Patch "bcache: only permit to recovery read error when cache device is clean" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

bcache: only permit to recovery read error when cache device is clean

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bcache-only-permit-to-recovery-read-error-when-cache-device-is-clean.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Coly Li <[email protected]>
Date: Mon, 30 Oct 2017 14:46:31 -0700
Subject: bcache: only permit to recovery read error when cache device is clean

From: Coly Li <[email protected]>

commit d59b23795933678c9638fd20c942d2b4f3cd6185 upstream.

When bcache does read I/Os, for example in writeback or writethrough mode, if a read request on cache device is failed, bcache will try to recovery the request by reading from cached device. If the data on cached device is not synced with cache device, then requester will get a stale data.

For critical storage system like database, providing stale data from recovery may result an application level data corruption, which is unacceptible.

With this patch, for a failed read request in writeback or writethrough mode, recovery a recoverable read request only happens when cache device is clean. That is to say, all data on cached device is up to update.

For other cache modes in bcache, read request will never hit cached_dev_read_error(), they don't need this patch.

Please note, because cache mode can be switched arbitrarily in run time, a writethrough mode might be switched from a writeback mode. Therefore checking dc->has_data in writethrough mode still makes sense.

Changelog:
V4: Fix parens error pointed by Michael Lyle.
v3: By response from Kent Oversteet, he thinks recovering stale data is a
bug to fix, and option to permit it is unnecessary. So this version the sysfs file is removed.
v2: rename sysfs entry from allow_stale_data_on_failure to
allow_stale_data_on_failure, and fix the confusing commit log.
v1: initial patch posted.

[small change to patch comment spelling by mlyle]

---
 drivers/md/bcache/request.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

@@ -705,8 +705,16 @@ static void cached_dev_read_error(struct
 {
 	struct search *s = container_of(cl, struct search, cl);
 	struct bio *bio = &s->bio.bio;
+	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);
 
-	if (s->recoverable) {
+	/*
+	 * If cache device is dirty (dc->has_dirty is non-zero), then
+	 * recovery a failed read request from cached device may get a
+	 * stale data back. So read failure recovery is only permitted
+	 * when cache device is clean.
+	 */
+	if (s->recoverable &&
+	    (dc && !atomic_read(&dc->has_dirty))) {
 		/* Retry from the backing device: */
 		trace_bcache_read_retry(s->orig_bio);
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/bcache-only-permit-to-recovery-read-error-when-cache-device-is-clean.patch queue-3.18/bcache-recover-data-from-backing-when-data-is-clean.patch