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SubjectRe: [PATCH] fix GFP zone modifier interators
--On 24 June 2004 15:23 -0700 Andrew Morton <akpm@osdl.org> wrote:

> Andy Whitcroft <apw@shadowen.org> wrote:
>>
>> For each node there are a defined list of MAX_NR_ZONES zones.
>> These are selected as a result of the __GFP_DMA and __GFP_HIGHMEM
>> zone modifier flags being passed to the memory allocator as part of
>> the GFP mask. Each node has a set of zone lists, node_zonelists,
>> which defines the list and order of zones to scan for each flag
>> combination. When initialising these lists we iterate over
>> modifier combinations 0 .. MAX_NR_ZONES. However, this is only
>> correct when there are at most ZONES_SHIFT flags. If another flag
>> is introduced zonelists for it would not be initialised.
>
> I don't get it. If you were going to add a new zone, identified by
> __GFP_WHATEVER then you'd need to increase MAX_NR_ZONES
> anyway, wouldn't you?
>
> I'm sure you're right, but I haven't worked on this stuff in months and
> it's obscure. Care to explain a little more?

If you added a new zone you would increase MAX_NR_ZONES from 3 to 4, you
would add __GFP_NEWONE as 0x4 as those are bit flags and GFP_ZONEMASK to
0x7. Now to build the zonelists we need to scan from 0-7 in 'Zone
Modifier' space to cover all the combinations, but MAX_NR_ZONES is only 4.
So we don't build the zonelists for them.

There is a question of whether we should be scanning 0..MAX_NR_ZONES and
assuming the selector is 1<<N. That would mean that there would be no
support for the use of more than one such 'Zone Modifier' at a time.
Currently there is no such usage. My gut feeling is to not rule them out
and to build the zonelists for all combinations (even if they are empty).

>> This patch introduces GFP_ZONEMODS (based on GFP_ZONEMASK) as a
>> bound for the number of modifier combinations.
>
> The "ZONEMODS" identifier doesn't really grab me. ZONETYPES, or
> something?

Zone types is fine with me. I took the name from the comments in mmzone.h,
I have no attachment to it.

> Either way, please add a big fat comment over it, explaining to the poor
> reader what its semantic meaning is.

I'll add some more commentary and see how it looks.

-apw


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