linux-kernel - Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG

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Message-ID: <CAPcyv4gXAC7imJs5qfsDNLeN545Rqyd2b1M9s5kr4f9y48LXgQ@mail.gmail.com>
Date:   Tue, 26 Jun 2018 14:28:04 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kani, Toshi" <toshi.kani@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode

On Tue, Jun 26, 2018 at 2:23 PM, Kani, Toshi <toshi.kani@....com> wrote:
> On Tue, 2018-06-26 at 14:02 -0700, Dan Williams wrote:
>> On Tue, Jun 26, 2018 at 1:54 PM, Kani, Toshi <toshi.kani@....com> wrote:
[..]
>> > When this dm change was made, the pmem driver supported DAX for both raw
>> > and memory modes (note: sector mode does not use the pmem driver).  I
>> > think the issue was introduced when we dropped DAX support from raw
>> > mode.
>>
>> Still DAX with raw mode never really worked any way. It was also
>> something that was broken from day one. So what happens to someone who
>> happened to avoid all the problems with page-less DAX and enabled
>> device-mapper on top? That failure mode detail needs to be added to
>> this changelog if we want to propose this for -stable.
>
> My point is that the behavior should be consistent between pmem and
> device-mapper.  When -o dax succeeds on a pmem, then it should succeed
> on a device-mapper on top of that pmem.
>
> Has the drop of dax support from raw mode made to -stable back to the
> baseline accepted 545ed20e6df6?  It will introduce inconsistency,
> otherwise.

That commit, 569d0365f571 "dax: require 'struct page' by default for
filesystem dax", has not been tagged for -stable.