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>

1 files changed, 21 insertions, 0 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 7aa184493aea..b750000b438a 100644
--- a/fs/btrfs/file.c
+++ b/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)
 {