linux-kernel - Re: [RFC] SLUB - define OO_ macro instead of hardcoded numbers

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Date:	Wed, 22 Oct 2008 22:42:12 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] SLUB - define OO_ macro instead of hardcoded numbers

[Christoph Lameter - Wed, Oct 22, 2008 at 11:24:58AM -0700]
> On Wed, 22 Oct 2008, Cyrill Gorcunov wrote:
>
>> [Christoph Lameter - Wed, Oct 22, 2008 at 11:10:56AM -0700]
>>> On Wed, 22 Oct 2008, Cyrill Gorcunov wrote:
>>>
>>>> +#define OO_SHIFT	16
>>>> +#define OO_MASK		((1 << OO_SHIFT) - 1)
>>>> +#define MAX_OBJS_PER_PAGE	65535 /* see struct page.objects */
>>>
>>> This is == OO_MASK right? Otherwise things will break when we change
>>> OO_SHIFT.
>>>
>>
>> Yes, I set it 65535 directly to distinguish it from OO_MASK
>> meaning not value and point to page.objects since they are
>> u16. Which meant that is the point where all limits start.
>> So it we set OO_SHIFT to say 14 it will not be a problem
>> but if we exceed 16 bits it will break SLUB. Am I right?
>> (I become scratching the head :)
>
> If you set it > 16 then the size of the field in struct page is violated.
>
> So
>
> #define MAX_OBJ_PER_PAGE MIN(1 << bits_in(page.objects) - 1, OO_MASK)
>
> ?
>
>

I think the patch becomes more complicated as it should be.
Maybe just use:

#define MAX_OBJ_PER_PAGE 65535 /* since page.objects is u16 */

		- Cyrill -
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