[PATCH] bcache: only recovery I/O error for writethrough mode

[Coly Li <colyli@suse.de>]

If a read bio to cache device gets failed, bcache will try to recovery it
by forward the read bio to backing device. If backing device responses
read request successfully then the bio contains data from backing device
will be returned to uppper layer.

The recovery effort in cached_dev_read_error() is not correct, and there
is report that corrupted data may returned when a dirty cache device goes
offline during reading I/O.

For writeback cache mode, before dirty data are wrote back to backing
device, data blocks on backing device are not updated and consistent. If
a dirty cache device dropped and a read bio gets failed, bcache will
return its stale version from backing device. This is mistaken behavior
that applications don't expected, especially for data base workload.

This patch fixes the issue by only permit recoverable I/O when cached
device is in writethough mode, and s->recoverable is set. For other cache
mode, recovery I/O failure by reading backing device does not make sense,
bache just simply returns -EIO immediately.

Reported-by: Arne Wolf <awolf@lenovo.com>
Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/bcache/request.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 019b3df9f1c6..6edacac9b00d 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -702,8 +702,11 @@ static void cached_dev_read_error(struct closure *cl)
 {
 	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);
+	unsigned mode = cache_mode(dc, NULL);
 
-	if (s->recoverable) {
+	if (s->recoverable &&
+	    (mode == CACHE_MODE_WRITETHROUGH)) {
 		/* Retry from the backing device: */
 		trace_bcache_read_retry(s->orig_bio);
 
-- 
2.12.0