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

[Keith Busch]

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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