linux-kernel - [PATCH 08/11] fscache: Add a comment explaining how page-release optimisation works

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <164251408479.3435901.9540165422908194636.stgit@warthog.procyon.org.uk>
Date:   Tue, 18 Jan 2022 13:54:44 +0000
From:   David Howells <dhowells@...hat.com>
To:     linux-cachefs@...hat.com
Cc:     Jeff Layton <jlayton@...nel.org>, dhowells@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <smfrench@...il.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 08/11] 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@...hat.com>
Reviewed-by: Jeff Layton <jlayton@...nel.org>
cc: linux-cachefs@...hat.com
Link: https://lore.kernel.org/r/043a206f03929c2667a465314144e518070a9b2d.camel@kernel.org/ [1]
---

 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))