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

[David Hildenbrand]

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.

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

--
Thanks,

David / dhildenb