Re: [rfc][patch 2/2] xip: support non-struct page memory

[Carsten Otte <cotte@de.ibm.com>]

Nick Piggin wrote:
>>> AFAIK, sparsemem keeps track of all sections for pfn_valid(), which would
>>> work. Any plans to convert s390 to it? ;)
>> I think vmem_map is superior to sparsemem, because a 
>> single-dimensional mem_map array is faster work with (single step 
>> lookup). And we've got plenty of virtual address space for the 
>> vmem_map array on 64bit.
> 
> But it doesn't still retain sparsemem sections behind that? Ie. so that
> pfn_valid could be used? (I admittedly don't know enough eabout the memory
> model code).
Not as far as I know. But arch/s390/mm/vmem.c has:

struct memory_segment {
         struct list_head list;
         unsigned long start;
         unsigned long size;
};

static LIST_HEAD(mem_segs);

This is maintained every time we map a segment/unmap a segment. And we 
could add a bit to struct memory_segment meaning "refcount this one". 
This way, we could tell core mm whether or not a pfn should be refcounted.

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