[PATCH v1 07/10] bcache: set error_limit correctly

Coly Li
2018-01-03T14:04:26Z

Struct cache uses io_errors for two purposes,
- Error decay: when cache set error_decay is set, io_errors is used to
generate a small piece of delay when I/O error happens.
- I/O errors counter: in order to generate big enough value for error
decay, I/O errors counter value is stored by left shifting 20 bits (a.k.a
IO_ERROR_SHIFT).

In function bch_count_io_errors(), if I/O errors counter reaches cache set
error limit, bch_cache_set_error() will be called to retire the whold cache
set. But current code is problematic when checking the error limit, see the
following code piece from bch_count_io_errors(),

90 if (error) {
91 char buf[BDEVNAME_SIZE];
92 unsigned errors = atomic_add_return(1 << IO_ERROR_SHIFT,
93 &ca->io_errors);
94 errors >>= IO_ERROR_SHIFT;
95
96 if (errors < ca->set->error_limit)
97 pr_err("%s: IO error on %s, recovering",
98 bdevname(ca->bdev, buf), m);
99 else
100 bch_cache_set_error(ca->set,
101 "%s: too many IO errors %s",
102 bdevname(ca->bdev, buf), m);
103 }

At line 94, errors is right shifting IO_ERROR_SHIFT bits, now it is real
errors counter to compare at line 96. But ca->set->error_limit is initia-
lized with an amplified value in bch_cache_set_alloc(),
1545 c->error_limit = 8 << IO_ERROR_SHIFT;

It means by default, in bch_count_io_errors(), before 8<<20 errors happened
bch_cache_set_error() won't be called to retire the problematic cache
device. If the average request size is 64KB, it means bcache won't handle
failed device until 512GB data is requested. This is too large to be an I/O
threashold. So I believe the correct error limit should be much less.

This patch sets default cache set error limit to 8, then in
bch_count_io_errors() when errors counter reaches 8 (if it is default
value), function bch_cache_set_error() will be called to retire the whole
cache set. This patch also removes bits shifting when store or show
io_error_limit value via sysfs interface.

Nowadays most of SSDs handle internal flash failure automatically by LBA
address re-indirect mapping. If an I/O error can be observed by upper layer
code, it will be a notable error because that SSD can not re-indirect
map the problematic LBA address to an available flash block. This situation
indicates the whole SSD will be failed very soon. Therefore setting 8 as
the default io error limit value makes sense, it is enough for most of
cache devices.

Signed-off-by: Coly Li <colyli@suse.de>
---
drivers/md/bcache/bcache.h | 1 +
drivers/md/bcache/super.c | 2 +-
drivers/md/bcache/sysfs.c | 4 ++--
3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 395b87942a2f..a31dc3737dae 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -654,6 +654,7 @@ struct cache_set {
ON_ERROR_UNREGISTER,
ON_ERROR_PANIC,
} on_error;
+#define DEFAULT_IO_ERROR_LIMIT 8
unsigned error_limit;
unsigned error_decay;

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 8912be4165c5..02d9d7110769 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1561,7 +1561,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)

c->congested_read_threshold_us = 2000;
c->congested_write_threshold_us = 20000;
- c->error_limit = 8 << IO_ERROR_SHIFT;
+ c->error_limit = DEFAULT_IO_ERROR_LIMIT;

return c;
err:
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index b4184092c727..d7ce9a05b304 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -556,7 +556,7 @@ SHOW(__bch_cache_set)

/* See count_io_errors for why 88 */
sysfs_print(io_error_halflife, c->error_decay * 88);
- sysfs_print(io_error_limit, c->error_limit >> IO_ERROR_SHIFT);
+ sysfs_print(io_error_limit, c->error_limit);

sysfs_hprint(congested,
((uint64_t) bch_get_congested(c)) << 9);
@@ -656,7 +656,7 @@ STORE(__bch_cache_set)
}

if (attr == &sysfs_io_error_limit)
- c->error_limit = strtoul_or_return(buf) << IO_ERROR_SHIFT;
+ c->error_limit = strtoul_or_return(buf);

/* See count_io_errors() for why 88 */
if (attr == &sysfs_io_error_halflife)
--
2.15.1