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

[David Howells <dhowells@redhat.com>]

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