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

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Message-ID: <20160615014658.GA5443@redhat.com>
Date:	Tue, 14 Jun 2016 21:46:58 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	"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>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"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  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")

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?

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