bcache: add readahead cache policy options via sysfs interface

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: [email protected]
Signed-off-by: Coly Li <[email protected]>
Acked-by: Eric Wheeler <[email protected]>
Cc: Michael Lyle <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Coly Li

drivers/md/bcache/bcache.h

@@ -330,6 +330,9 @@ 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;
 

drivers/md/bcache/request.c

@@ -379,13 +379,20 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
 		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)) {

drivers/md/bcache/sysfs.c

@@ -27,6 +27,12 @@ static const char * const bch_cache_modes[] = {
 	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_us);
 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);
@@ -168,6 +175,11 @@ 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,
@@ -353,6 +365,15 @@ 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)
@@ -467,6 +488,7 @@ static struct attribute *bch_cached_dev_files[] = {
 	&sysfs_data_csum,
 #endif
 	&sysfs_cache_mode,
+	&sysfs_readahead_cache_policy,
 	&sysfs_stop_when_cache_set_failed,
 	&sysfs_writeback_metadata,
 	&sysfs_writeback_running,