Re: [patch 2/2] mm: handle unaligned zones

[Mel Gorman <mel@csn.ul.ie>]

On Sun, 21 May 2006, Nick Piggin wrote:

> Nick Piggin wrote:
>> Andrew Morton wrote:
>> 
>>> How about just throwing the pages away?  It sounds like a pretty rare
>>> problem.
>> 
>> 
>> Well that's what many architectures will end up doing, yes. But on
>> small or embedded platforms, 4MB - 1 is a whole lot of memory to be
>> throwing away.
>> 
>> Also, I'm not sure it is something we can be doing in generic code,
>> because some architectures apparently have very strange zone setups
>> (eg. zones from several pages interleaved within a single zone's
>> ->spanned_pages). So it doesn't sound like a simple matter of trying
>> to override the zones' intervals.
>
> Oh I see, yeah I guess you could throw away the pages forming the
> present fraction of the MAX_ORDER buddy...
>

As Andy points out in another thread, the need to check unaligned zones is 
heavily relaxed (if not redundant) once the node_mem_map is aligned by 
patch "[PATCH 1/2] Align the node_mem_map endpoints to a MAX_ORDER 
boundary".

Once the node_mem_map is aligned, we know that we'll be checking a valid 
struct page. If the zones are not aligned, the unused struct pages forming 
the absent fraction of the MAX_ORDER buddy will be marked reserved since 
memmap_init_zone(). This will be caught by free_pages_check() and the 
buddies will not be merged.

I don't think there is any need to do these complex zone boundary checks 
once the node_mem_map is aligned for CONFIG_FLAT_NODE_MEM_MAP and 
SPARSEMEM already gets this right.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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