linux-kernel - Re: [Bug #13319] Page allocation failures with b43 and p54usb

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Date:	Tue, 30 Jun 2009 11:24:53 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Johannes Berg <johannes@...solutions.net>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [Bug #13319] Page allocation failures with b43 and p54usb

Hi David,

On Tue, 30 Jun 2009, Pekka Enberg wrote:
>> > I'd disagree with disabling slub debugging by default for caches where
>> > oo_order(s->min) increases as the result of using it.  This particular
>> > page allocation failure is happening for, presumably, kmalloc-4096, and
>> > the system has 4K pages.  Disabling debugging for that cache (and any of
>> > its aliases) implicitly will lead to errors going undiagnosed as a result.
>>
>> Well, I obviously don't agree here because kmalloc-4096 debugging
>> causes problems in the real world.

On Tue, Jun 30, 2009 at 10:47 AM, David Rientjes<rientjes@...gle.com> wrote:
> I don't think CONFIG_SLUB_DEBUG_ON is generally the configuration used in
> the real world.

It is, hence the epic bug report that's eaten too many man hours
already! Look, we encourage _testers_ to turn all as much as debugging
options as possible so we catch bugs early. That why the only sane
defaults are the ones that don't cause other problems!

I don't know why you want to argue this. It's simply not an option to
say "stupid user, fix your config" in core code like the slab
allocator. Enabling CONFIG_SLUB_DEBUG_ON is a very reasonable thing to
do when you are a tester looking for bugs.

On Tue, 30 Jun 2009, Pekka Enberg wrote:
>> So we should fix SLUB debugging as outlined by Mel Gorman and
>> Christoph Lameter. I simply haven't had the time to do it. Patches are
>> welcome!

On Tue, Jun 30, 2009 at 10:47 AM, David Rientjes<rientjes@...gle.com> wrote:
> You're referring to `slub_debug=A'?  I think CONFIG_SLUB_DEBUG_ON should
> continue to enable debugging on all slab caches and in instances where it
> causes page allocation failures such in Larry's case because
> oo_order(s->min) with debugging on is greater than oo_order(s->min) with
> debugging off, you can emit a friendly warning in your recently added
> slab_out_of_memory() about using `slab_debug=-,<cache>'.
>
> We have a disagreement about which is the default behavior, but I would
> opt on the side of adding exemptions to a debug configuration option as
> opposed to requiring additional command line parameters to be fully
> enabled.

Yup, I was referring to slub_debug=A and no, I don't agree with you
that it should be on by default. Only people who know what they're
doing should enable the option and a random tester by definition
doesn't (no offence to Mr. Random Tester).

                                                   Pekka
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