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authorDavid Howells <[email protected]>2022-01-07 09:28:41 +0000
committerDavid Howells <[email protected]>2022-01-21 21:36:28 +0000
commitc522e3ad296b7b692ed3960dfde467f2a34b434f (patch)
tree62d967b182c1daf2a36b3b396b9e0073ee535603
parent6633213139d827fb9abf9a9a280f3d9e89fc7091 (diff)
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 <[email protected]> Reviewed-by: Jeff Layton <[email protected]> cc: [email protected] Link: https://lore.kernel.org/r/[email protected]/ [1] Link: https://lore.kernel.org/r/164251408479.3435901.9540165422908194636.stgit@warthog.procyon.org.uk/ # v1
-rw-r--r--include/linux/fscache.h5
1 files changed, 5 insertions, 0 deletions
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))