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

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Date:	Fri, 10 May 2013 21:38:47 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	cl@...ux.com
Cc:	glommer@...allels.com, penberg@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [linux-next-20130422] Bug in SLAB?

Tetsuo Handa wrote:
> Can we manage with allocating only 26 elements when MAX_ORDER + PAGE_SHIFT > 26
> (e.g. PAGE_SIZE == 256 * 1024) ?
> 
> Can kmalloc_index()/kmalloc_size()/kmalloc_slab() etc. work correctly when
> MAX_ORDER + PAGE_SHIFT > 26 (e.g. PAGE_SIZE == 256 * 1024) ?
> 
Today I compared SLAB/SLUB code. If I understood correctly, the line

  if (size <=  64 * 1024 * 1024) return 26;

in kmalloc_index() is redundant (in fact, kmalloc_caches[26] is out of range)
and conflicts with what the comment

  * The largest kmalloc size supported by the SLAB allocators is
  * 32 megabyte (2^25) or the maximum allocatable page order if that is
  * less than 32 MB.

says, and 0 <= kmalloc_index() <= 25 is always true for SLAB and
0 <= kmalloc_index() <= PAGE_SHIFT+1 is always true for SLUB.

Therefore, towards 3.10-rc1,

> > -	for (i = 1; i < PAGE_SHIFT + MAX_ORDER; i++) {
> > +	for (i = 1; i =< KMALLOC_SHIFT_HIGH; i++) {
> 
-+	for (i = 1; i =< KMALLOC_SHIFT_HIGH; i++) {
++	for (i = 1; i <= KMALLOC_SHIFT_HIGH; i++) {

would be the last fix for me. (I don't know why kmalloc_caches[0] is excluded.)
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