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SubjectRe: [PATCH v3 01/11] block: make generic_make_request handle arbitrarily sized bios
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Ming Lin <mlin@kernel.org> writes:

> On Mon, May 18, 2015 at 9:52 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
>> Ming Lin <mlin@kernel.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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