bcachefs: Don't set btree nodes as accessed on fill

Prevent jobs that do lots of scanning (i.e. evacuatee, scrub) from
causing OOMs.

The shrinker code seems to be having issues when it doesn't do any
freeing because it's just flipping off the acccessed bit - and the
accessed bit shouldn't be set on first use anyways.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/btree_cache.c

@@ -852,7 +852,6 @@ struct btree *bch2_btree_node_mem_alloc(struct btree_trans *trans, bool pcpu_rea
 	b->sib_u64s[1]		= 0;
 	b->whiteout_u64s	= 0;
 	bch2_btree_keys_init(b);
-	set_btree_node_accessed(b);
 
 	bch2_time_stats_update(&c->times[BCH_TIME_btree_node_mem_alloc],
 			       start_time);
@@ -1286,6 +1285,10 @@ struct btree *bch2_btree_node_get_noiter(struct btree_trans *trans,
 			six_unlock_read(&b->c.lock);
 			goto retry;
 		}
+
+		/* avoid atomic set bit if it's not needed: */
+		if (!btree_node_accessed(b))
+			set_btree_node_accessed(b);
 	}
 
 	/* XXX: waiting on IO with btree locks held: */
@@ -1301,10 +1304,6 @@ struct btree *bch2_btree_node_get_noiter(struct btree_trans *trans,
 		prefetch(p + L1_CACHE_BYTES * 2);
 	}
 
-	/* avoid atomic set bit if it's not needed: */
-	if (!btree_node_accessed(b))
-		set_btree_node_accessed(b);
-
 	if (unlikely(btree_node_read_error(b))) {
 		six_unlock_read(&b->c.lock);
 		b = ERR_PTR(-BCH_ERR_btree_node_read_err_cached);