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

[David Hildenbrand]

On 02.05.23 19:22, 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.

I can only talk about anon pages in this specific call order here (check 
first, then test if the PTE changed in the meantime): we don't care if 
we get two different values. If we get a different value the second 
time, surely we (temporarily) pinned an anon page that is no longer 
mapped (freed in the meantime). But in that case (even if we read 
garbage folio->mapping and made the wrong call here), we'll detect 
afterwards that the PTE changed, and unpin what we (temporarily) pinned. 
As folio_test_anon() only checks two bits in folio->mapping it's fine, 
because we won't dereference garbage folio->mapping.

With folio_mapping() on !anon and READ_ONCE() ... good question. Kirill 
said it would be fairly stable, but I suspect that it could change 
(especially if we call it before validating if the PTE changed as I 
described further below).

Now, if we read folio->mapping after checking if the page we pinned is 
still mapped (PTE unchanged), at least the page we pinned cannot be 
reused in the meantime. I suspect that we can still read "NULL" on the 
second read. But whatever we dereference from the first read should 
still be valid, even if the second read would have returned NULL ("rcu 
freeing").

--
Thanks,

David / dhildenb