From: David Howells <dhowells@redhat.com>
Date: Fri, 7 Jan 2022 09:28:41 +0000
Subject: fscache: Add a comment explaining how page-release optimisation works
Git-commit: c522e3ad296b7b692ed3960dfde467f2a34b434f
Patch-mainline: v5.17-rc1
References: jsc#SES-1880
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
Acked-by: Luis Henriques <lhenriques@suse.com>
---
include/linux/fscache.h | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index ede50406bcb0..296c5f1d9f35 100644
--- a/include/linux/fscache.h
+++ b/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))