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

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Date:	Wed, 22 Oct 2008 11:24:58 -0700 (PDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] SLUB - define OO_ macro instead of hardcoded numbers

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)

?


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