linux-kernel - Re: [PATCH v7] NVMe: conversion to blk-mq

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Date:	Tue, 10 Jun 2014 15:21:18 -0600 (MDT)
From:	Keith Busch <keith.busch@...el.com>
To:	Jens Axboe <axboe@...com>
cc:	Keith Busch <keith.busch@...el.com>,
	Matias Bjørling <m@...rling.me>,
	"willy@...ux.intel.com" <willy@...ux.intel.com>,
	"sbradshaw@...ron.com" <sbradshaw@...ron.com>,
	"tom.leiming@...il.com" <tom.leiming@...il.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>
Subject: Re: [PATCH v7] NVMe: conversion to blk-mq

On Tue, 10 Jun 2014, Jens Axboe wrote:
> On 06/10/2014 03:10 PM, Keith Busch wrote:
>> On Tue, 10 Jun 2014, Jens Axboe wrote:
>>> On 06/10/2014 01:29 PM, Keith Busch wrote:
>>>> I have two devices, one formatted 4k, the other 512. The 4k is used as
>>>> the TEST_DEV and 512 is used as SCRATCH_DEV. I'm always hitting a BUG
>>>> when
>>>> unmounting the scratch dev in xfstests generic/068. The bug looks like
>>>> nvme was trying to use an SGL that doesn't map correctly to a PRP.
>>>
>>> I'm guessing it's some of the coalescing settings, since the driver is
>>> now using the generic block rq mapping.
>>
>> Ok, sounds right. I mentioned in a way earlier review it doesn't look
>> like a request that doesn't conform to a PRP list would get split anymore,
>> and this test seems to confirm that.
>>
>> Can we create something that will allow a driver to add DMA constraints to
>> a request queue with the rules of a PRP list?
>
> I haven't even looked at the rules - can you briefly outline them? From
> a quick look, seems it does prp chaining for every 512 entries. But
> nvme_setup_prps() looks like voodoo to an uninitiated, it could have
> used a comment or two :-)

Yeah, nvme_setup_prps is probably the least readable code in this driver.
Maybe some comments are in order here...

There are two rules for an SGL to be mappable to a PRP:

1. Every element must have zero page offset, except the first.

2. Every element must end on a page boundary, except the last.
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