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

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Message-Id: <201305100654.FCI28926.HOtMVOJLFFFSQO@I-love.SAKURA.ne.jp>
Date:	Fri, 10 May 2013 06:54:04 +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?

Christoph Lameter wrote:
> -	for (i = 1; i < PAGE_SHIFT + MAX_ORDER; i++) {
> +	for (i = 1; i =< KMALLOC_SHIFT_HIGH; i++) {

  mm/slab.c: In function 'init_node_lock_keys':
  mm/slab.c:568:17: error: expected expression before '<' token

What I'm worrying is:

  /*
   * 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.
   *
   * WARNING: Its not easy to increase this value since the allocators have
   * to do various tricks to work around compiler limitations in order to
   * ensure proper constant folding.
   */
  #define KMALLOC_SHIFT_HIGH      ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
                                  (MAX_ORDER + PAGE_SHIFT - 1) : 25)

  extern struct kmem_cache *kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];

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) ?
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