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

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Date:	Tue, 21 Jun 2016 09:25:14 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Kani, Toshimitsu" <toshi.kani@....com>
Cc:	"axboe@...com" <axboe@...com>,
	"snitzer@...hat.com" <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"sandeen@...hat.com" <sandeen@...hat.com>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"agk@...hat.com" <agk@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: Re: [PATCH 0/6] Support DAX for device-mapper dm-linear devices

On Tue, Jun 21, 2016 at 8:44 AM, Kani, Toshimitsu <toshi.kani@....com> wrote:
> On Tue, 2016-06-21 at 09:41 -0400, Mike Snitzer wrote:
>> On Mon, Jun 20 2016 at  6:22pm -0400,
>> Mike Snitzer <snitzer@...hat.com> wrote:
>> >
>> > On Mon, Jun 20 2016 at  5:28pm -0400,
>> > Kani, Toshimitsu <toshi.kani@....com> wrote:
>> >
>  :
>> > Looks good, I folded it in and tested it to work.  Pushed to my 'wip'
>> > branch.
>> >
>> > No longer seeing any corruption in my test that was using partitions to
>> > span pmem devices with a dm-linear device.
>> >
>> > Jens, any chance you'd be open to picking up the first 2 patches in this
>> > series?  Or would you like to see them folded or something different?
>>
>> I'm now wondering if we'd be better off setting a new QUEUE_FLAG_DAX
>> rather than establish GENHD_FL_DAX on the genhd?
>>
>> It'd be quite a bit easier to allow upper layers (e.g. XFS and ext4) to
>> check for a queue flag.
>
> I think GENHD_FL_DAX is more appropriate since DAX does not use a request
> queue, except for protecting the underlining device being disabled while
> direct_access() is called (b2e0d1625e19).
>
> About protecting direct_access, this patch assumes that the underlining
> device cannot be disabled until dtr() is called.  Is this correct?  If not,
> I will need to call dax_map_atomic().

Kernel internal usages of dax should be using dax_map_atomic() to
safely resolve device removal races.