For read cache missing, bcache defines a readahead size for the read I/O
request to the backing device for the missing data. This readahead size
is initialized to 0, and almost no one uses it to avoid unnecessary read
amplifying onto backing device and write amplifying onto cache device.
Considering upper layer file system code has readahead logic allready
and works fine with readahead_cache_policy sysfile interface, we don't
have to keep bcache self-defined readahead anymore.
This patch removes the bcache self-defined readahead for cache missing
request for backing device, and the readahead sysfs file interfaces are
removed as well.
This is the preparation for next patch to fix potential kernel panic due
to oversized request in a simpler method.
Reported-by: Alexander Ullrich <[email protected]>
Reported-by: Diego Ercolani <[email protected]>
Reported-by: Jan Szubiak <[email protected]>
Reported-by: Marco Rebhan <[email protected]>
Reported-by: Matthias Ferdinand <[email protected]>
Reported-by: Thorsten Knabe <[email protected]>
Reported-by: Victor Westerhuis <[email protected]>
Reported-by: Vojtech Pavlik <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Cc: [email protected]
Cc: Christoph Hellwig <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Nix <[email protected]>
Cc: Takashi Iwai <[email protected]>
---
Changlog,
v1, the initial version by hint from Christoph Hellwig.
drivers/md/bcache/bcache.h | 1 -
drivers/md/bcache/request.c | 13 +------------
drivers/md/bcache/stats.c | 14 --------------
drivers/md/bcache/stats.h | 1 -
drivers/md/bcache/sysfs.c | 4 ----
5 files changed, 1 insertion(+), 32 deletions(-)
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 0a4551e165ab..5fc989a6d452 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -364,7 +364,6 @@ struct cached_dev {
/* The rest of this all shows up in sysfs */
unsigned int sequential_cutoff;
- unsigned int readahead;
unsigned int io_disable:1;
unsigned int verify:1;
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 29c231758293..ab8ff18df32a 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -880,7 +880,6 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
struct bio *bio, unsigned int sectors)
{
int ret = MAP_CONTINUE;
- unsigned int reada = 0;
struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);
struct bio *miss, *cache_bio;
@@ -892,14 +891,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
goto out_submit;
}
- if (!(bio->bi_opf & REQ_RAHEAD) &&
- !(bio->bi_opf & (REQ_META|REQ_PRIO)) &&
- s->iop.c->gc_stats.in_use < CUTOFF_CACHE_READA)
- reada = min_t(sector_t, dc->readahead >> 9,
- get_capacity(bio->bi_bdev->bd_disk) -
- bio_end_sector(bio));
-
- s->insert_bio_sectors = min(sectors, bio_sectors(bio) + reada);
+ s->insert_bio_sectors = min(sectors, bio_sectors(bio));
s->iop.replace_key = KEY(s->iop.inode,
bio->bi_iter.bi_sector + s->insert_bio_sectors,
@@ -933,9 +925,6 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
if (bch_bio_alloc_pages(cache_bio, __GFP_NOWARN|GFP_NOIO))
goto out_put;
- if (reada)
- bch_mark_cache_readahead(s->iop.c, s->d);
-
s->cache_miss = miss;
s->iop.bio = cache_bio;
bio_get(cache_bio);
diff --git a/drivers/md/bcache/stats.c b/drivers/md/bcache/stats.c
index 503aafe188dc..4c7ee5fedb9d 100644
--- a/drivers/md/bcache/stats.c
+++ b/drivers/md/bcache/stats.c
@@ -46,7 +46,6 @@ read_attribute(cache_misses);
read_attribute(cache_bypass_hits);
read_attribute(cache_bypass_misses);
read_attribute(cache_hit_ratio);
-read_attribute(cache_readaheads);
read_attribute(cache_miss_collisions);
read_attribute(bypassed);
@@ -64,7 +63,6 @@ SHOW(bch_stats)
DIV_SAFE(var(cache_hits) * 100,
var(cache_hits) + var(cache_misses)));
- var_print(cache_readaheads);
var_print(cache_miss_collisions);
sysfs_hprint(bypassed, var(sectors_bypassed) << 9);
