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]>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 93e495d2d477..dd022b065614 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4073,6 +4073,8 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
 	struct address_space * const mapping = folio->mapping;
 
 	BUG_ON(!folio_test_locked(folio));
+	if (!folio_needs_release(folio))
+		return true;
 	if (folio_test_writeback(folio))
 		return false;