Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes]

On Tue, May 02, 2023 at 07:22:32PM +0200, Peter Zijlstra wrote:
> On Tue, May 02, 2023 at 07:13:49PM +0200, David Hildenbrand wrote:
> > [...]
> >
> > > +{
> > > +	struct address_space *mapping;
> > > +
> > > +	/*
> > > +	 * GUP-fast disables IRQs - this prevents IPIs from causing page tables
> > > +	 * to disappear from under us, as well as preventing RCU grace periods
> > > +	 * from making progress (i.e. implying rcu_read_lock()).
> > > +	 *
> > > +	 * This means we can rely on the folio remaining stable for all
> > > +	 * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
> > > +	 * and those that do not.
> > > +	 *
> > > +	 * We get the added benefit that given inodes, and thus address_space,
> > > +	 * objects are RCU freed, we can rely on the mapping remaining stable
> > > +	 * here with no risk of a truncation or similar race.
> > > +	 */
> > > +	lockdep_assert_irqs_disabled();
> > > +
> > > +	/*
> > > +	 * If no mapping can be found, this implies an anonymous or otherwise
> > > +	 * non-file backed folio so in this instance we permit the pin.
> > > +	 *
> > > +	 * shmem and hugetlb mappings do not require dirty-tracking so we
> > > +	 * explicitly whitelist these.
> > > +	 *
> > > +	 * Other non dirty-tracked folios will be picked up on the slow path.
> > > +	 */
> > > +	mapping = folio_mapping(folio);
> > > +	return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio);
> >
> > "Folios in the swap cache return the swap mapping" -- you might disallow
> > pinning anonymous pages that are in the swap cache.
> >
> > I recall that there are corner cases where we can end up with an anon page
> > that's mapped writable but still in the swap cache ... so you'd fallback to
> > the GUP slow path (acceptable for these corner cases, I guess), however
> > especially the comment is a bit misleading then.
> >
> > So I'd suggest not dropping the folio_test_anon() check, or open-coding it
> > ... which will make this piece of code most certainly easier to get when
> > staring at folio_mapping(). Or to spell it out in the comment (usually I
> > prefer code over comments).
>
> So how stable is folio->mapping at this point? Can two subsequent reads
> get different values? (eg. an actual mapping and NULL)
>
> If so, folio_mapping() itself seems to be missing a READ_ONCE() to avoid
> the compiler from emitting the load multiple times.

Yes that actually feels like a bit of a flaw in folio_mapping(). I suppose
we run the risk of mapping being reset (e.g. to NULL) even if any mapping
we get being guaranteed to be safe due to the RCU thing.

Based on David's feedback I think I'll recode to something more direct
where we READ_ONCE() the mapping, then check mapping flags for anon, avoid
the swap case + check shmem.