#undef var
@@ -86,7 +84,6 @@ static struct attribute *bch_stats_files[] = {
&sysfs_cache_bypass_hits,
&sysfs_cache_bypass_misses,
&sysfs_cache_hit_ratio,
- &sysfs_cache_readaheads,
&sysfs_cache_miss_collisions,
&sysfs_bypassed,
NULL
@@ -113,7 +110,6 @@ void bch_cache_accounting_clear(struct cache_accounting *acc)
acc->total.cache_misses = 0;
acc->total.cache_bypass_hits = 0;
acc->total.cache_bypass_misses = 0;
- acc->total.cache_readaheads = 0;
acc->total.cache_miss_collisions = 0;
acc->total.sectors_bypassed = 0;
}
@@ -145,7 +141,6 @@ static void scale_stats(struct cache_stats *stats, unsigned long rescale_at)
scale_stat(&stats->cache_misses);
scale_stat(&stats->cache_bypass_hits);
scale_stat(&stats->cache_bypass_misses);
- scale_stat(&stats->cache_readaheads);
scale_stat(&stats->cache_miss_collisions);
scale_stat(&stats->sectors_bypassed);
}
@@ -168,7 +163,6 @@ static void scale_accounting(struct timer_list *t)
move_stat(cache_misses);
move_stat(cache_bypass_hits);
move_stat(cache_bypass_misses);
- move_stat(cache_readaheads);
move_stat(cache_miss_collisions);
move_stat(sectors_bypassed);
@@ -209,14 +203,6 @@ void bch_mark_cache_accounting(struct cache_set *c, struct bcache_device *d,
mark_cache_stats(&c->accounting.collector, hit, bypass);
}
-void bch_mark_cache_readahead(struct cache_set *c, struct bcache_device *d)
-{
- struct cached_dev *dc = container_of(d, struct cached_dev, disk);
-
- atomic_inc(&dc->accounting.collector.cache_readaheads);
- atomic_inc(&c->accounting.collector.cache_readaheads);
-}
-
void bch_mark_cache_miss_collision(struct cache_set *c, struct bcache_device *d)
{
struct cached_dev *dc = container_of(d, struct cached_dev, disk);
diff --git a/drivers/md/bcache/stats.h b/drivers/md/bcache/stats.h
index abfaabf7e7fc..ca4f435f7216 100644
--- a/drivers/md/bcache/stats.h
+++ b/drivers/md/bcache/stats.h
@@ -7,7 +7,6 @@ struct cache_stat_collector {
atomic_t cache_misses;
atomic_t cache_bypass_hits;
atomic_t cache_bypass_misses;
- atomic_t cache_readaheads;
atomic_t cache_miss_collisions;
atomic_t sectors_bypassed;
};
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index cc89f3156d1a..05ac1d6fbbf3 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -137,7 +137,6 @@ rw_attribute(io_disable);
rw_attribute(discard);
rw_attribute(running);
rw_attribute(label);
-rw_attribute(readahead);
rw_attribute(errors);
rw_attribute(io_error_limit);
rw_attribute(io_error_halflife);
@@ -260,7 +259,6 @@ SHOW(__bch_cached_dev)
var_printf(partial_stripes_expensive, "%u");
var_hprint(sequential_cutoff);
- var_hprint(readahead);
sysfs_print(running, atomic_read(&dc->running));
sysfs_print(state, states[BDEV_STATE(&dc->sb)]);
@@ -365,7 +363,6 @@ STORE(__cached_dev)
sysfs_strtoul_clamp(sequential_cutoff,
dc->sequential_cutoff,
0, UINT_MAX);
- d_strtoi_h(readahead);
if (attr == &sysfs_clear_stats)
bch_cache_accounting_clear(&dc->accounting);
@@ -538,7 +535,6 @@ static struct attribute *bch_cached_dev_files[] = {
&sysfs_running,
&sysfs_state,
&sysfs_label,
- &sysfs_readahead,
#ifdef CONFIG_BCACHE_DEBUG
&sysfs_verify,
&sysfs_bypass_torture_test,
--
2.26.2
In the cache missing code path of cached device, if a proper location
from the internal B+ tree is matched for a cache miss range, function
cached_dev_cache_miss() will be called in cache_lookup_fn() in the
following code block,
[code block 1]
526 unsigned int sectors = KEY_INODE(k) == s->iop.inode
527 ? min_t(uint64_t, INT_MAX,
528 KEY_START(k) - bio->bi_iter.bi_sector)
529 : INT_MAX;
530 int ret = s->d->cache_miss(b, s, bio, sectors);
Here s->d->cache_miss() is the call backfunction pointer initialized as
cached_dev_cache_miss(), the last parameter 'sectors' is an important
hint to calculate the size of read request to backing device of the
missing cache data.
