[PATCH v2] f2fs: change write_hint for hot/warm nodes

[Daejun Park]

In the fs-based mode of F2FS, the mapping of hot/warm node to
WRITE_LIFE_NOT_SET should be changed to WRITE_LIFE_SHORT.

As a result of analyzing the write pattern of f2fs using real workload,
hot/warm nodes have high update ratio close to hot data.[*]
However, F2FS passes write hints for hot/warm nodes as WRITE_LIFE_NOT_SET.

Furthermore, WRITE_LIFE_NOT_SET is a default value of write hint when it is
not provided from the file system.
In storage, write_hint is used to distinguish streams (e.g. NVMe).
So, the hot/warm node of F2FS is not distinguished from other write_hints,
which can make the wrong stream seperation.

* Liang, Yu, et al. "An empirical study of F2FS on mobile devices." 2017
IEEE 23rd International Conference on Embedded and Real-Time Computing
Systems and Applications (RTCSA).

Signed-off-by: Daejun Park <daejun7.park@xxxxxxxxxxx>
---
v2: update documentation and comments
---
 Documentation/filesystems/f2fs.rst | 4 ++--
 fs/f2fs/segment.c                  | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index b8ee761c9922..afa3da7cfade 100644
--- a/Documentation/filesystems/f2fs.rst
+++ b/Documentation/filesystems/f2fs.rst
@@ -717,8 +717,8 @@ WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG
 ===================== ======================== ===================
 User                  F2FS                     Block
 ===================== ======================== ===================
-                      META                     WRITE_LIFE_MEDIUM;
-                      HOT_NODE                 WRITE_LIFE_NOT_SET
+                      META                     WRITE_LIFE_MEDIUM
+                      HOT_NODE                 WRITE_LIFE_SHORT
                       WARM_NODE                "
                       COLD_NODE                WRITE_LIFE_NONE
 ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 1596502f7375..577ab7516c6b 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -3160,8 +3160,8 @@ int f2fs_rw_hint_to_seg_type(enum rw_hint hint)
  *
  * User                  F2FS                     Block
  * ----                  ----                     -----
- *                       META                     WRITE_LIFE_MEDIUM;
- *                       HOT_NODE                 WRITE_LIFE_NOT_SET
+ *                       META                     WRITE_LIFE_MEDIUM
+ *                       HOT_NODE                 WRITE_LIFE_SHORT
  *                       WARM_NODE                "
  *                       COLD_NODE                WRITE_LIFE_NONE
  * ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
@@ -3208,7 +3208,7 @@ enum rw_hint f2fs_io_type_to_rw_hint(struct f2fs_sb_info *sbi,
 				return WRITE_LIFE_EXTREME;
 		} else if (type == NODE) {
 			if (temp == WARM || temp == HOT)
-				return WRITE_LIFE_NOT_SET;
+				return WRITE_LIFE_SHORT;
 			else if (temp == COLD)
 				return WRITE_LIFE_NONE;
 		} else if (type == META) {
-- 
2.25.1