btrfs: handle U64_MAX for shrink_delalloc

Data allocations are going to want to pass in U64_MAX for flushing
space, adjust shrink_delalloc to handle this properly.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -530,8 +530,19 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info, u64 to_reclaim,
 	int loops;
 
 	/* Calc the number of the pages we need flush for space reservation */
-	items = calc_reclaim_items_nr(fs_info, to_reclaim);
-	to_reclaim = items * EXTENT_SIZE_PER_ITEM;
+	if (to_reclaim == U64_MAX) {
+		items = U64_MAX;
+	} else {
+		/*
+		 * to_reclaim is set to however much metadata we need to
+		 * reclaim, but reclaiming that much data doesn't really track
+		 * exactly, so increase the amount to reclaim by 2x in order to
+		 * make sure we're flushing enough delalloc to hopefully reclaim
+		 * some metadata reservations.
+		 */
+		items = calc_reclaim_items_nr(fs_info, to_reclaim) * 2;
+		to_reclaim = items * EXTENT_SIZE_PER_ITEM;
+	}
 
 	trans = (struct btrfs_trans_handle *)current->journal_info;
 	space_info = btrfs_find_space_info(fs_info, BTRFS_BLOCK_GROUP_METADATA);
@@ -742,7 +753,7 @@ static void flush_space(struct btrfs_fs_info *fs_info,
 		break;
 	case FLUSH_DELALLOC:
 	case FLUSH_DELALLOC_WAIT:
-		shrink_delalloc(fs_info, num_bytes * 2,
+		shrink_delalloc(fs_info, num_bytes,
 				state == FLUSH_DELALLOC_WAIT);
 		break;
 	case FLUSH_DELAYED_REFS_NR: