Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes]

On Tue, May 02, 2023 at 02:47:22PM +0200, David Hildenbrand wrote:
> On 02.05.23 14:40, Peter Zijlstra wrote:
> > On Tue, May 02, 2023 at 02:08:10PM +0200, Peter Zijlstra wrote:
> >
> > > > >
> > > > >
> > > > > 	if (folio_test_anon(folio))
> > > > > 		return true;
> > > >
> > > > This relies on the mapping so belongs below the lockdep assert imo.
> > >
> > > Oh, right you are.
> > >
> > > > >
> > > > > 	/*
> > > > > 	 * Having IRQs disabled (as per GUP-fast) also inhibits RCU
> > > > > 	 * grace periods from making progress, IOW. they imply
> > > > > 	 * rcu_read_lock().
> > > > > 	 */
> > > > > 	lockdep_assert_irqs_disabled();
> > > > >
> > > > > 	/*
> > > > > 	 * Inodes and thus address_space are RCU freed and thus safe to
> > > > > 	 * access at this point.
> > > > > 	 */
> > > > > 	mapping = folio_mapping(folio);
> > > > > 	if (mapping && shmem_mapping(mapping))
> > > > > 		return true;
> > > > >
> > > > > 	return false;
> > > > >
> > > > > > +}
> >
> > So arguably you should do *one* READ_ONCE() load of mapping and
> > consistently use that, this means open-coding both folio_test_anon() and
> > folio_mapping().
>
> Open-coding folio_test_anon() should not be required. We only care about
> PAGE_MAPPING_FLAGS stored alongside folio->mapping, that will stick around
> until the anon page was freed.
>

Ack, good point!

> @Lorenzo, you might also want to special-case hugetlb directly using
> folio_test_hugetlb().
>

I already am :) I guess you mean when I respin along these lines? Will port
that across to.

> --
> Thanks,
>
> David / dhildenb
>