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

[Christoph Lameter]

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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