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

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Date:	Tue, 14 Jun 2016 00:30:58 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Kani, Toshimitsu" <toshi.kani@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"agk@...hat.com" <agk@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"snitzer@...hat.com" <snitzer@...hat.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"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 Mon, Jun 13, 2016 at 5:02 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Mon, Jun 13, 2016 at 4:59 PM, Kani, Toshimitsu <toshi.kani@....com> wrote:
>> On Mon, 2016-06-13 at 16:18 -0700, Dan Williams wrote:
>>> Thanks Toshi!
>>>
>>> On Mon, Jun 13, 2016 at 3:21 PM, Toshi Kani <toshi.kani@....com> wrote:
>>> >
>>> > This patch-set adds DAX support to device-mapper dm-linear devices
>>> > used by LVM.  It works with LVM commands as follows:
>>> >  - Creation of a logical volume with all DAX capable devices (such
>>> >    as pmem) sets the logical volume DAX capable as well.
>>> >  - Once a logical volume is set to DAX capable, the volume may not
>>> >    be extended with non-DAX capable devices.
>>>
>>> I don't mind this, but it seems a policy decision that the kernel does
>>> not need to make.  A sufficiently sophisticated user could cope with
>>> DAX being available at varying LBAs.  Would it be sufficient to move
>>> this policy decision to userspace tooling?
>>
>> I think this is a kernel restriction.  When a block device is declared as
>> DAX capable, it should mean that the whole device is DAX capable.  So, I
>> think we need to assure the same to a mapped device.
>
> Hmm, but we already violate this with badblocks.  The device is DAX
> capable, but certain LBAs will return an error if direct_access is
> attempted.

Nevermind, for this to be useful we would need to fallback to regular
mmap for a portion of the linear span.  That's different than the
badblocks case.