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

Kai Krakow
2017-07-10T21:46:39Z

Am Mon, 10 Jul 2017 19:18:28 +0800
schrieb 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.

Shouldn't write-around mode be okay for the same reason, i.e. there's
no stale version on disk?

So the patch should read "mode != CACHE_MODE_WRITEBACK" (not sure about
the constant), that would also match your textual description.

> 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);
>

--
Regards,
Kai

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