linux-kernel - Re: block: correctly fallback for zeroout

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Date:	Thu, 2 Jun 2016 23:54:15 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>, Shaohua Li <shli@...com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	sitsofe@...oo.com, axboe@...com, Kernel-team@...com
Subject: Re: block: correctly fallback for zeroout

On Thu, Jun 02 2016 at 11:06pm -0400,
Martin K. Petersen <martin.petersen@...cle.com> wrote:

> >>>>> "Christoph" == Christoph Hellwig <hch@...radead.org> writes:
> 
> Christoph> As part of that I also removed the strange EOPNOTSUPP ignore,
> Christoph> but Mike reverted it just because it changed something in the
> Christoph> dm testsuite.
> 
> Mike?

Yes? ;)

Seems there is some serious confusion going on here.  The entirety of
hch's post (to which you quoted a subset) makes little sense to me.

shli's patch builds ontop of latest blk-lib.c code yet hch said this::
"We've split blkdev_issue_discard into __blkdev_issue_discard and a
small wrapper around in for 4.7, so this will need a bit of an update."

And hch never "removed the strange EOPNOTSUPP ignore".  He preserved it
(see his commit 38f25255330's "return ret != -EOPNOTSUPP ? ret : 0;"
that I adjusted in commit bbd848e0f -- _and_ he expanded it to eat the
early return that I restored).

So I can only infer that hch is still missing why my revert fixes
historic blkdev_issue_discard() behavior that his commit regressed.
Please read commit bbd848e0f's header.  That at least details the early
vs late -EOPNOTSUPP blkdev_issue_discard() return.

But all that nuance aside, AFAICT my commit bbd848e0f ("block: reinstate
early return of -EOPNOTSUPP from blkdev_issue_discard") really has
_nothing_ to do with the issue shli is addressing with his fix.

> Christoph> I still believe we should never ignore it in this helper, and
> Christoph> only do so in callers that believe it's the right thing.
> 
> Yeah.

Hmm...

You agreed to what hch said there about how we should probably always
return EOPNOTSUPP but then you immediately elaborated with details that
mean you don't agree:

> I really wish EOPNOTSUPP would just go away except for ioctl callers.
> Now that we have real bi_error I don't understand why we need it.

But hch was originally in favor of _always_ dropping EOPNOTSUPP on the
floor (that is what his commit 38f25255330 did).  Then he said he
disagrees with these interfaces playing games with masking EOPNOTSUPP --
to which you seemingly really don't agree.  Unless I'm completely
misreading you.

Anyway, shli is at least making it so that blkdev_issue_zerout() can
fallback to other mechanisms as needed.