linux-kernel - Re: [PATCH 2/2] SLUB: Use GFP_PANIC for early-boot allocations

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Date:	Sat, 9 May 2009 10:30:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <cl@...ux-foundation.org>,
	akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	mel@....ul.ie, riel@...hat.com, linux-kernel@...r.kernel.org,
	rientjes@...gle.com
Subject: Re: [PATCH 2/2] SLUB: Use GFP_PANIC for early-boot allocations


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2009-05-08 at 20:15 +0400, Cyrill Gorcunov wrote:
> > [Peter Zijlstra - Fri, May 08, 2009 at 05:50:58PM +0200]
> > | On Fri, 2009-05-08 at 18:45 +0300, Pekka Enberg wrote:
> > | 
> > | > On Fri, 2009-05-08 at 17:42 +0200, Peter Zijlstra wrote:
> > | > > BUG_ON((gfp & __GFP_PANIC) && (system_state != STATE_BOOTING));
> > | > 
> > | > There's no technical reason not to use GFP_PANIC when system_state !=
> > | > STATE_BOOTING so I don't think it's needed. It's just that GFP_PANIC
> > | > (and BUG_ON) is IMHO too harsh for create_unique_id().
> > | 
> > | Shouldn't we handle every allocation failure after booting?
> > 
> > Definitely
> > 
> > | 
> > | I think it _is_ a bug to panic on allocation failures once we're
> > | running.
> > |
> > 
> > But Peter I believe there was no suggestion to use GFP_PANIC everywhere
> > to get rid of error handling. But rather to use it in case if kmalloc is
> > followed by BUG_ON.
> 
> Well, what I'm saying is that that either is a genuine bug we 
> should fix, or its boot code, which is exactly what my assertion 
> above tests for.

This is about boot code - and only about boot code.

This is a really simple thing. Replace existing x86 platform code 
early boot allocaton patterns of:

	x = alloc(GFP_KERNEL, ...);
	BUG_ON(!x);

	y = alloc(GFP_KERNEL, ...);
	BUG_ON(!y);

With:

	x = alloc(GFP_KERNEL | __GFP_PANIC, ...);

	y = alloc(GFP_KERNEL | __GFP_PANIC, ...);

it makes code shorter, smaller and easier to read. Nothing more, 
nothing less. And yes, i agree that we should disallow this after 
bootup has finished - but the boot-alloc case Cyrill and me would 
like to handle via this is still fully valid.

	Ingo
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