bcachefs: Add a workqueue for journal reclaim

journal reclaim writes btree nodes, which can end up waiting for in
flight btree writes to complete, and btree write completions run out of
workqueues - so we can't run out of the same workqueue or we risk
deadlock
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -600,6 +600,7 @@ struct bch_fs {
 	struct workqueue_struct	*wq;
 	/* copygc needs its own workqueue for index updates.. */
 	struct workqueue_struct	*copygc_wq;
+	struct workqueue_struct	*journal_reclaim_wq;
 
 	/* ALLOCATION */
 	struct delayed_work	pd_controllers_update;

fs/bcachefs/journal_reclaim.c

@@ -336,7 +336,7 @@ void bch2_journal_reclaim_work(struct work_struct *work)
 		mutex_unlock(&j->reclaim_lock);
 
 	if (!test_bit(BCH_FS_RO, &c->flags))
-		queue_delayed_work(system_freezable_wq, &j->reclaim_work,
+		queue_delayed_work(c->journal_reclaim_wq, &j->reclaim_work,
 				   msecs_to_jiffies(j->reclaim_delay_ms));
 }
 

fs/bcachefs/super.c

@@ -388,6 +388,8 @@ static void bch2_fs_free(struct bch_fs *c)
 	kfree(c->replicas_gc.entries);
 	kfree(rcu_dereference_protected(c->disk_groups, 1));
 
+	if (c->journal_reclaim_wq)
+		destroy_workqueue(c->journal_reclaim_wq);
 	if (c->copygc_wq)
 		destroy_workqueue(c->copygc_wq);
 	if (c->wq)
@@ -602,6 +604,8 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 				WQ_FREEZABLE|WQ_MEM_RECLAIM|WQ_HIGHPRI, 1)) ||
 	    !(c->copygc_wq = alloc_workqueue("bcache_copygc",
 				WQ_FREEZABLE|WQ_MEM_RECLAIM|WQ_HIGHPRI, 1)) ||
+	    !(c->journal_reclaim_wq = alloc_workqueue("bcache_journal",
+				WQ_FREEZABLE|WQ_MEM_RECLAIM|WQ_HIGHPRI, 1)) ||
 	    percpu_ref_init(&c->writes, bch2_writes_disabled, 0, GFP_KERNEL) ||
 	    mempool_init_kmalloc_pool(&c->btree_reserve_pool, 1,
 				      sizeof(struct btree_reserve)) ||