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From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Mike Snitzer <snitzer@redhat.com>, Toshi Kani <toshi.kani@hpe.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>,
	dm-devel@redhat.com, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-xfs@vger.kernel.orgw
Subject: Re: [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode
Date: Mon, 4 Jun 2018 17:24:16 -0600	[thread overview]
Message-ID: <20180604232416.GB10666@linux.intel.com> (raw)
In-Reply-To: <20180601220443.GB18712@redhat.com>

On Fri, Jun 01, 2018 at 06:04:43PM -0400, Mike Snitzer wrote:
> On Tue, May 29 2018 at  3:51pm -0400,
> Ross Zwisler <ross.zwisler@linux.intel.com> wrote:
> 
> > The DM_TYPE_DAX_BIO_BASED dm_queue_mode was introduced to prevent DM
> > devices that could possibly support DAX from transitioning into DM devices
> > that cannot support DAX.
> > 
> > For example, the following transition will currently fail:
> > 
> >  dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw]
> > 	      DM_TYPE_DAX_BIO_BASED       DM_TYPE_BIO_BASED
> > 
> > but these will both succeed:
> > 
> >  dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw]
> >  	      DM_TYPE_DAX_BIO_BASED        DM_TYPE_BIO_BASED
> > 
> 
> I fail to see how this succeeds given
> drivers/md/dm-ioctl.c:is_valid_type() only allows transitions from:
> 
> DM_TYPE_BIO_BASED => DM_TYPE_DAX_BIO_BASED

Right, sorry, that was a typo.  What I meant was:

> For example, the following transition will currently fail:
> 
>  dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw]
>               DM_TYPE_DAX_BIO_BASED       DM_TYPE_BIO_BASED
> 
> but these will both succeed:
> 
>  dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw]
>                 DM_TYPE_BIO_BASED        DM_TYPE_BIO_BASED
> 
>  dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem]
>                 DM_TYPE_BIO_BASED        DM_TYPE_DAX_BIO_BASED

So we allow 2 of the 3 transitions, but the reason that we disallow the third
isn't fully clear to me.

> >  dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem]
> >  		DM_TYPE_BIO_BASED        DM_TYPE_DAX_BIO_BASED
> > 
> > This seems arbitrary, as really the choice on whether to use DAX happens at
> > filesystem mount time.  There's no guarantee that the in the first case
> > (double fsdax pmem) we were using the dax mount option with our file
> > system.
> > 
> > Instead, get rid of DM_TYPE_DAX_BIO_BASED and all the special casing around
> > it, and instead make the request queue's QUEUE_FLAG_DAX be our one source
> > of truth.  If this is set, we can use DAX, and if not, not.  We keep this
> > up to date in table_load() as the table changes.  As with regular block
> > devices the filesystem will then know at mount time whether DAX is a
> > supported mount option or not.
> 
> If you don't think you need this specialization that is fine.. but DM
> devices supporting suspending (as part of table reloads) so is there any
> risk that there will be inflight IO (say if someone did 'dmsetup suspend
> --noflush').. and then upon reload the device type changed out from
> under us.. anyway, I don't have all the PMEM DAX stuff paged back into
> my head yet.
> 
> But this just seems like we really shouldn't be allowing the
> transition from what was DM_TYPE_DAX_BIO_BASED back to DM_TYPE_BIO_BASED

I admit I don't fully understand all the ways that DM supports suspending and
resuming devices.  Is there actually a case where we can change out the DM
devices while I/O is running, and somehow end up trying to issue a DAX I/O to
a device that doesn't support DAX?

Toshi, do you have a test case that shows this somehow?

  reply	other threads:[~2018-06-04 23:24 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
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 [this message]
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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