bcache: ignore read-ahead request failure on backing device

When md raid device (e.g. raid456) is used as backing device, read-ahead
requests on a degrading and recovering md raid device might be failured
immediately by md raid code, but indeed this md raid array can still be
read or write for normal I/O requests. Therefore such failed read-ahead
request are not real hardware failure. Further more, after degrading and
recovering accomplished, read-ahead requests will be handled by md raid
array again.

For such condition, I/O failures of read-ahead requests don't indicate
real health status (because normal I/O still be served), they should not
be counted into I/O error counter dc->io_errors.

Since there is no simple way to detect whether the backing divice is a
md raid device, this patch simply ignores I/O failures for read-ahead
bios on backing device, to avoid bogus backing device failure on a
degrading md raid array.

Suggested-and-tested-by: Thorsten Knabe <linux@thorsten-knabe.de>
Signed-off-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
index c250979683194c69f8ea1b9585e13907c877eb3a..4d93f07f63e515c23e0792331d87f3c2cea1c485 100644 (file)
--- a/drivers/md/bcache/io.c
+++ b/drivers/md/bcache/io.c
@@ -58,6 +58,18 @@ void bch_count_backing_io_errors(struct cached_dev *dc, struct bio *bio)
 
        WARN_ONCE(!dc, "NULL pointer of struct cached_dev");
 
+       /*
+        * Read-ahead requests on a degrading and recovering md raid
+        * (e.g. raid6) device might be failured immediately by md
+        * raid code, which is not a real hardware media failure. So
+        * we shouldn't count failed REQ_RAHEAD bio to dc->io_errors.
+        */
+       if (bio->bi_opf & REQ_RAHEAD) {
+               pr_warn_ratelimited("%s: Read-ahead I/O failed on backing device, ignore",
+                                   dc->backing_dev_name);
+               return;
+       }
+
        errors = atomic_add_return(1, &dc->io_errors);
        if (errors < dc->error_limit)
                pr_err("%s: IO error on backing device, unrecoverable",