linux-kernel - Re: [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue

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Date:   Mon, 4 Jun 2018 20:46:28 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Toshi Kani <toshi.kani@....com>, dm-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode

On Mon, Jun 04 2018 at  7:24pm -0400,
Ross Zwisler <ross.zwisler@...ux.intel.com> wrote:

> 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@...ux.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?

Yes, provided root permissions, it's very easy to dmsetup suspend/load/resume
to replace any portion of the DM device's logical address space to map to an
entirely different DM target (with a different backing store).  It's
pretty intrusive to do such things, but easily done and powerful.

Mike