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

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Message-ID: <20180626191346.GA7233@redhat.com>
Date:   Tue, 26 Jun 2018 15:13:46 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Toshi Kani <toshi.kani@....com>,
        device-mapper development <dm-devel@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode

On Tue, Jun 26 2018 at  3:07pm -0400,
Dan Williams <dan.j.williams@...el.com> wrote:

> On Tue, Jun 26, 2018 at 11:58 AM, Mike Snitzer <snitzer@...hat.com> wrote:
> > On Tue, Jun 26 2018 at  2:52pm -0400,
> > Dan Williams <dan.j.williams@...el.com> wrote:
> >
> >> On Tue, Jun 26, 2018 at 10:59 AM, Ross Zwisler
> >> <ross.zwisler@...ux.intel.com> wrote:
> >> > QUEUE_FLAG_DAX is an indication that a given block device supports
> >> > filesystem DAX and should not be set for PMEM namespaces which are in "raw"
> >> > or "sector" modes.  These namespaces lack struct page and are prevented
> >> > from participating in filesystem DAX.
> >> >
> >> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> >> > Suggested-by: Mike Snitzer <snitzer@...hat.com>
> >> > Cc: stable@...r.kernel.org
> >>
> >> Why is this cc: stable? What is the user visible impact of this change
> >> especially given the requirement to validate QUEUE_FLAG_DAX with
> >> bdev_dax_supported()? Patch looks good, but it's just a cosmetic fixup
> >> afaics.
> >
> > This isn't cosmetic when you consider that stacking up a DM device is
> > looking at this flag to determine whether a table does or does _not_
> > support DAX.
> >
> > So this patch, in conjunction with the other changes in the series, is
> > certainly something I'd consider appropriate for stable.
> 
> I think this classifies as something that never worked correctly and
> is not a regression. It does not identify which commit it is repairing
> or the user visible failure mode.

So you're taking issue with making stacked dax configs work in older
kernels?  That's fine.  We can drop the stable cc if you like.

But I mean we intended for this to work.. so the Fixes commit references
can easily be added, e.g.: 545ed20e6df68a4d2584a29a2a28ee8b2f7e9547
("dm: add infrastructure for DAX support")