Andy Whitcroft wrote:
> The memory allocator buddy location algorithm has an implicit assumption
> that the memory map will be contigious and valid out to MAX_ORDER. ie
> that we can do relative arithmetic on a page* for a page to find its
> buddy at all times. The allocator never looks outside a MAX_ORDER
> block, aligned to MAX_ORDER in physical pages. SPARSEMEM's
> implementation by it nature breaks up the mem_map at the section size.
> Thus for the buddy to work a section must be >= MAX_ORDER in size to
> maintain the contiguity constraint.
thanks for the explanation. But still something I'm missing, how can a
MAX_ORDER block be allocated in a memory whose size is only 128Ko ?
Can't it be detected by the buddy allocatorvery early without doing any
relative arithmetic on a page* ?
> However, just because you have a small memory block in your memory map
> doesn't mean that the sparsemem section size needs to be that small to
> match. If there is any valid memory in any section that section will be
> instantiated and the valid memory marked within it, any invalid memory
> is marked reserved.
ah ok but that means that pfn_valid() will still returns ok for invalid page which
are in a invalid memory marked as reserved. Is it not risky ?
> The section size bounds the amount of internal
> fragmentation we can have in the mem_map. SPARSEMEM as its name
> suggests wins biggest when memory is very sparsly populate.
sorry but I don't understand. I would say that sparsemem section size should
be chosen to make mem_map[] and mem_section[] sizes as small as possible.
> If I am
> reading correctly your memory is actually contigious.
well there're big holes in address space.
thanks
Francis
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