On 2017/7/12 上午8:34, tang.junhui@zte.com.cn wrote:
quoted
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 <redacted>
Signed-off-by: Coly Li <redacted>
---
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)
quoted
{
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);
quoted
+ 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);
--
No, Poly,
Cache mode can change dynamically,
So maybe the bcache device is still dirty
when the cache mode is changed to CACHE_MODE_WRITETHROUGH,
so I think this patch can not solve this question fundamentally.
Hi Junnhui,
Yes I agree with you, if cache mode is switched from writeback to
writethrough or writearound mode, and then the cache device is
disconnected, this patch does not help us.
When I replied question from Kai Krakow, I expressed similar opinion as
yours. This patch is a best-effort fix, it is very necessary for data
base applications which always use writeback mode and not switch to
other mode during all their online time.
Thanks.
Coly