[RFC PATCH v2] f2fs: map data writes to FDP streams

[Wenjie Qi]

From: Wenjie Qi <qwjhust@xxxxxxxxx>

From: Wenjie Qi <qiwenjie@xxxxxxxxxx>

F2FS already classifies DATA writes using its existing hot, warm and cold
temperature policy, but it only passes that intent down as a write hint.
That hint alone is not sufficient for NVMe FDP placement, because the
current NVMe command path consumes `bio->bi_write_stream` rather than
`bio->bi_write_hint` when selecting a placement ID.

When the target block device exposes write streams, map the existing F2FS
DATA temperature classes onto stream IDs and set `bio->bi_write_stream`
for both buffered and direct writes. If the device exposes no write
streams, keep the current behavior by leaving the stream unset.

The stream mapping is evaluated against the target block device of each
bio, so the existing per-device fallback behavior stays unchanged for
multi-device filesystems. Existing blkzoned restrictions also remain in
place.

The mapping is intentionally small and deterministic:

- 1 stream: hot, warm and cold all use stream 1
- 2 streams: hot/warm use 1, cold uses 2
- 3+ streams: hot uses 1, warm uses 2, cold uses 3

Signed-off-by: Wenjie Qi <qwjhust@xxxxxxxxx>
---
v1 -> v2:
- document the default active_logs=6 DATA stream mapping and non-FDP fallback
- replace open-coded stream mapping values with existing F2FS symbols where possible

RFC notes:
- Scope is intentionally limited to DATA writes only.
- Validation used stock QEMU 8.2.2 NVMe FDP emulation with a 16G
  namespace.
- FDP enabled: buffered and direct writes each produced 3 dominant
  NVMe placement tuples.
- FDP disabled: buffered and direct writes each collapsed to 1
  dominant tuple.

 Documentation/filesystems/f2fs.rst |  9 +++++++++
 fs/f2fs/data.c                     |  2 ++
 fs/f2fs/f2fs.h                     |  2 ++
 fs/f2fs/file.c                     |  2 ++
 fs/f2fs/segment.c                  | 13 +++++++++++++
 5 files changed, 28 insertions(+)

diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index 7e4031631..8c4a14ae4 100644
--- a/Documentation/filesystems/f2fs.rst
+++ b/Documentation/filesystems/f2fs.rst
@@ -137,6 +137,15 @@ noacl			 Disable POSIX Access Control List. Note: acl is enabled
 active_logs=%u		 Support configuring the number of active logs. In the
 			 current design, f2fs supports only 2, 4, and 6 logs.
 			 Default number is 6.
+			 When the underlying block device exposes write
+			 streams, the default active_logs=6 configuration
+			 maps hot, warm, and cold DATA writes to streams 1,
+			 2, and 3, respectively. If only one or two write
+			 streams are available, f2fs falls back to mapping
+			 all DATA writes to stream 1 or mapping hot/warm
+			 to stream 1 and cold to stream 2. If no write
+			 streams are exposed, f2fs leaves the stream
+			 unset.
 disable_ext_identify	 Disable the extension list configured by mkfs, so f2fs
 			 is not aware of cold files such as media files.
 inline_xattr		 Enable the inline xattrs feature.
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 338df7a2a..df9f4a230 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -508,6 +508,8 @@ static struct bio *__bio_alloc(struct f2fs_io_info *fio, int npages)
 		bio->bi_private = sbi;
 		bio->bi_write_hint = f2fs_io_type_to_rw_hint(sbi,
 						fio->type, fio->temp);
+		bio->bi_write_stream = f2fs_io_type_to_write_stream(bdev, fio->type,
+								    fio->temp);
 	}
 	iostat_alloc_and_bind_ctx(sbi, bio, NULL);
 
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index bb34e864d..2f0777478 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -4056,6 +4056,8 @@ void f2fs_destroy_segment_manager_caches(void);
 int f2fs_rw_hint_to_seg_type(struct f2fs_sb_info *sbi, enum rw_hint hint);
 enum rw_hint f2fs_io_type_to_rw_hint(struct f2fs_sb_info *sbi,
 			enum page_type type, enum temp_type temp);
+u8 f2fs_io_type_to_write_stream(struct block_device *bdev,
+				enum page_type type, enum temp_type temp);
 unsigned int f2fs_usable_segs_in_sec(struct f2fs_sb_info *sbi);
 unsigned int f2fs_usable_blks_in_seg(struct f2fs_sb_info *sbi,
 			unsigned int segno);
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index c8a2f17a8..a3186cfc9 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -5064,6 +5064,8 @@ static void f2fs_dio_write_submit_io(const struct iomap_iter *iter,
 	enum temp_type temp = f2fs_get_segment_temp(sbi, type);
 
 	bio->bi_write_hint = f2fs_io_type_to_rw_hint(sbi, DATA, temp);
+	bio->bi_write_stream =
+		f2fs_io_type_to_write_stream(bio->bi_bdev, DATA, temp);
 	blk_crypto_submit_bio(bio);
 }
 
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 6a97fe767..902fc654d 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -3626,6 +3626,19 @@ enum rw_hint f2fs_io_type_to_rw_hint(struct f2fs_sb_info *sbi,
 	}
 }
 
+u8 f2fs_io_type_to_write_stream(struct block_device *bdev,
+				enum page_type type, enum temp_type temp)
+{
+	unsigned short nr = bdev_max_write_streams(bdev);
+
+	if (type != DATA || !nr)
+		return 0;
+	if (nr < NR_TEMP_TYPE)
+		return temp == COLD ? nr : HOT + 1;
+
+	return temp + 1;
+}
+
 static int __get_segment_type_2(struct f2fs_io_info *fio)
 {
 	if (fio->type == DATA)
-- 
2.43.0