[RFC PATCH RESEND 19/28] mm: disallow do_swap_page to handle page faults under VMA lock

[Peter Xu]

On Tue, Sep 06, 2022 at 01:08:10PM -0700, Suren Baghdasaryan wrote:
> On Tue, Sep 6, 2022 at 12:39 PM Peter Xu <peterx at redhat.com> wrote:
> >
> > On Thu, Sep 01, 2022 at 10:35:07AM -0700, Suren Baghdasaryan wrote:
> > > Due to the possibility of do_swap_page dropping mmap_lock, abort fault
> > > handling under VMA lock and retry holding mmap_lock. This can be handled
> > > more gracefully in the future.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> > > ---
> > >  mm/memory.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 9ac9944e8c62..29d2f49f922a 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3738,6 +3738,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > >       vm_fault_t ret = 0;
> > >       void *shadow = NULL;
> > >
> > > +     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > > +             ret = VM_FAULT_RETRY;
> > > +             goto out;
> > > +     }
> > > +
> >
> > May want to fail early similarly for handle_userfault() too for similar
> > reason.  Thanks,
> 
> I wasn't aware of a similar issue there. Will have a closer look. Thanks!

Sure.

Just in case this would be anything helpful - handle_userfault() will both
assert at the entry (mmap_assert_locked) and will in most cases release
read lock along the way when waiting for page fault resolutions.

And userfaultfd should work on anonymous memory for either missing mode or
write protect mode.

Thanks,

-- 
Peter Xu