linux-kernel - Re: [PATCH 1/5] block: Make generic_make

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Message-ID: <20131126072128.GF9244@kmo-pixel>
Date:	Mon, 25 Nov 2013 23:21:28 -0800
From:	Kent Overstreet <kmo@...erainc.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	axboe@...nel.dk, Neil Brown <neilb@...e.de>,
	Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com
Subject: Re: [PATCH 1/5] block: Make generic_make_request handle arbitrary
 sized bios

On Mon, Nov 25, 2013 at 10:09:21PM -0800, Christoph Hellwig wrote:
> > +	q->bio_split = bioset_create(4, 0);
> > +	if (!q->bio_split)
> > +		goto fail_id;
> 
> How did we arrive at a mempool size of 4 to make sure we can always make
> progress with arbitrarily sized bios?  Shouldn't we document the design
> decision somewhere?

It just has to be nonzero to guarantee forward progress - the
bio_alloc_bioset() rescuer thing I did awhile back guarantees that.

> > +static struct bio *blk_bio_discard_split(struct request_queue *q,
> > +					 struct bio *bio,
> > +					 struct bio_set *bs)
> > +{
> 
> One fallout of this should be that it would allows us to impement
> a range UNMAP/TRIP much more easily, which would help dramatically
> with TRIM performance!

Cool!
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