fscache: Add a comment explaining how page-release optimisation works

Add a comment into fscache_note_page_release() to explain how the
page-release optimisation logic works[1].  It's not entirely obvious as it
has nothing to do with whether or not the netfs file contains data.

FSCACHE_COOKIE_NO_DATA_TO_READ is set if we have no data in the cache yet
(ie. the backing file lookup was negative, the file is 0 length or the
cookie got invalidated).  It means that we have no data in the cache, not
that the file is necessarily empty on the server.

FSCACHE_COOKIE_HAVE_DATA is set once we've stored data in the backing file.
From that point on, we have data we *could* read - however, it's covered by
pages in the netfs pagecache until at such time one of those covering pages
is released.

So if we've written data to the cache (HAVE_DATA) and there wasn't any data
in the cache when we started (NO_DATA_TO_READ), it may no longer be true
that we can skip reading from the cache.

Read skipping is done by cachefiles_prepare_read().

Note that tracking is not done on a per-page basis, but only on a per-file
basis.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/043a206f03929c2667a465314144e518070a9b2d.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251408479.3435901.9540165422908194636.stgit@warthog.procyon.org.uk/ # v1

include/linux/fscache.h

@@ -665,6 +665,11 @@ static inline void fscache_clear_inode_writeback(struct fscache_cookie *cookie,
 static inline
 void fscache_note_page_release(struct fscache_cookie *cookie)
 {
+	/* If we've written data to the cache (HAVE_DATA) and there wasn't any
+	 * data in the cache when we started (NO_DATA_TO_READ), it may no
+	 * longer be true that we can skip reading from the cache - so clear
+	 * the flag that causes reads to be skipped.
+	 */
 	if (cookie &&
 	    test_bit(FSCACHE_COOKIE_HAVE_DATA, &cookie->flags) &&
 	    test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))