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

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Date:	Wed, 20 May 2015 14:58:41 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lin <mlin@...nel.org>
Cc:	lkml <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:

> On Mon, May 18, 2015 at 9:52 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> 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.
>
> Kent,
>
> Is there specific reason to choose number 4?
> If no, I may change it to BIO_POOL_SIZE which is 2.

Here's what he had to say last time around:

  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.

link:  https://lkml.org/lkml/2013/11/26/47

Cheers,
Jeff
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