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Date: Tue, 15 Sep 2020 15:56:56 -0400
From: Mike Snitzer <snitzer@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Subject: Re: =?utf-8?B?5Zue5aSN77yacmVncmVzc2lvbiBj?=
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 =?utf-8?Q?=28=22dax=3A_fix_detectio?= =?utf-8?Q?n?= of dax support for
 non-persistent memory block?? devices")
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CC: Jan Kara <jack@suse.cz>, "colyli@suse.de" <colyli@suse.de>, Adrian Huang <ahuang12@lenovo.com>, Jan Kara <jack@suse.com>, Pankaj Gupta <pankaj.gupta.linux@gmail.com>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, Mikulas Patocka <mpatocka@redhat.com>
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On Tue, Sep 15 2020 at  3:49pm -0400,
Dan Williams <dan.j.williams@intel.com> wrote:

> On Tue, Sep 15, 2020 at 1:01 AM Jan Kara <jack@suse.cz> wrote:
> >
> > Hi!
> >
> > On Tue 15-09-20 11:03:29, colyli@suse.de wrote:
> > > Could you please to take a look? I am offline in the next two weeks.
> >
> > I just had a look into this. IMHO the justification in 6180bb446a "dax: fix
> > detection of dax support for non-persistent memory block devices" is just
> > bogus and people got confused by the previous condition
> >
> > if (!dax_dev && !bdev_dax_supported(bdev, blocksize))
> >
> > which was bogus as well. bdev_dax_supported() always returns false for bdev
> > that doesn't have dax_dev (naturally so). So in the original condition
> > there was no point in calling bdev_dax_supported() if we know dax_dev is
> > NULL.
> >
> > Then this was changed to:
> >
> > if (!dax_dev || !bdev_dax_supported(bdev, blocksize))
> >
> > which looks more sensible at the first sight. But only at the first sight -
> > if you look at wider context, __generic_fsdax_supported() is the bulk of
> > code that decides whether a device supports DAX so calling
> > bdev_dax_supported() from it indeed doesn't look as such a great idea. So
> > IMO the condition should be just:
> >
> > if (!dax_dev)
> >
> > I'll send a fix for this.
> 
> If you beat me to it, great, but you might be sleeping now. I agree
> the original condition was bogus and looks to be a result of previous
> non-thorough refactoring on my part. I think we can move that !dax_dev
> into dax_supported(). I'll take a look.

You trimmed the relevant portion of Jan's reply but: can you also
weigh-in one whether DM is using the wrong function to test for DAX?

Mike
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