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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 26 Jun 2018 14:51:52 -0700
Message-ID: <CAPcyv4ifHX95fCM9Xj87CugwNWzgq=y-4LxPQ5L6RdjX0jdctQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode
To:     "Kani, Toshi" <toshi.kani@hpe.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
        "snitzer@redhat.com" <snitzer@redhat.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "ross.zwisler@linux.intel.com" <ross.zwisler@linux.intel.com>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>
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On Tue, Jun 26, 2018 at 2:31 PM, Kani, Toshi <toshi.kani@hpe.com> wrote:
> On Tue, 2018-06-26 at 14:28 -0700, Dan Williams wrote:
>> On Tue, Jun 26, 2018 at 2:23 PM, Kani, Toshi <toshi.kani@hpe.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@hpe.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.
>
> Then, Fixes tag should be set to 569d0365f571 to keep the behavior
> consistent.

Sure, and the failure mode is...? I'm thinking the commit log should say:

"Starting with commit 569d0365f571 "dax: require 'struct page' by
default for filesystem dax", dax is no longer supported for page-less
configurations. However, device-mapper sees the QUEUE_FLAG_DAX still
being set and falsely assumes that DAX is enabled, this leads to
<insert user visible failure mode details here>"