[PATCH v3 4/4] btrfs: Add comments for btrfs_reserve_flush_enum

[Qu Wenruo <wqu@suse.com>]

This enum is the the interface exposed to developers.

Although we have a detailed comment explaining the whole idea of space
flushing at the head of space-info.c, the exposed enum interface doesn't
have any comment.

Some corner cases, like BTRFS_RESERVE_FLUSH_ALL and
BTRFS_RESERVE_FLUSH_ALL_STEAL can be interrupted by fatal signals, are
not explained at all.

So add some simple comments for these enums as a quick reference.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ctree.h | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index c96c27f66999..9e5cb19aaf54 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2563,16 +2563,46 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 int btrfs_extent_readonly(struct btrfs_fs_info *fs_info, u64 bytenr);
 void btrfs_clear_space_info_full(struct btrfs_fs_info *info);
 
+/*
+ * Different levels for to flush space when doing reserve.
+ *
+ * The higher the level, the more methods we try to reclaim space.
+ */
 enum btrfs_reserve_flush_enum {
 	/* If we are in the transaction, we can't flush anything.*/
 	BTRFS_RESERVE_NO_FLUSH,
+
 	/*
-	 * Flushing delalloc may cause deadlock somewhere, in this
-	 * case, use FLUSH LIMIT
+	 * Flush space by:
+	 * - Running delayed inode items
+	 * - Allocating a new chunk
 	 */
 	BTRFS_RESERVE_FLUSH_LIMIT,
+
+	/*
+	 * Flushing space by:
+	 * - Running delayed inode items
+	 * - Running delayed refs
+	 * - Running delalloc and wait for ordered extents
+	 * - Allocating a new chunk
+	 */
 	BTRFS_RESERVE_FLUSH_EVICT,
+
+	/*
+	 * Flush space by above mentioned methods and:
+	 * - Running delayed iputs
+	 * - Commiting transaction
+	 *
+	 * Can be interruped by fatal signal.
+	 */
 	BTRFS_RESERVE_FLUSH_ALL,
+
+	/*
+	 * Pretty much the same as FLUSH_ALL, but can also steal space
+	 * from global rsv.
+	 *
+	 * Can be interruped by fatal signal.
+	 */
 	BTRFS_RESERVE_FLUSH_ALL_STEAL,
 };
 
-- 
2.27.0