Btrfs: fix preallocate vs double nocow write

We can not release the reserved metadata space for the first write if we
find the write position is pre-allocated. Because the kernel might write
the data on the disk before we do the second write but after the can-nocow
check, if we release the space for the first write, we might fail to update
the metadata because of no space.

Fix this problem by end nocow write if there is dirty data in the range whose
space is pre-allocated.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/file.c

@@ -1427,16 +1427,11 @@ static noinline int check_can_nocow(struct inode *inode, loff_t pos,
 
 	num_bytes = lockend - lockstart + 1;
 	ret = can_nocow_extent(inode, lockstart, &num_bytes, NULL, NULL, NULL);
-	if (ret <= 0) {
+	if (ret <= 0)
 		ret = 0;
-	} else {
-		clear_extent_bit(&BTRFS_I(inode)->io_tree, lockstart, lockend,
-				 EXTENT_DIRTY | EXTENT_DELALLOC |
-				 EXTENT_DO_ACCOUNTING | EXTENT_DEFRAG, 0, 0,
-				 NULL, GFP_NOFS);
+	else
 		*write_bytes = min_t(size_t, *write_bytes ,
 				     num_bytes - pos + lockstart);
-	}
 
 	unlock_extent(&BTRFS_I(inode)->io_tree, lockstart, lockend);
 

fs/btrfs/inode.c

@@ -6557,6 +6557,7 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 	int ret;
 	struct extent_buffer *leaf;
 	struct btrfs_root *root = BTRFS_I(inode)->root;
+	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
 	struct btrfs_file_extent_item *fi;
 	struct btrfs_key key;
 	u64 disk_bytenr;
@@ -6633,6 +6634,20 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 
 	if (btrfs_extent_readonly(root, disk_bytenr))
 		goto out;
+
+	num_bytes = min(offset + *len, extent_end) - offset;
+	if (!nocow && found_type == BTRFS_FILE_EXTENT_PREALLOC) {
+		u64 range_end;
+
+		range_end = round_up(offset + num_bytes, root->sectorsize) - 1;
+		ret = test_range_bit(io_tree, offset, range_end,
+				     EXTENT_DELALLOC, 0, NULL);
+		if (ret) {
+			ret = -EAGAIN;
+			goto out;
+		}
+	}
+
 	btrfs_release_path(path);
 
 	/*
@@ -6661,7 +6676,6 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 	 */
 	disk_bytenr += backref_offset;
 	disk_bytenr += offset - key.offset;
-	num_bytes = min(offset + *len, extent_end) - offset;
 	if (csum_exist_in_range(root, disk_bytenr, num_bytes))
 				goto out;
 	/*