linux-kernel - Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME

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Date:	Wed, 16 Jul 2014 10:38:27 -0700
From:	"hch@...radead.org" <hch@...radead.org>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	"hch@...radead.org" <hch@...radead.org>,
	KY Srinivasan <kys@...rosoft.com>,
	James Bottomley <jbottomley@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16

On Wed, Jul 16, 2014 at 11:44:18AM -0400, Martin K. Petersen wrote:
> There are lots of devices out there that support WRITE SAME(10) or (16)
> without the UNMAP bit. And there are devices that support WRITE SAME w/
> UNMAP functionality but not "regular" WRITE SAME.

Oh, we actually have devices that support WRITE SAME with unmap, but not
without?  That's defintively a little strange.

> no_write_same is there to prevent the REQ_WRITE_SAME use case (for which
> we have really weak heuristics). Your patch overloads no_write_same so
> it also governs a REQ_DISCARD use case.

Yes, and it did this intentionally.  I really wouldn't expect devices
to support WRITE SAME with UNMAP but blow up on a WRITE SAME without
it (and not just simple fail it in an orderly way).

> My proposed black list patch fixes the hyperv discard issue. So I don't
> see why we'd need to overload no_write_same which was meant for an
> entirely different purpose.

It definitively seems odd to default to trying WRITE SAME for unmap
for a device that explicitly tells us that it doesn't support WRITE
SAME.

Note that I'm not against your patch - I suspect forcing us to read
EVPD pages even for devices that claim to be SPC-2 will come in useful
in various scenarios.
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