Re: [PATCH 01/17] block: export bio_split_rw

From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
	Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	Qu Wenruo <wqu@suse.com>, Jens Axboe <axboe@kernel.dk>,
	"Darrick J. Wong" <djwong@kernel.org>,
	linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 01/17] block: export bio_split_rw
Date: Thu, 1 Sep 2022 16:54:32 +0800	[thread overview]
Message-ID: <de16bd58-3f14-01d9-9de5-6a79792c62c7@gmx.com> (raw)
In-Reply-To: <20220901074216.1849941-2-hch@lst.de>

On 2022/9/1 15:42, Christoph Hellwig wrote:
> bio_split_rw can be used by file systems to split and incoming write
> bio into multiple bios fitting the hardware limit for use as ZONE_APPEND
> bios.  Export it for initial use in btrfs.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   block/blk-merge.c   | 3 ++-
>   include/linux/bio.h | 4 ++++
>   2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index ff04e9290715a..e68295462977b 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -267,7 +267,7 @@ static bool bvec_split_segs(struct queue_limits *lim, const struct bio_vec *bv,
>    * responsible for ensuring that @bs is only destroyed after processing of the
>    * split bio has finished.
>    */
> -static struct bio *bio_split_rw(struct bio *bio, struct queue_limits *lim,
> +struct bio *bio_split_rw(struct bio *bio, struct queue_limits *lim,

I found the queue_limits structure pretty scary, while we only have very
limited members used in this case:

- lim->virt_boundary_mask
   Used in bvec_gap_to_prev()

- lim->max_segments

- lim->seg_boundary_mask
- lim->max_segment_size
   Used in bvec_split_segs()

- lim->logical_block_size

Not familiar with block layer, thus I'm wondering do btrfs really need a
full queue_limits structure to call bio_split_rw().

Or can we have a simplified wrapper?

IIRC inside btrfs we only need two cases for bio split:

- Split for stripe boundary

- Split for OE/zoned boundary

Thanks,
Qu

>   		unsigned *segs, struct bio_set *bs, unsigned max_bytes)
>   {
>   	struct bio_vec bv, bvprv, *bvprvp = NULL;
> @@ -317,6 +317,7 @@ static struct bio *bio_split_rw(struct bio *bio, struct queue_limits *lim,
>   	bio_clear_polled(bio);
>   	return bio_split(bio, bytes >> SECTOR_SHIFT, GFP_NOIO, bs);
>   }
> +EXPORT_SYMBOL_GPL(bio_split_rw);
>
>   /**
>    * __bio_split_to_limits - split a bio to fit the queue limits
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index ca22b06700a94..46890f8235401 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -12,6 +12,8 @@
>
>   #define BIO_MAX_VECS		256U
>
> +struct queue_limits;
> +
>   static inline unsigned int bio_max_segs(unsigned int nr_segs)
>   {
>   	return min(nr_segs, BIO_MAX_VECS);
> @@ -375,6 +377,8 @@ static inline void bip_set_seed(struct bio_integrity_payload *bip,
>   void bio_trim(struct bio *bio, sector_t offset, sector_t size);
>   extern struct bio *bio_split(struct bio *bio, int sectors,
>   			     gfp_t gfp, struct bio_set *bs);
> +struct bio *bio_split_rw(struct bio *bio, struct queue_limits *lim,
> +		unsigned *segs, struct bio_set *bs, unsigned max_bytes);
>
>   /**
>    * bio_next_split - get next @sectors from a bio, splitting if necessary