bcache: remove bcache device self-defined readahead

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: Victor Westerhuis <[email protected]>
Reported-by: Vojtech Pavlik <[email protected]>
Reported-and-tested-by: Rolf Fokkens <[email protected]>
Reported-and-tested-by: Thorsten Knabe <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: [email protected]
Cc: Kent Overstreet <[email protected]>
Cc: Nix <[email protected]>
Cc: Takashi Iwai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Coly Li

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;

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);

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);

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;
 };

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,