Messages in this thread | | | Date | Wed, 23 Nov 2011 15:06:37 -0800 (PST) | From | David Rientjes <> | Subject | Re: Fwd: Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()? |
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On Wed, 23 Nov 2011, zhihua che wrote:
> I know what you mean, that is, a slab can only store no more than > MAX_OBJS_PER_PAGE, actually 0x7FFF, objects. > > But get_order(size * MAX_OBJS_PER_PAGE) already returns the order > which reserves no_more_than size * MAX_OBJS_PER_PAGE memory. Right? >
Yes, but it reserves too much memory if the conditional is true.
> So I think there is no need to subtract one. >
If we didn't subtract one, then the order of a slab page would allow for _more_ than MAX_OBJS_PER_PAGE to be allocated and that's not allowed because of the restrictions in struct page.
Consider a page size of 4K and an object size of 8 bytes. get_order(8 * 32767) would be 6, so that's a 4K * 2^6 = 256K slab page without the subtraction and could allocate (256K * 1024 / 8) = 32768 which is greater than MAX_OBJS_PER_PAGE and not allowed.
So we subtract one so the compound slab page is guaranteed to allocate less than MAX_OBJS_PER_PAGE. | |