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 <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]/ [1]
Link: https://lore.kernel.org/r/[e-mail]/ # v1

include/linux/fscache.h

···
 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))