btrfs: do not BUG_ON after failure to migrate space during truncation

During truncation we reserve 2 metadata units when starting a transaction
(reserved space goes to fs_info->trans_block_rsv) and then attempt to
migrate 1 unit (min_size bytes) from fs_info->trans_block_rsv into the
local block reserve. If we ever fail we trigger a BUG_ON(), which should
never happen, because we reserved 2 units. However if we happen to fail
for some reason, we don't need to be so dire and hit a BUG_ON(), we can
just error out the truncation and, since this is highly unexpected,
surround the error check with WARN_ON(), to get an informative stack
trace and tag the branh as 'unlikely'. Also make the 'min_size' variable
const, since it's not supposed to ever change and any accidental change
could possibly make the space migration not so unlikely to fail.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/inode.c

@@ -8344,7 +8344,7 @@ static int btrfs_truncate(struct btrfs_inode *inode, bool skip_writeback)
 	int ret;
 	struct btrfs_trans_handle *trans;
 	u64 mask = fs_info->sectorsize - 1;
-	u64 min_size = btrfs_calc_metadata_size(fs_info, 1);
+	const u64 min_size = btrfs_calc_metadata_size(fs_info, 1);
 
 	if (!skip_writeback) {
 		ret = btrfs_wait_ordered_range(&inode->vfs_inode,
@@ -8401,7 +8401,15 @@ static int btrfs_truncate(struct btrfs_inode *inode, bool skip_writeback)
 	/* Migrate the slack space for the truncate to our reserve */
 	ret = btrfs_block_rsv_migrate(&fs_info->trans_block_rsv, rsv,
 				      min_size, false);
-	BUG_ON(ret);
+	/*
+	 * We have reserved 2 metadata units when we started the transaction and
+	 * min_size matches 1 unit, so this should never fail, but if it does,
+	 * it's not critical we just fail truncation.
+	 */
+	if (WARN_ON(ret)) {
+		btrfs_end_transaction(trans);
+		goto out;
+	}
 
 	trans->block_rsv = rsv;
 
@@ -8449,7 +8457,14 @@ static int btrfs_truncate(struct btrfs_inode *inode, bool skip_writeback)
 		btrfs_block_rsv_release(fs_info, rsv, -1, NULL);
 		ret = btrfs_block_rsv_migrate(&fs_info->trans_block_rsv,
 					      rsv, min_size, false);
-		BUG_ON(ret);	/* shouldn't happen */
+		/*
+		 * We have reserved 2 metadata units when we started the
+		 * transaction and min_size matches 1 unit, so this should never
+		 * fail, but if it does, it's not critical we just fail truncation.
+		 */
+		if (WARN_ON(ret))
+			break;
+
 		trans->block_rsv = rsv;
 	}