Date:	Wed, 30 Jun 2010 11:26:48 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	axboe@...nel.dk, linux-scsi@...r.kernel.org,
	martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/2] block: fix leaks associated with discard request
 payload

On Wed, 2010-06-30 at 11:36 -0400, Mike Snitzer wrote:
> On Wed, Jun 30 2010 at 10:22am -0400,
> James Bottomley <James.Bottomley@...e.de> wrote:
> 
> > On Tue, 2010-06-29 at 20:11 -0400, Mikulas Patocka wrote:
> > > > > If the layering violation spans only scsi code, it can be eventually 
> > > > > fixed, but this, much worse "layering violation" that will be spanning all 
> > > > > block device midlayers, won't ever be fixed.
> > > > > 
> > > > > Imagine for example --- a discard request arrivers at a dm-snapshot 
> > > > > device. The driver splits it into chunks, remaps each chunk to the 
> > > > > physical chunk, submits the requests, the elevator merges adjacent 
> > > > > requests and submits fewer bigger requests to the device. Now, if you had 
> > > > > to allocate a zeroed page each time you are splitting the request, that 
> > > > > would exhaust memory and burn cpu needlessly. You delete a 100MB file? --- 
> > > > > fine, allocate a 100MB of zeroed pages.
> > > > 
> > > > This is a straw man:  You've tried to portray a position I've never
> > > > taken as mine then attack it ... with what is effectively another bogus
> > > > argument.
> > > >
> > > > It's not an either/or choice.
> > > 
> > > It is either/or choice. If the interface isn't fixed NOW, the existing 
> > > flawed zeroed-page-allocation interface gets into RHEL
> > 
> > That's a false dichotomy.  You might see an either apply this hack now
> > or support the interface choice with RHEL, but upstream has the option
> > to fix stuff correctly.  RHEL has never needed my blessing to apply
> > random crap to their kernel before ... why is this patch any different?
> > 
> > > and I and others will have to support it for 7 years.
> > 
> > It's called a business model ... I believe it's what they pay you for.
> > 
> > > > I've asked the relevant parties to
> > > > combine the approaches and see if a REQ_TYPE_FS path that does the
> > > > allocations in the appropriate place, likely the ULD, produces a good
> > > > design.
> > > 
> > > OK, but before you do this research, fix the interface.
> > 
> > So even in the RHEL world, I think you'd find that analysing the problem
> > *before* comping up with a fix is a good way of doing things.
> > 
> > > > > So I say --- let there be a layering violation in the scsi code, but don't 
> > > > > put this problem with a page allocation to all the other bio midlayer 
> > > > > developers.
> > > > 
> > > > Thanks for explaining that you have nothing to contribute, I'll make
> > > > sure you're not on my list of relevant parties.
> > > 
> > > You misunderstand what I meant. You admit that there are design problems 
> > > in SCSI.
> > 
> > No I didn't.
> > 
> > And the rest of this rubbish is based on that false premise.  It might
> > help you to take off your SCSI antipathy and see this as a system
> > problem: it actually originates in block and spills out from there.
> > Thus it requires a system solution.
> 
> As fun as it is for the others monitoring these lists to see redhat.com
> vs suse.de banter I think framing this discussion like you (and Mikulas)
> continue to do is a complete distraction.

Well, it's not SUSE v Red Hat, it's upstream v Enterprise ... and it's
partly my job to explain why upstream does correct fixes not enterprise
workarounds (whether the enterprise is RHEL or SLES).  But I agree it's
becoming a pointless distraction.

> I tried to elevate (and defuse) the discussion yesterday.  But simply
> put: patches speak volumes.  I look forward to working with Tomo, hch
> and anyone else who has something to contribute that moves us toward a
> real fix for discards.

Right, so thanks for that.

Most of our problem is tied up in the fact that we need to allocate in
the prepare path, but we don't have a corresponding clear unprepare path
to do the deallocation in.  Introducing that into block might sort out
this tangle better ... error handling on the backend is very convoluted
and we can't really free the page until after it's complete (and the
->done function doesn't mark completion of error handling terms).  I
think the bones of a solution to this might be that
scsi_unprep_request() needs to call into block (instead of setting the
flags itself), say blk_unprep_request.  Block also needs to call
blk_unprep_request based on the REQ_DONTPREP status in its completion
path.  This would then give us a hook to hang the deallocation correctly
on.

James


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