Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode

[Dan Williams]

On Thu, Jun 28, 2018 at 10:48 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Thu, Jun 28 2018 at  1:42pm -0400,
> Kani, Toshi <toshi.kani@xxxxxxx> wrote:
>
>> On Tue, 2018-06-26 at 16:04 -0600, Ross Zwisler wrote:
>> > On Tue, Jun 26, 2018 at 02:51:52PM -0700, Dan Williams wrote:
>> > > On Tue, Jun 26, 2018 at 2:31 PM, Kani, Toshi <toshi.kani@xxxxxxx> 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@xxxxxxx> 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@xxxxxxx> 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>"
>> >
>> > Dan is correct that there is no user visible change for this.  It is the right
>> > thing to do for consistency and sanity, but it doesn't actually have user
>> > visible behavior that needs to be backported to stable.
>> >
>> > Toshi is correct that this change is only for raw mode namespaces, not btt
>> > namespaces.
>> >
>> > I'll adjust the changelog and remove the stable flag for v5, and I'll add a
>> > Fixes: tag for patch 2.
>>
>> Hi Ross,
>>
>> Your patches look good.  But I am still not clear about the Fixes &
>> stable handling.  Talking about user visible behavior, I do not think we
>> had any issue until dax support was dropped from raw mode.  Until then,
>> the pmem driver supported dax for all modes, and the check for
>> direct_access worked.
>
> I've staged the changes to send to Linus shortly.
>
> The first patch has:
>
> Fixes: 569d0365f571 ("dax: require 'struct page' by default for filesystem dax")
> Cc: stable@xxxxxxxxxxxxxxx
>
> As that is the right thing to do given the other 2 patches are marked
> for stable.  We don't want to have a stable kernel with the last 2
> patches but not the first.

Ok, I'm still grumbling about the changelog being more clear about
what the problem was, but let's just go with what you got.