RE: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: James Bottomley [mailto:jbottomley@parallels.com]
> Sent: Wednesday, July 9, 2014 3:27 PM
> To: KY Srinivasan
> Cc: linux-kernel@vger.kernel.org; mkp@mkp.net; hch@infradead.org;
> devel@linuxdriverproject.org; apw@canonical.com; stable@vger.kernel.org;
> linux-scsi@vger.kernel.org; ohering@suse.com; jasowang@redhat.com
> Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16
> 
> On Wed, 2014-07-09 at 21:14 +0000, KY Srinivasan wrote:
> >
> > > -----Original Message-----
> > > From: James Bottomley [mailto:jbottomley@parallels.com]
> > > Sent: Wednesday, July 9, 2014 12:57 PM
> > > To: KY Srinivasan
> > > Cc: linux-kernel@vger.kernel.org; hch@infradead.org;
> > > devel@linuxdriverproject.org; apw@canonical.com;
> > > stable@vger.kernel.org; linux-scsi@vger.kernel.org;
> > > ohering@suse.com; jasowang@redhat.com
> > > Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter
> > > WRITE_SAME_16
> > >
> > > On Wed, 2014-07-09 at 19:52 +0000, KY Srinivasan wrote:
> > > >
> > > > > -----Original Message-----
> > > > > From: Christoph Hellwig [mailto:hch@infradead.org]
> > > > > Sent: Wednesday, July 9, 2014 1:43 AM
> > > > > To: KY Srinivasan
> > > > > Cc: linux-kernel@vger.kernel.org; devel@linuxdriverproject.org;
> > > > > ohering@suse.com; jbottomley@parallels.com;
> jasowang@redhat.com;
> > > > > apw@canonical.com; linux-scsi@vger.kernel.org;
> > > > > stable@vger.kernel.org
> > > > > Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter
> > > > > WRITE_SAME_16
> > > > >
> > > > > On Tue, Jul 08, 2014 at 05:46:48PM -0700, K. Y. Srinivasan wrote:
> > > > > > Host does not handle WRITE_SAME_16; filter this command out.
> > > > > > This patch is required to handle large devices (greater than 2 TB
> disks).
> > > > >
> > > > > Storvsc already sets the no_write_same flag, where is the
> > > > > command coming from?
> > > >
> > > > In spite of this flag,  I see WRITE_SAME_16 being issued when I
> > > > format a device bigger than 2 TB; I tried both xfs and ext4.
> > > > Windows hosts currently do not handle unsupported commands
> > > > correctly - The information returned is not sufficient to effect recovery
> in the Linux guest.
> > > While this may be addressed in future hosts, this patch fixes the problem.
> > >
> > > What Christoph means is that this looks like a bug somewhere in SCSI
> itself.
> > > That means we need to find it and kill it, not add workarounds to
> > > every driver that sets no_write_same ...
> >
> > James,
> >
> > I will try to isolate this issue in the SCSI stack. If it is ok with
> > you guys, I would still want to filter WRITE_SAME_16 (as we currently
> > do WRITE_SAME) in our driver since this would address the problem for a
> large number of customers on our platform.
> 
> If we fix it at source, why would there be any need to filter?  That's the
> reason the no_write_same flag was introduced.  If we can find and fix the
> bug, it can go back into the stable trees as a bug fix, hence nothing should
> ever emit write_same(10 or 16) and additional driver code is redundant (and
> counter productive, since if this ever breaks again you're our best canary).
> 
> This looks like it might be the problem but Martin should confirm (I think the
> problem comes to us from the RC16 code which unconditionally sets WS16).

Ok; I am concerned about older kernels that do not have no_write_same flag.
I suppose I can work directly with these Distros and give them a choice: either implement
the no_write_same flag or filter the command in our driver. I will not include this patch when
I resubmit this patch-set.

Thanks,

K. Y