linux-kernel - Re: [linux-next-20130422] Bug in SLAB?

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Date:	Tue, 30 Apr 2013 17:27:53 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
cc:	glommer@...allels.com, penberg@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [linux-next-20130422] Bug in SLAB?


On Wed, 1 May 2013, Tetsuo Handa wrote:

> Christoph Lameter wrote:
> > On Tue, 30 Apr 2013, Tetsuo Handa wrote:
> >
> > > The testcases still trigger BUG() at 32M:
> >
> > I thought we established that MAX_ORDER only allows a maximum of 8M sized
> > allocations? Why are you trying 32M?
>
> Only for regression testing. At least until Linux 3.9, requesting too large
> size didn't trigger oops, did it? I'm not expecting kmalloc() to trigger oops
> for Linux 3.10 and future kernels.

It did for SLUB. SLAB returned NULL for some cases.

> "kmalloc() returning NULL for these allocations" is needed by "try kmalloc()
> first, fallback to vmalloc()" allocation. There are kernel modules which expect
> kmalloc() to return NULL rather than oops when the requested size is larger
> than KMALLOC_MAX_SIZE bytes. If kmalloc() suddenly starts triggering oops, such
> modules will break.

This behavior has been in there for years. Why try a kmalloc that
always fails since the size is too big?

> Anyway, there is a regression we want to fix : we won't be able to boot
> Linux 3.10-rc1 for x86_32 built with CONFIG_DEBUG_SLAB=y &&
> CONFIG_DEBUG_SPINLOCK=y && CONFIG_DEBUG_PAGEALLOC=y .
> ("Fix off by one error in slab.h" did not fix the regression.)

Hmm... Where does this fail? In slab?


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