linux-kernel - RE: [PATCH 1/5] LLVMLinux: Correct size_index table before replacing the bootstrap kmem_cache

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Date:	Wed, 4 Feb 2015 20:38:56 +0000
From:	Daniel Sanders <Daniel.Sanders@...tec.com>
To:	Pekka Enberg <penberg@....fi>
CC:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/5] LLVMLinux: Correct size_index table before
 replacing the bootstrap kmem_cache_node.

> -----Original Message-----
> From: Pekka Enberg [mailto:penberg@....fi]
> Sent: 04 February 2015 19:33
> To: Daniel Sanders
> Cc: Christoph Lameter; Pekka Enberg; David Rientjes; Joonsoo Kim; Andrew
> Morton; linux-mm@...ck.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH 1/5] LLVMLinux: Correct size_index table before
> replacing the bootstrap kmem_cache_node.
> 
> On 2/3/15 3:37 PM, Daniel Sanders wrote:
> > This patch moves the initialization of the size_index table slightly
> > earlier so that the first few kmem_cache_node's can be safely allocated
> > when KMALLOC_MIN_SIZE is large.
> 
> The patch looks OK to me but how is this related to LLVM?
>
> - Pekka

I don't believe the bug to be LLVM specific but GCC doesn't normally encounter the problem. I haven't been able to identify exactly what GCC is doing better (probably inlining) but it seems that GCC is managing to optimize  to the point that it eliminates the problematic allocations. This theory is supported by the fact that GCC can be made to fail in the same way by changing inline, __inline, __inline__, and __always_inline in include/linux/compiler-gcc.h such that they don't actually inline things.
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