bcachefs: Fix a deadlock on journal reclaim

Flushing the btree key cache needs to use allocation reserves - journal
reclaim depends on flushing the btree key cache for making forward
progress, and the allocator and copygc depend on journal reclaim making
forward progress.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_key_cache.c

@@ -386,12 +386,18 @@ static int btree_key_cache_flush_pos(struct btree_trans *trans,
 		goto evict;
 	}
 
+	/*
+	 * Since journal reclaim depends on us making progress here, and the
+	 * allocator/copygc depend on journal reclaim making progress, we need
+	 * to be using alloc reserves:
+	 * */
 	ret   = bch2_btree_iter_traverse(b_iter) ?:
 		bch2_trans_update(trans, b_iter, ck->k, BTREE_TRIGGER_NORUN) ?:
 		bch2_trans_commit(trans, NULL, NULL,
 				  BTREE_INSERT_NOUNLOCK|
 				  BTREE_INSERT_NOCHECK_RW|
 				  BTREE_INSERT_NOFAIL|
+				  BTREE_INSERT_USE_RESERVE|
 				  (ck->journal.seq == journal_last_seq(j)
 				   ? BTREE_INSERT_JOURNAL_RESERVED
 				   : 0)|

fs/bcachefs/journal_reclaim.c

@@ -634,7 +634,7 @@ static int __bch2_journal_reclaim(struct journal *j, bool direct)
 			       msecs_to_jiffies(j->reclaim_delay_ms)))
 			min_nr = 1;
 
-		if (j->prereserved.reserved * 2 > j->prereserved.remaining)
+		if (j->prereserved.reserved * 4 > j->prereserved.remaining)
 			min_nr = 1;
 
 		if (fifo_free(&j->pin) <= 32)

fs/bcachefs/movinggc.c

@@ -87,9 +87,20 @@ static enum data_cmd copygc_pred(struct bch_fs *c, void *arg,
 		if (i >= 0 &&
 		    p.ptr.offset < h->data[i].offset + ca->mi.bucket_size &&
 		    p.ptr.gen == h->data[i].gen) {
+			/*
+			 * We need to use the journal reserve here, because
+			 *  - journal reclaim depends on btree key cache
+			 *    flushing to make forward progress,
+			 *  - which has to make forward progress when the
+			 *    journal is pre-reservation full,
+			 *  - and depends on allocation - meaning allocator and
+			 *    copygc
+			 */
+
 			data_opts->target		= io_opts->background_target;
 			data_opts->nr_replicas		= 1;
-			data_opts->btree_insert_flags	= BTREE_INSERT_USE_RESERVE;
+			data_opts->btree_insert_flags	= BTREE_INSERT_USE_RESERVE|
+				BTREE_INSERT_JOURNAL_RESERVED;
 			data_opts->rewrite_dev		= p.ptr.dev;
 
 			if (p.has_ec)