bcache: remove member accessed from struct btree

The member 'accessed' of struct btree is used in bch_mca_scan() when
shrinking btree node caches. The original idea is, if b->accessed is
set, clean it and look at next btree node cache from c->btree_cache
list, and only shrink the caches whose b->accessed is cleaned. Then
only cold btree node cache will be shrunk.

But when I/O pressure is high, it is very probably that b->accessed
of a btree node cache will be set again in bch_btree_node_get()
before bch_mca_scan() selects it again. Then there is no chance for
bch_mca_scan() to shrink enough memory back to slub or slab system.

This patch removes member accessed from struct btree, then once a
btree node ache is selected, it will be immediately shunk. By this
change, bch_mca_scan() may release btree node cahce more efficiently.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -754,14 +754,12 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
 		b = list_first_entry(&c->btree_cache, struct btree, list);
 		list_rotate_left(&c->btree_cache);
 
-		if (!b->accessed &&
-		    !mca_reap(b, 0, false)) {
+		if (!mca_reap(b, 0, false)) {
 			mca_bucket_free(b);
 			mca_data_free(b);
 			rw_unlock(true, b);
 			freed++;
-		} else
-			b->accessed = 0;
+		}
 	}
 out:
 	mutex_unlock(&c->bucket_lock);
@@ -1069,7 +1067,6 @@ struct btree *bch_btree_node_get(struct cache_set *c, struct btree_op *op,
 	BUG_ON(!b->written);
 
 	b->parent = parent;
-	b->accessed = 1;
 
 	for (; i <= b->keys.nsets && b->keys.set[i].size; i++) {
 		prefetch(b->keys.set[i].tree);
@@ -1160,7 +1157,6 @@ struct btree *__bch_btree_node_alloc(struct cache_set *c, struct btree_op *op,
 		goto retry;
 	}
 
-	b->accessed = 1;
 	b->parent = parent;
 	bch_bset_init_next(&b->keys, b->keys.set->data, bset_magic(&b->c->sb));
 

drivers/md/bcache/btree.h

@@ -121,8 +121,6 @@ struct btree {
 	/* Key/pointer for this btree node */
 	BKEY_PADDED(key);
 
-	/* Single bit - set when accessed, cleared by shrinker */
-	unsigned long		accessed;
 	unsigned long		seq;
 	struct rw_semaphore	lock;
 	struct cache_set	*c;