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SubjectRe: [13/14] vcompound: Use vcompound for swap_map
On Fri, 21 Mar 2008, Andi Kleen wrote:

> > is larger then there is no way around the use of vmalloc.
>
> Have you considered the potential memory wastage from rounding up
> to the next page order now? (similar in all the other patches
> to change vmalloc). e.g. if the old size was 64k + 1 byte it will
> suddenly get 128k now. That is actually not a uncommon situation
> in my experience; there are often power of two buffers with
> some small headers.

Yes the larger the order the more significant the problem becomes.

> A long time ago (in 2.4-aa) I did something similar for module loading
> as an experiment to avoid too many TLB misses. The module loader
> would first try to get a continuous range in the direct mapping and
> only then fall back to vmalloc.
>
> But I used a simple trick to avoid the waste problem: it allocated a
> continuous range rounded up to the next page-size order and then freed
> the excess pages back into the page allocator. That was called
> alloc_exact(). If you replace vmalloc with alloc_pages you should
> use something like that too I think.

That trick is still in use for alloc_large_system_hash....

But cutting off the tail of compound pages would make treating them as
order N pages difficult. The vmalloc fallback situation is easy to deal
with.

Maybe we can think about making compound pages being N consecutive pages
of PAGE_SIZE rather than an order O page? The api would be a bit
different then and it would require changes to the page allocator. More
fragmentation if pages like that are freed.



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