Re: [PATCHv4 6/9] block/merge: count bytes instead of sectors

Damien Le Moal
2022-05-26T01:57:37Z

On 2022/05/26 10:06, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
>
> Individual bv_len's may not be a sector size.
>
> Signed-off-by: Keith Busch <kbusch@kernel.org>
> ---
> v3->v4:
>
> Use sector shift
>
> Add comment explaing the ALIGN_DOWN
>
> block/blk-merge.c | 41 ++++++++++++++++++++++++-----------------
> 1 file changed, 24 insertions(+), 17 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 7771dacc99cb..9ff0cb9e4840 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -201,11 +201,11 @@ static inline unsigned get_max_segment_size(const struct request_queue *q,
> * @nsegs: [in,out] Number of segments in the bio being built. Incremented
> * by the number of segments from @bv that may be appended to that
> * bio without exceeding @max_segs
> - * @sectors: [in,out] Number of sectors in the bio being built. Incremented
> - * by the number of sectors from @bv that may be appended to that
> - * bio without exceeding @max_sectors
> + * @bytes: [in,out] Number of bytes in the bio being built. Incremented
> + * by the number of bytes from @bv that may be appended to that
> + * bio without exceeding @max_bytes
> * @max_segs: [in] upper bound for *@nsegs
> - * @max_sectors: [in] upper bound for *@sectors
> + * @max_bytes: [in] upper bound for *@bytes
> *
> * When splitting a bio, it can happen that a bvec is encountered that is too
> * big to fit in a single segment and hence that it has to be split in the
> @@ -216,10 +216,10 @@ static inline unsigned get_max_segment_size(const struct request_queue *q,
> */
> static bool bvec_split_segs(const struct request_queue *q,
> const struct bio_vec *bv, unsigned *nsegs,
> - unsigned *sectors, unsigned max_segs,
> - unsigned max_sectors)
> + unsigned *bytes, unsigned max_segs,
> + unsigned max_bytes)
> {
> - unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
> + unsigned max_len = min(max_bytes, UINT_MAX) - *bytes;
> unsigned len = min(bv->bv_len, max_len);
> unsigned total_len = 0;
> unsigned seg_size = 0;
> @@ -237,7 +237,7 @@ static bool bvec_split_segs(const struct request_queue *q,
> break;
> }
>
> - *sectors += total_len >> 9;
> + *bytes += total_len;
>
> /* tell the caller to split the bvec if it is too big to fit */
> return len > 0 || bv->bv_len > max_len;
> @@ -269,8 +269,8 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> {
> struct bio_vec bv, bvprv, *bvprvp = NULL;
> struct bvec_iter iter;
> - unsigned nsegs = 0, sectors = 0;
> - const unsigned max_sectors = get_max_io_size(q, bio);
> + unsigned nsegs = 0, bytes = 0;
> + const unsigned max_bytes = get_max_io_size(q, bio) << 9;

You missed one SECTOR_SHIFT here :)

Also, this get_max_io_size() function is now super confusing. It really should
be get_max_io_sectors() and we could add also get_max_io_bytes(), no ?

> const unsigned max_segs = queue_max_segments(q);
>
> bio_for_each_bvec(bv, bio, iter) {
> @@ -282,12 +282,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> goto split;
>
> if (nsegs < max_segs &&
> - sectors + (bv.bv_len >> 9) <= max_sectors &&
> + bytes + bv.bv_len <= max_bytes &&
> bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
> nsegs++;
> - sectors += bv.bv_len >> 9;
> - } else if (bvec_split_segs(q, &bv, &nsegs, &sectors, max_segs,
> - max_sectors)) {
> + bytes += bv.bv_len;
> + } else if (bvec_split_segs(q, &bv, &nsegs, &bytes, max_segs,
> + max_bytes)) {
> goto split;
> }
>
> @@ -300,13 +300,20 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> split:
> *segs = nsegs;
>
> + /*
> + * Individual bvecs may not be logical block aligned, so round down to
> + * that size to ensure both sides of the split bio are appropriately
> + * sized.
> + */
> + bytes = ALIGN_DOWN(bytes, queue_logical_block_size(q));
> +
> /*
> * Bio splitting may cause subtle trouble such as hang when doing sync
> * iopoll in direct IO routine. Given performance gain of iopoll for
> * big IO can be trival, disable iopoll when split needed.
> */
> bio_clear_polled(bio);
> - return bio_split(bio, sectors, GFP_NOIO, bs);
> + return bio_split(bio, bytes >> SECTOR_SHIFT, GFP_NOIO, bs);
> }
>
> /**
> @@ -375,7 +382,7 @@ EXPORT_SYMBOL(blk_queue_split);
> unsigned int blk_recalc_rq_segments(struct request *rq)
> {
> unsigned int nr_phys_segs = 0;
> - unsigned int nr_sectors = 0;
> + unsigned int bytes = 0;
> struct req_iterator iter;
> struct bio_vec bv;
>
> @@ -398,7 +405,7 @@ unsigned int blk_recalc_rq_segments(struct request *rq)
> }
>
> rq_for_each_bvec(bv, rq, iter)
> - bvec_split_segs(rq->q, &bv, &nr_phys_segs, &nr_sectors,
> + bvec_split_segs(rq->q, &bv, &nr_phys_segs, &bytes,
> UINT_MAX, UINT_MAX);
> return nr_phys_segs;
> }

--
Damien Le Moal
Western Digital Research