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Subject[PATCH 5.4 195/309] bcache: add readahead cache policy options via sysfs interface
Date
From: Coly Li <colyli@suse.de>

commit 038ba8cc1bffc51250add4a9b9249d4331576d8f upstream.

In year 2007 high performance SSD was still expensive, in order to
save more space for real workload or meta data, the readahead I/Os
for non-meta data was bypassed and not cached on SSD.

In now days, SSD price drops a lot and people can find larger size
SSD with more comfortable price. It is unncessary to alway bypass
normal readahead I/Os to save SSD space for now.

This patch adds options for readahead data cache policies via sysfs
file /sys/block/bcache<N>/readahead_cache_policy, the options are,
- "all": cache all readahead data I/Os.
- "meta-only": only cache meta data, and bypass other regular I/Os.

If users want to make bcache continue to only cache readahead request
for metadata and bypass regular data readahead, please set "meta-only"
to this sysfs file. By default, bcache will back to cache all read-
ahead requests now.

Cc: stable@vger.kernel.org
Signed-off-by: Coly Li <colyli@suse.de>
Acked-by: Eric Wheeler <bcache@linux.ewheeler.net>
Cc: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/bcache/bcache.h | 3 +++
drivers/md/bcache/request.c | 17 ++++++++++++-----
drivers/md/bcache/sysfs.c | 22 ++++++++++++++++++++++
3 files changed, 37 insertions(+), 5 deletions(-)

--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -329,6 +329,9 @@ struct cached_dev {
*/
atomic_t has_dirty;

+#define BCH_CACHE_READA_ALL 0
+#define BCH_CACHE_READA_META_ONLY 1
+ unsigned int cache_readahead_policy;
struct bch_ratelimit writeback_rate;
struct delayed_work writeback_rate_update;

--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -391,13 +391,20 @@ static bool check_should_bypass(struct c
goto skip;

/*
- * Flag for bypass if the IO is for read-ahead or background,
- * unless the read-ahead request is for metadata
+ * If the bio is for read-ahead or background IO, bypass it or
+ * not depends on the following situations,
+ * - If the IO is for meta data, always cache it and no bypass
+ * - If the IO is not meta data, check dc->cache_reada_policy,
+ * BCH_CACHE_READA_ALL: cache it and not bypass
+ * BCH_CACHE_READA_META_ONLY: not cache it and bypass
+ * That is, read-ahead request for metadata always get cached
* (eg, for gfs2 or xfs).
*/
- if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) &&
- !(bio->bi_opf & (REQ_META|REQ_PRIO)))
- goto skip;
+ if ((bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND))) {
+ if (!(bio->bi_opf & (REQ_META|REQ_PRIO)) &&
+ (dc->cache_readahead_policy != BCH_CACHE_READA_ALL))
+ goto skip;
+ }

if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) ||
bio_sectors(bio) & (c->sb.block_size - 1)) {
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -27,6 +27,12 @@ static const char * const bch_cache_mode
NULL
};

+static const char * const bch_reada_cache_policies[] = {
+ "all",
+ "meta-only",
+ NULL
+};
+
/* Default is 0 ("auto") */
static const char * const bch_stop_on_failure_modes[] = {
"auto",
@@ -100,6 +106,7 @@ rw_attribute(congested_write_threshold_u
rw_attribute(sequential_cutoff);
rw_attribute(data_csum);
rw_attribute(cache_mode);
+rw_attribute(readahead_cache_policy);
rw_attribute(stop_when_cache_set_failed);
rw_attribute(writeback_metadata);
rw_attribute(writeback_running);
@@ -167,6 +174,11 @@ SHOW(__bch_cached_dev)
bch_cache_modes,
BDEV_CACHE_MODE(&dc->sb));

+ if (attr == &sysfs_readahead_cache_policy)
+ return bch_snprint_string_list(buf, PAGE_SIZE,
+ bch_reada_cache_policies,
+ dc->cache_readahead_policy);
+
if (attr == &sysfs_stop_when_cache_set_failed)
return bch_snprint_string_list(buf, PAGE_SIZE,
bch_stop_on_failure_modes,
@@ -352,6 +364,15 @@ STORE(__cached_dev)
}
}

+ if (attr == &sysfs_readahead_cache_policy) {
+ v = __sysfs_match_string(bch_reada_cache_policies, -1, buf);
+ if (v < 0)
+ return v;
+
+ if ((unsigned int) v != dc->cache_readahead_policy)
+ dc->cache_readahead_policy = v;
+ }
+
if (attr == &sysfs_stop_when_cache_set_failed) {
v = __sysfs_match_string(bch_stop_on_failure_modes, -1, buf);
if (v < 0)
@@ -466,6 +487,7 @@ static struct attribute *bch_cached_dev_
&sysfs_data_csum,
#endif
&sysfs_cache_mode,
+ &sysfs_readahead_cache_policy,
&sysfs_stop_when_cache_set_failed,
&sysfs_writeback_metadata,
&sysfs_writeback_running,

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