linux-kernel - Re: [PATCH v3 01/11] block: make generic_make

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Date:	Mon, 18 May 2015 12:52:03 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lin <mlin@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Dongsu Park <dpark@...teo.net>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ming Lei <ming.lei@...onical.com>, Neil Brown <neilb@...e.de>,
	Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	drbd-user@...ts.linbit.com, Jiri Kosina <jkosina@...e.cz>,
	Geoff Levand <geoff@...radead.org>, Jim Paris <jim@...n.com>,
	Joshua Morris <josh.h.morris@...ibm.com>,
	Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>
Subject: Re: [PATCH v3 01/11] block: make generic_make_request handle arbitrarily sized bios

Ming Lin <mlin@...nel.org> writes:

> diff --git a/block/blk-core.c b/block/blk-core.c
> index fd154b9..909f317 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -617,6 +617,10 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
>  	if (q->id < 0)
>  		goto fail_q;
>  
> +	q->bio_split = bioset_create(4, 0);
> +	if (!q->bio_split)
> +		goto fail_id;
> +

Arbitrary numbers should be documented.

> +static struct bio *blk_bio_discard_split(struct request_queue *q,
> +					 struct bio *bio,
> +					 struct bio_set *bs)
> +{
> +	unsigned int max_discard_sectors, granularity;
> +	int alignment;
> +	sector_t tmp;
> +	unsigned split_sectors;
> +
> +	/* Zero-sector (unknown) and one-sector granularities are the same.  */
> +	granularity = max(q->limits.discard_granularity >> 9, 1U);
> +
> +	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
> +	max_discard_sectors -= max_discard_sectors % granularity;
> +
> +	if (unlikely(!max_discard_sectors)) {
> +		/* XXX: warn */
> +		return NULL;
> +	}
> +
> +	if (bio_sectors(bio) <= max_discard_sectors)
> +		return NULL;
> +
> +	split_sectors = max_discard_sectors;
> +
> +	/*
> +	 * If the next starting sector would be misaligned, stop the discard at
> +	 * the previous aligned sector.
> +	 */
> +	alignment = (q->limits.discard_alignment >> 9) % granularity;
> +
> +	tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
> +	tmp = sector_div(tmp, granularity);
> +
> +	if (split_sectors > tmp)
> +		split_sectors -= tmp;
> +
> +	return bio_split(bio, split_sectors, GFP_NOIO, bs);

Much of this function is cut-n-paste from blk-lib.c.  Is there any way
to factor it out?

> +static struct bio *blk_bio_segment_split(struct request_queue *q,
> +					 struct bio *bio,
> +					 struct bio_set *bs)
> +{

Funny name for a function that also merges segments.  ;-)  I admit I am
at a loss for coming up with a better name that won't overflow 80
columns.

Other than that, this looks sane.

Cheers,
Jeff
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