mm: merge folio_has_private()/filemap_release_folio() call pairs

mm: merge folio_has_private()/filemap_release_folio() call pairs

Patch series "mm, netfs, fscache: Stop read optimisation when folio
removed from pagecache", v7.

This fixes an optimisation in fscache whereby we don't read from the cache
for a particular file until we know that there's data there that we don't
have in the pagecache.  The problem is that I'm no longer using PG_fscache
(aka PG_private_2) to indicate that the page is cached and so I don't get
a notification when a cached page is dropped from the pagecache.

The first patch merges some folio_has_private() and
filemap_release_folio() pairs and introduces a helper,
folio_needs_release(), to indicate if a release is required.

The second patch is the actual fix.  Following Willy's suggestions[1], it
adds an AS_RELEASE_ALWAYS flag to an address_space that will make
filemap_release_folio() always call ->release_folio(), even if
PG_private/PG_private_2 aren't set.  folio_needs_release() is altered to
add a check for this.

This patch (of 2):

Make filemap_release_folio() check folio_has_private().  Then, in most
cases, where a call to folio_has_private() is immediately followed by a
call to filemap_release_folio(), we can get rid of the test in the pair.

There are a couple of sites in mm/vscan.c that this can't so easily be
done.  In shrink_folio_list(), there are actually three cases (something
different is done for incompletely invalidated buffers), but
filemap_release_folio() elides two of them.

In shrink_active_list(), we don't have have the folio lock yet, so the
check allows us to avoid locking the page unnecessarily.

A wrapper function to check if a folio needs release is provided for those
places that still need to do it in the mm/ directory.  This will acquire
additional parts to the condition in a future patch.

After this, the only remaining caller of folio_has_private() outside of
mm/ is a check in fuse.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Reported-by: Rohith Surabattula <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
Signed-off-by: David Howells <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Steve French <[email protected]>
Cc: Shyam Prasad N <[email protected]>
Cc: Rohith Surabattula <[email protected]>
Cc: Dave Wysochanski <[email protected]>
Cc: Dominique Martinet <[email protected]>
Cc: Ilya Dryomov <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: Xiubo Li <[email protected]>
Cc: Jingbo Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>