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SubjectRe: Generic approach to customizable zones - was: Re: [PATCH v7 0/7] Introduce ZONE_CMA
I didn't read this thoughly yet because I will be travelling shortly but
this point alone just made ask, because it seems there is some

On Fri 28-04-17 11:04:27, Igor Stoppa wrote:
> * if one is happy to have a 64bits type, allow for as many zones as
> it's possible to fit, or anyway more than what is possible with
> the 32 bit mask.

zones are currently placed in struct page::flags. And that already is
64b size on 64b arches. And we do not really have any room spare there.
We encode page flags, zone id, numa_nid/sparse section_nr there. How can
you add more without enlarging the struct page itself or using external
means to store the same information (page_ext comes to mind)? Even if
the later would be possible then note thatpage_zone() is used in many
performance sensitive paths and making it perform well with special
casing would be far from trivial.
Michal Hocko

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