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

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Date:   Tue, 26 Jun 2018 20:54:03 +0000
From:   "Kani, Toshi" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>
CC:     "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode

On Tue, 2018-06-26 at 15:13 -0400, Mike Snitzer wrote:
> 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")

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.

Thanks,
-Toshi