[PATCH] bcache: add sysfs option to enable/disable read caching

[Jianjian Huo]

A few of bcache users are requesting a bcache option to enable/disable
read caching, in order to fine tune their bcache use cases. This patch
will do the work. The new sysfs setting is within backing device sysfs
folders, for example /sys/block/bcache0/bcache/cache_read_enable.

By default, it's enabled, since it's typical cache use case. And it works
with all three cache policies, writearound, writeback and writethrough.

This patch is based on 3.15, has been tested for several related test cases.

Signed-off-by: Jianjian Huo <***@gmail.com>
---
drivers/md/bcache/bcache.h | 2 ++
drivers/md/bcache/request.c | 5 +++--
drivers/md/bcache/super.c | 1 +
drivers/md/bcache/sysfs.c | 4 ++++
4 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 82c9c5d..b9ce6c1 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -365,6 +365,8 @@ struct cached_dev {
unsigned sequential_cutoff;
unsigned readahead;

+ unsigned cache_read_enable:1;
+
unsigned verify:1;
unsigned bypass_torture_test:1;

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 15fff4f..f382223 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -368,6 +368,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
unsigned sectors, congested = bch_get_congested(c);
struct task_struct *task = current;
struct io *i;
+ int rw = bio_data_dir(bio);

if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) ||
c->gc_stats.in_use > CUTOFF_CACHE_ADD ||
@@ -375,8 +376,8 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
goto skip;

if (mode == CACHE_MODE_NONE ||
- (mode == CACHE_MODE_WRITEAROUND &&
- (bio->bi_rw & REQ_WRITE)))
+ (mode == CACHE_MODE_WRITEAROUND && (bio->bi_rw & REQ_WRITE)) ||
+ (!rw && !dc->cache_read_enable))
goto skip;

if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) ||
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 926ded8..ed57be7 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1126,6 +1126,7 @@ static int cached_dev_init(struct cached_dev *dc, unsigned block_size)
bch_cache_accounting_init(&dc->accounting, &dc->disk.cl);

dc->sequential_cutoff = 4 << 20;
+ dc->cache_read_enable = true;

for (io = dc->io; io < dc->io + RECENT_IO; io++) {
list_add(&io->lru, &dc->io_lru);
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index b3ff57d..e15f888 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -73,6 +73,7 @@ rw_attribute(congested_write_threshold_us);
rw_attribute(sequential_cutoff);
rw_attribute(data_csum);
rw_attribute(cache_mode);
+rw_attribute(cache_read_enable);
rw_attribute(writeback_metadata);
rw_attribute(writeback_running);
rw_attribute(writeback_percent);
@@ -124,6 +125,7 @@ SHOW(__bch_cached_dev)
var_printf(bypass_torture_test, "%i");
var_printf(writeback_metadata, "%i");
var_printf(writeback_running, "%i");
+ var_printf(cache_read_enable, "%i");
var_print(writeback_delay);
var_print(writeback_percent);
sysfs_hprint(writeback_rate, dc->writeback_rate.rate << 9);
@@ -205,6 +207,7 @@ STORE(__cached_dev)
d_strtoul(writeback_metadata);
d_strtoul(writeback_running);
d_strtoul(writeback_delay);
+ d_strtoul(cache_read_enable);

sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent, 0, 40);

@@ -312,6 +315,7 @@ static struct attribute *bch_cached_dev_files[] = {
&sysfs_data_csum,
#endif
&sysfs_cache_mode,
+ &sysfs_cache_read_enable,
&sysfs_writeback_metadata,
&sysfs_writeback_running,
&sysfs_writeback_delay,