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

[Suren Baghdasaryan]

On Tue, Sep 6, 2022 at 1:22 PM Peter Xu <peterx at redhat.com> wrote:
>
> 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.

Got it. Thanks for the explanation. It definitely helps!

>
> Thanks,
>
> --
> Peter Xu
>
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