[RFC PATCH v2 14/32] x86: mm: Provide support to use memblock when spliting large pages

[Paolo Bonzini]

On 17/03/2017 11:17, Borislav Petkov wrote:
> 
>> I also don't really like the patch as is (plus it fails modpost), but
>> IMO reusing __change_page_attr and __split_large_page is the right thing
>> to do.
> 
> Right, so teaching pageattr.c about memblock could theoretically come
> around and bite us later when a page allocated with memblock gets freed
> with free_page().

Theoretically or practically?

> And looking at this more, we have all this kernel pagetable preparation
> code down the init_mem_mapping() call and the pagetable setup in
> arch/x86/mm/init_{32,64}.c

It only looks at the E820 map, doesn't it?  Why does it have to do
anything with percpu memory areas?

Paolo

> And that code even does some basic page splitting. Oh and it uses
> alloc_low_pages() which knows whether to do memblock reservation or the
> common __get_free_pages() when slabs are up.
> 
> So what would be much cleaner, IMHO, is if one would reuse that code to
> change init_mm.pgd mappings early without copying pageattr.c.
> 
> init_mem_mapping() gets called before kvm_guest_init() in setup_arch()
> so the guest would simply fixup its pagetable right there.