Re: [RFC PATCH v1] f2fs: map data writes to FDP streams

[Chao Yu <chao@kernel.org>]

On 4/15/2026 4:14 PM, Wenjie Qi wrote:
> From: Wenjie Qi <qwjhust@gmail.com>
> 
> From: Wenjie Qi <qiwenjie@xiaomi.com>
> 
> 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

Hi Wenjie,

Can we add doc to describe the default config?

> 
> Signed-off-by: Wenjie Qi <qwjhust@gmail.com>
> 
> ---
> 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.
> 
>   fs/f2fs/data.c    |  2 ++
>   fs/f2fs/f2fs.h    |  2 ++
>   fs/f2fs/file.c    |  2 ++
>   fs/f2fs/segment.c | 24 ++++++++++++++++++++++++
>   4 files changed, 30 insertions(+)
> 
> 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..ef8a3ac55 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -3626,6 +3626,30 @@ 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 == 1)
> +		return 1;
> +	if (nr == 2)
> +		return temp == COLD ? 2 : 1;
> +
> +	switch (temp) {
> +	case HOT:
> +		return 1;
> +	case WARM:
> +		return 2;
> +	case COLD:
> +		return 3;
> +	default:
> +		return 0;

How about using existing macros if there is instead magic numbers?

Thanks,

> +	}
> +}
> +
>   static int __get_segment_type_2(struct f2fs_io_info *fio)
>   {
>   	if (fio->type == DATA)