Re: [RFC PATCH v2] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache

David Howells
2022-11-15T09:42:00Z

Matthew Wilcox <willy@infradead.org> wrote:

> On Mon, Nov 14, 2022 at 04:02:20PM +0000, David Howells wrote:
> > +++ b/mm/filemap.c
> > @@ -3941,6 +3941,10 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
> > struct address_space * const mapping = folio->mapping;
> >
> > BUG_ON(!folio_test_locked(folio));
> > + if ((!mapping || !mapping_release_always(mapping))
> > + && !folio_test_private(folio) &&
> > + !folio_test_private_2(folio))
> > + return true;
>
> Why do you need to test 'mapping' here?

Why does the function do:

if (mapping && mapping->a_ops->release_folio)

later then? There are callers of the function, such as shrink_folio_list(),
that seem to think that folio->mapping might be NULL.

> Also this is the most inconsistent style ...

Yeah, I accidentally pushed the '&&' onto the next line.

> > @@ -276,7 +275,7 @@ static long mapping_evict_folio(struct address_space *mapping,
> > if (folio_ref_count(folio) >
> > folio_nr_pages(folio) + folio_has_private(folio) + 1)
>
> I think this line is incorrect, right? You don't increment the folio
> refcount just because the folio has private2 set, do you?

Errr, yes:

static inline void folio_start_fscache(struct folio *folio)
{
VM_BUG_ON_FOLIO(folio_test_private_2(folio), folio);
folio_get(folio);
folio_set_private_2(folio);
}

Someone insisted - might even have been you;-)

I'm working on getting rid of the use of PG_private_2 from the network
filesystems, but it's still in progress. Kind of blocked on the iov_iter
stuff.

> > return 0;
> > - if (folio_has_private(folio) && !filemap_release_folio(folio, 0))
> > + if (!filemap_release_folio(folio, 0))
> > return 0;
> >
> > return remove_mapping(mapping, folio);
>
> Can we get rid of folio_has_private()

That would be nice, but there are still places that check it, and until we get
rid of the use of PG_private_2, we can't reduce it to just a check on
PG_private. Truncate, for example, checks it to see if it should can
->invalidate_folio().

It's only used in mm/, so it could be moved into mm/internal.h.

> / page_has_private() now?

That's used in some a number of places outside of mm/. The arch/s390/ usage
is just to calculate the expected refcount. I wonder if calculation of the
expected refcount could be potted into a function as it's performed in a
number of places - though the expectation isn't always the same.

Ext3 and fuse both use it - but those probably need to check PG_private_2 and
could use a "folio_test_private()" function when fully foliated.

David