't change. This is important because
> > __anon_vma_prepare() uses find_mergeable_anon_vma() that needs the
> > neighboring VMAs to be stable. Per-VMA lock guarantees stability of
> > the VMA we locked but not of its neighbors, therefore holding per-VMA
> > lock while calling anon_vma_prepare() is not enough. The solution
> > Lokesh suggests would call anon_vma_prepare() under mmap_read_lock and
> > therefore would avoid the issue.
> >

..

> anon_vma_prepare() is also called in validate_move_areas() via move_pages().

Probably worth doing it unconditionally and have a comment as to why it
is necessary.

Does this avoid your locking workaround?

Thanks,
Liam