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SubjectRe: The IBM order relaxation patch
Date
On February 6, 2002 10:50 pm, Ulrich Weigand wrote:
> Pete Zaitcev wrote:
> >It's a stupid question, but: why can we not simply
> >wait until a desired unfragmented memory area is available,
> >with a GPF flag? What they describe does not happen in an
> >interrupt context, so we can sleep.
>
> Because nobody even *tries* to free adjacent pages to build up
> a free order-2 area. You could wait really long ...
>
> This looks hard to fix with the current mm layer. Maybe Rik's
> rmap method could help here, because with reverse mappings we
> can at least try to free adjacent areas (because we then at least
> *know* who's using the pages).

Yes, that's one of leading reasons for wanting rmap. (Number one and two
reasons are: allow forcible unmapping of multiply referenced pages for
swapout; get more reliable hardware ref bit readings.)

Note that even if we can do forcible freeing we still have to deal with the
issue of fragmentation due to pinned pages, e.g., slab cache, admittedly a
rarer problem.

--
Daniel
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