btrfs: add comments for btrfs_check_can_nocow() and can_nocow_extent()

These two functions have extra conditions that their callers need to
meet, and some not-that-common parameters used for return value.

So adding some comments may save reviewers some time.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -1533,6 +1533,27 @@ lock_and_cleanup_extent_if_need(struct btrfs_inode *inode, struct page **pages,
 	return ret;
 }
 
+/*
+ * Check if we can do nocow write into the range [@pos, @pos + @write_bytes)
+ *
+ * @pos:	 File offset
+ * @write_bytes: The length to write, will be updated to the nocow writeable
+ *		 range
+ * @nowait:	 Whether this function could sleep
+ *
+ * This function will flush ordered extents in the range to ensure proper
+ * nocow checks for (nowait == false) case.
+ *
+ * Return:
+ * >0		and update @write_bytes if we can do nocow write
+ *  0		if we can't do nocow write
+ * -EAGAIN	if we can't get the needed lock or there are ordered extents
+ * 		for * (nowait == true) case
+ * <0		if other error happened
+ *
+ * NOTE: For wait (nowait == false) calls, callers need to release the drew
+ * write lock of inode->root->snapshot_lock when return value > 0.
+ */
 int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 			  size_t *write_bytes, bool nowait)
 {

fs/btrfs/inode.c

@@ -6945,8 +6945,25 @@ static struct extent_map *btrfs_new_extent_direct(struct inode *inode,
 }
 
 /*
- * returns 1 when the nocow is safe, < 1 on error, 0 if the
- * block must be cow'd
+ * Check if we can do nocow write into the range [@offset, @offset + @len)
+ *
+ * @offset:	File offset
+ * @len:	The length to write, will be updated to the nocow writeable
+ *		range
+ * @orig_start:	(optional) Return the original file offset of the file extent
+ * @orig_len:	(optional) Return the original on-disk length of the file extent
+ * @ram_bytes:	(optional) Return the ram_bytes of the file extent
+ *
+ * This function will flush ordered extents in the range to ensure proper
+ * nocow checks for (nowait == false) case.
+ *
+ * Return:
+ * >0	and update @len if we can do nocow write
+ *  0	if we can't do nocow write
+ * <0	if error happened
+ *
+ * NOTE: This only checks the file extents, caller is responsible to wait for
+ *	 any ordered extents.
  */
 noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 			      u64 *orig_start, u64 *orig_block_len,