Current calculation in above code block may generate oversized value of
'sectors', which consequently may trigger 2 different potential kernel
panics by BUG() or BUG_ON() as listed below,
1) BUG_ON() inside bch_btree_insert_key(),
[code block 2]
886 BUG_ON(b->ops->is_extents && !KEY_SIZE(k));
2) BUG() inside biovec_slab(),
[code block 3]
51 default:
52 BUG();
53 return NULL;
All the above panics are original from cached_dev_cache_miss() by the
oversized parameter 'sectors'.
Inside cached_dev_cache_miss(), parameter 'sectors' is used to calculate
the size of data read from backing device for the cache missing. This
size is stored in s->insert_bio_sectors by the following lines of code,
[code block 4]
909 s->insert_bio_sectors = min(sectors, bio_sectors(bio) + reada);
Then the actual key inserting to the internal B+ tree is generated and
stored in s->iop.replace_key by the following lines of code,
[code block 5]
911 s->iop.replace_key = KEY(s->iop.inode,
912 bio->bi_iter.bi_sector + s->insert_bio_sectors,
913 s->insert_bio_sectors);
The oversized parameter 'sectors' may trigger panic 1) by BUG_ON() from
the above code block.
And the bio sending to backing device for the missing data is allocated
with hint from s->insert_bio_sectors by the following lines of code,
[code block 6]
926 cache_bio = bio_alloc_bioset(GFP_NOWAIT,
927 DIV_ROUND_UP(s->insert_bio_sectors, PAGE_SECTORS),
928 &dc->disk.bio_split);
The oversized parameter 'sectors' may trigger panic 2) by BUG() from the
agove code block.
Now let me explain how the panics happen with the oversized 'sectors'.
In code block 5, replace_key is generated by macro KEY(). From the
definition of macro KEY(),
[code block 7]
71 #define KEY(inode, offset, size) \
72 ((struct bkey) { \
73 .high = (1ULL << 63) | ((__u64) (size) << 20) | (inode), \
74 .low = (offset) \
75 })
Here 'size' is 16bits width embedded in 64bits member 'high' of struct
bkey. But in code block 1, if "KEY_START(k) - bio->bi_iter.bi_sector" is
very probably to be larger than (1<<16) - 1, which makes the bkey size
calculation in code block 5 is overflowed. In one bug report the value
of parameter 'sectors' is 131072 (= 1 << 17), the overflowed 'sectors'
results the overflowed s->insert_bio_sectors in code block 4, then makes
size field of s->iop.replace_key to be 0 in code block 5. Then the 0-
sized s->iop.replace_key is inserted into the internal B+ tree as cache
missing check key (a special key to detect and avoid a racing between
normal write request and cache missing read request) as,
[code block 8]
915 ret = bch_btree_insert_check_key(b, &s->op, &s->iop.replace_key);
Then the 0-sized s->iop.replace_key as 3rd parameter triggers the bkey
size check BUG_ON() in code block 2, and causes the kernel panic 1).
Another kernel panic is from code block 6, is by the bvecs number
oversized value s->insert_bio_sectors from code block 4,
min(sectors, bio_sectors(bio) + reada)
There are two possibility for oversized reresult,
- bio_sectors(bio) is valid, but bio_sectors(bio) + reada is oversized.
