[PATCH v3 1/3] btrfs: add comments for check_can_nocow() and can_nocow_extent()

[Qu Wenruo <wqu@suse.com>]

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>
---
 fs/btrfs/file.c  | 19 +++++++++++++++++++
 fs/btrfs/inode.c | 19 +++++++++++++++++--
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index fccf5862cd3e..0e4f57fb2737 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1533,6 +1533,25 @@ 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)
+ *
+ * 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 into the range.
+ * Return 0 if we can't do nocow write.
+ * Return -EAGAIN if we can't get the needed lock, or for (nowait == true) case,
+ * there are ordered extents need to be flushed.
+ * Return <0 for if other error happened.
+ *
+ * NOTE: For wait (nowait==false) calls, callers need to release the drew write
+ * 	 lock of inode->root->snapshot_lock if return value > 0.
+ *
+ * @pos:	 File offset of the range
+ * @write_bytes: The length of the range to check, also contains the nocow
+ * 		 writable length if we can do nocow write
+ */
 static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 				    size_t *write_bytes, bool nowait)
 {
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 86f7aa377da9..48e16eae7278 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6922,8 +6922,23 @@ 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 write into [@offset, @offset + @len) of @inode.
+ *
+ * Return >0 and update @len if we can do nocow write into [@offset, @offset +
+ * @len).
+ * Return 0 if we can't do nocow write.
+ * Return <0 if error happened.
+ *
+ * NOTE: This only checks the file extents, caller is responsible to wait for
+ *	 any ordered extents.
+ *
+ * @offset:	File offset
+ * @len:	The length to write, will be updated to the nocow writable
+ * 		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
  */
 noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 			      u64 *orig_start, u64 *orig_block_len,
-- 
2.27.0