On 12/12/2013 09:46 AM, Christoph Lameter wrote: > On Wed, 11 Dec 2013, Dave Hansen wrote: >> The write-argument to cmpxchg_double() must be 16-byte aligned. >> We used to align 'struct page' itself in order to guarantee this, >> but that wastes 8-bytes per page. Instead, we take 8-bytes >> internal to the page before page->counters and move freelist >> between there and the existing 8-bytes after counters. That way, >> no matter how 'stuct page' itself is aligned, we can ensure that >> we have a 16-byte area with which to to this cmpxchg. > > Well this adds additional branching to the fast paths.
I don't think it *HAS* to inherently. The reason here is really that we swap the _order_ of the arguments to the cmpxchg() since their order in memory changes. Essentially, we do: | flags | freelist | counters | | | flags | | counters | freelist | I did this so I wouldn't have to make a helper for ->counters. But, if we also move counters around, we can do: | flags | counters | freelist | | | flags | | counters | freelist | I believe we can do that all with plain pointer arithmetic and masks so that it won't cost any branches. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
