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From: Mike Snitzer <snitzer@redhat.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Toshi Kani <toshi.kani@hpe.com>,
	dm-devel@redhat.com, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH v2 3/7] dm: fix test for DAX device support
Date: Fri, 1 Jun 2018 16:46:04 -0400	[thread overview]
Message-ID: <20180601204604.GB1144@redhat.com> (raw)
In-Reply-To: <20180601201924.GA1144@redhat.com>

On Fri, Jun 01 2018 at  4:19P -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, May 29 2018 at  3:51P -0400,
> Ross Zwisler <ross.zwisler@linux.intel.com> wrote:
> 
> > Currently device_supports_dax() just checks to see if the QUEUE_FLAG_DAX
> > flag is set on the device's request queue to decide whether or not the
> > device supports filesystem DAX.  This is insufficient because there are
> > devices like PMEM namespaces in raw mode which have QUEUE_FLAG_DAX set but
> > which don't actually support DAX.
> 
> Isn't that a PMEM bug then?
> 
> What is the point of setting QUEUE_FLAG_DAX if it cannot be trusted?
>  
> > This means that you could create a dm-linear device, for example, where the
> > first part of the dm-linear device was a PMEM namespace in fsdax mode and
> > the second part was a PMEM namespace in raw mode.  Both DM and the
> > filesystem you put on that dm-linear device would think the whole device
> > supports DAX, which would lead to bad behavior once your raw PMEM namespace
> > part using DAX needed struct page for something.
> 
> The PMEM namespace in raw mode shouldn't be setting QUEUE_FLAG_DAX, if
> it didn't then the stacked-up linear DM wouldn't 
> 
> > Fix this by using bdev_dax_supported() like filesystems do at mount time.
> > This checks for raw mode and also performs other tests like checking to
> > make sure the dax_direct_access() path works.
> 
> Sorry "This" does those things where?

I see you meant bdev_dax_supported() does these additional checks.

My previous question stands though.  Why is QUEUE_FLAG_DAX getting set
if the device hasn't already passed these checks?  Shouldn't setting
QUEUE_FLAG_DAX on request_queue depend on bdev_dax_supported() passing?

But looking at the drivers that do set QUEUE_FLAG_DAX: they
don't have the bdev readily available.  Anyway, just strikes me as
bizarre that a driver can set QUEUE_FLAG_DAX without having to have
ensured bdev_dax_supported() passes (even if not programatically, but
that the developer has verified the hooks, et al exist).

But I'll give up on this line of questioning..

My dilemma now is: how do I take these changes without first rebasing
linux-dm.git ontop of Darrick's xfs tree?

I probably should've reviewed faster and been the one to take the entire
set (with appropriate acks obviously).

  reply	other threads:[~2018-06-01 20:46 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-29 19:50 [PATCH v2 0/7] Fix DM DAX handling Ross Zwisler
2018-05-29 19:51 ` [PATCH v2 1/7] fs: allow per-device dax status checking for filesystems Ross Zwisler
2018-05-29 19:51 ` [PATCH v2 2/7] dax: change bdev_dax_supported() to support boolean returns Ross Zwisler
2018-05-29 21:25   ` Darrick J. Wong
2018-05-29 22:01     ` Ross Zwisler
2018-05-31 19:13       ` Darrick J. Wong
2018-05-31 20:34         ` Ross Zwisler
2018-05-31 20:35         ` Dan Williams
2018-05-31 20:41         ` Ross Zwisler
2018-05-31 20:52         ` Mike Snitzer
2018-05-31 22:26           ` [dm-devel] " Darrick J. Wong
2018-06-01 20:59             ` Ross Zwisler
2018-06-01  1:26         ` Dave Chinner
2018-06-01  1:57           ` Dan Williams
2018-06-01  2:24             ` Dave Chinner
2018-06-01  4:02               ` Dan Williams
2018-06-03 22:20                 ` Dave Chinner
2018-06-04  0:25                   ` Dave Chinner
2018-06-04  1:48                     ` Dan Williams
2018-06-04 23:40                       ` Dan Williams
2018-06-05  0:33                         ` Mike Snitzer
2018-06-05  5:55                           ` Dave Chinner
2018-06-05  3:32                         ` Dan Williams
2018-05-29 19:51 ` [PATCH v2 3/7] dm: fix test for DAX device support Ross Zwisler
2018-06-01 20:19   ` Mike Snitzer
2018-06-01 20:46     ` Mike Snitzer [this message]
2018-06-01 21:11       ` Ross Zwisler
2018-06-01 21:16       ` Dan Williams
2018-05-29 19:51 ` [PATCH v2 4/7] dm: prevent DAX mounts if not supported Ross Zwisler
2018-06-01 21:55   ` Mike Snitzer
2018-06-04 23:15     ` Ross Zwisler
2018-06-20 15:17       ` Mike Snitzer
2018-06-25 19:20         ` Ross Zwisler
2018-05-29 19:51 ` [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode Ross Zwisler
2018-06-01 22:04   ` Mike Snitzer
2018-06-04 23:24     ` Ross Zwisler
2018-06-04 23:49       ` Kani, Toshi
2018-06-05  0:46       ` Mike Snitzer
2018-06-06 17:24         ` Ross Zwisler
2018-06-06 22:29           ` Mike Snitzer
2018-05-29 19:51 ` [PATCH v2 6/7] dm-snap: remove unnecessary direct_access() stub Ross Zwisler
2018-05-29 19:51 ` [PATCH v2 7/7] dm-error: " Ross Zwisler

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