Re: [rfc][patch 1/4] include: add callbacks to toggle reference counting for VM_MIXEDMAP pages

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

Nick Piggin wrote:
> You know that pfn_valid() can be changed at runtime depending on what
> your intentions are for that page. It can remain false if you don't
> want struct pages for it, then you can switch a flag...
We would'nt need to switch at runtime: it is sufficient to make that 
decision when the segment gets attched.

> I've just been looking at putting everything together (including the
> pte_special patch). 
Yippieh. I am going to try it out next :-).

> I still hit one problem with your required modification
> to the filemap_xip patch.
 >
> You need to unconditionally do a vm_insert_pfn in xip_file_fault, and rely
> on the pte bit to tell the rest of the VM that the page has not been
> refcounted. For architectures without such a bit, this breaks VM_MIXEDMAP,
> because it relies on testing pfn_valid() rather than a pte bit here.
> We can go 2 ways here: either s390 can make pfn_valid() work like we'd
> like; or we can have a vm_insert_mixedmap_pfn(), which has
> #ifdef __HAVE_ARCH_PTE_SPECIAL
> in order to do the right thing (ie. those architectures which do have pte
> special can just do vm_insert_pfn, and those that don't will either do a
> vm_insert_pfn or vm_insert_page depending on the result of pfn_valid).
Of those two choices, I'd cleary favor vm_insert_mixedmap_pfn(). But 
we can #ifdef __HAVE_ARCH_PTE_SPECIAL in vm_insert_pfn() too, can't 
we? We can safely set the bit for both VM_MIXEDMAP and VM_PFNMAP. Did 
I miss something?

> The latter I guess is more efficient for those that do implement pte_special,
> however if anything I would rather investigate that as an incremental patch
> after the basics are working. It would also break the dependency of the
> xip stuff on the pte_special patch, and basically make everything much
> more likely to get merged IMO.
I'll talk to Martin and see what he thinks. I really hate doing list 
walk in pfn_valid(), it just does'nt feel right.

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