linux-kernel - Re: [PATCH 0/6] Support DAX for device-mapper dm-linear devices

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Date:	Tue, 14 Jun 2016 19:07:53 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	"Kani, Toshimitsu" <toshi.kani@....com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
	"agk@...hat.com" <agk@...hat.com>
Subject: Re: [PATCH 0/6] Support DAX for device-mapper dm-linear devices

On Tue, Jun 14, 2016 at 6:46 PM, Mike Snitzer <snitzer@...hat.com> wrote:
> On Tue, Jun 14 2016 at  4:19pm -0400,
> Jeff Moyer <jmoyer@...hat.com> wrote:
>
>> Mike Snitzer <snitzer@...hat.com> writes:
>>
>> > On Tue, Jun 14 2016 at  9:50am -0400,
>> > Jeff Moyer <jmoyer@...hat.com> wrote:
>> >
>> >> "Kani, Toshimitsu" <toshi.kani@....com> writes:
>> >>
>> >> >> I had dm-linear and md-raid0 support on my list of things to look at,
>> >> >> did you have raid0 in your plans?
>> >> >
>> >> > Yes, I hope to extend further and raid0 is a good candidate.
>> >>
>> >> dm-flakey would allow more xfstests test cases to run.  I'd say that's
>> >> more important than linear or raid0.  ;-)
>> >
>> > Regardless of which target(s) grow DAX support the most pressing initial
>> > concern is getting the DM device stacking correct.  And verifying that
>> > IO that cross pmem device boundaries are being properly split by DM
>> > core (via drivers/md/dm.c:__split_and_process_non_flush()'s call to
>> > max_io_len).
>>
>> That was a tongue-in-cheek comment.  You're reading way too much into
>> it.
>>
>> >> Also, the next step in this work is to then decide how to determine on
>> >> what numa node an LBA resides.  We had discussed this at a prior
>> >> plumbers conference, and I think the consensus was to use xattrs.
>> >> Toshi, do you also plan to do that work?
>> >
>> > How does the associated NUMA node relate to this?  Does the
>> > DM requests_queue need to be setup to only allocate from the NUMA node
>> > the pmem device is attached to?  I recently added support for this to
>> > DM.  But there will likely be some code need to propagate the NUMA node
>> > id accordingly.
>>
>> I assume you mean allocate memory (the volatile kind).  That should work
>> the same between pmem and regular block devices, no?
>
> This is the commit I made to train DM to be numa node aware:
> 115485e83f497fdf9b4 ("dm: add 'dm_numa_node' module parameter")

Hmm, but this is global for all DM device instances.

> As is the DM code is focused on memory allocations.  But I think blk-mq
> may use the NUMA node for via tag_set->numa_node.  But that is moot
> given pmem is bio-based right?

Right.

>
> Steps could be taken to make all threads DM creates for a a given device
> get pinned to the specified NUMA node too.

I think it would be useful if a DM instance inherited the numa node
from the component devices by default (assuming they're all from the
same node).  A "dev_to_node(disk_to_dev(disk))" conversion works for
pmem devices.

As far as I understand, Jeff wants to go further and have a linear
span across component devices from different nodes with an interface
to do an LBA-to-numa-node conversion.