- sectors < bio_sectors(bio) + reada, but sectors is oversized.
From a bug report the result of "DIV_ROUND_UP(s->insert_bio_sectors,
PAGE_SECTORS)" from code block 6 can be 344, 282, 946, 342 and many
other values which larther than BIO_MAX_VECS (a.k.a 256). When calling
bio_alloc_bioset() with such larger-than-256 value as the 2nd parameter,
this value will eventually be sent to biovec_slab() as parameter
'nr_vecs' in following code path,
bio_alloc_bioset() ==> bvec_alloc() ==> biovec_slab()
Because parameter 'nr_vecs' is larger-than-256 value, the panic by BUG()
in code block 3 is triggered inside biovec_slab().
From the above analysis, we know that the 4th parameter 'sector' sent
into cached_dev_cache_miss() may cause overflow in code block 5 and 6,
and finally cause kernel panic in code block 2 and 3. And if result of
bio_sectors(bio) + reada exceeds valid bvecs number, it may also trigger
kernel panic in code block 3 from code block 6.
Now the almost-useless readahead size for cache missing request back to
backing device is removed, this patch can fix the oversized issue with
more simpler method.
- add a local variable size_limit, set it by the minimum value from
the max bkey size and max bio bvecs number.
- set s->insert_bio_sectors by the minimum value from size_limit,
sectors, and the sectors size of bio.
- replace sectors by s->insert_bio_sectors to do bio_next_split.
By the above method with size_limit, s->insert_bio_sectors will never
result oversized replace_key size or bio bvecs number. And split bio
'miss' from bio_next_split() will always match the size of 'cache_bio',
that is the current maximum bio size we can sent to backing device for
fetching the cache missing data.
Current problmatic code can be partially found since Linux v3.13-rc1,
therefore all maintained stable kernels should try to apply this fix.
Reported-by: Alexander Ullrich <[email protected]>
Reported-by: Diego Ercolani <[email protected]>
Reported-by: Jan Szubiak <[email protected]>
Reported-by: Marco Rebhan <[email protected]>
Reported-by: Matthias Ferdinand <[email protected]>
Reported-by: Thorsten Knabe <[email protected]>
Reported-by: Victor Westerhuis <[email protected]>
Reported-by: Vojtech Pavlik <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Cc: [email protected]
Cc: Christoph Hellwig <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Nix <[email protected]>
Cc: Takashi Iwai <[email protected]>
---
Changelog,
v5, improvement and fix based on v4 comments from Christoph Hellwig
and Nix.
v4, not directly access BIO_MAX_VECS and reduce reada value to avoid
oversized bvecs number, by hint from Christoph Hellwig.
v3, fix typo in v2.
v2, fix the bypass bio size calculation in v1.
v1, the initial version.
drivers/md/bcache/request.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index ab8ff18df32a..d855a8882cbc 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -882,6 +882,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
int ret = MAP_CONTINUE;
struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);
struct bio *miss, *cache_bio;
+ unsigned int size_limit;
s->cache_missed = 1;
@@ -891,7 +892,10 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
goto out_submit;
}
- s->insert_bio_sectors = min(sectors, bio_sectors(bio));
+ /* Limitation for valid replace key size and cache_bio bvecs number */
+ size_limit = min_t(unsigned int, bio_max_segs(UINT_MAX) * PAGE_SECTORS,
+ (1 << KEY_SIZE_BITS) - 1);
+ s->insert_bio_sectors = min3(size_limit, sectors, bio_sectors(bio));
s->iop.replace_key = KEY(s->iop.inode,
bio->bi_iter.bi_sector + s->insert_bio_sectors,
@@ -903,7 +907,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
s->iop.replace = true;
- miss = bio_next_split(bio, sectors, GFP_NOIO, &s->d->bio_split);
+ miss = bio_next_split(bio, s->insert_bio_sectors, GFP_NOIO, &s->d->bio_split);
/* btree_search_recurse()'s btree iterator is no good anymore */
ret = miss == bio ? MAP_DONE : -EINTR;
--
2.26.2