Messages in this thread | | | Date | Thu, 24 Nov 2011 12:44:04 +0800 | Subject | Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()? | From | zhihua che <> |
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2011/11/24 David Rientjes <rientjes@google.com>: > 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.
Oh, sorry to bother you ... I guess I calculated a wrong result. I repeat your and my example carefully and you're right.
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