[RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

[David Hildenbrand (Arm)]

>> This makes a *ton* of sense in effective_prot() especially. Its entire
>> job is mirroring the hardware's job of inheriting permissions from
>> higher levels of the page tables and enforcing them on lower level leaf
>> entries. If a higher level is folded, there's nothing to inherit.
>>
>> I also think it's worth taking a brief pause on the coding to think
>> about what kind of design would actually be nice here. If the design
>> really is that the pgd is folded, the 'struct mm_walk_ops' code would
>> ideally not even call ->pgd_entry(). It would just (for example) *start*
>> at ->pud_entry() for a 3-level hardware page table.
>>
>> If there are no pgds, why bother calling ->pgd_entry()? It couldn't be
>> done transparently to mm_walk users of course but it could be done
>> incrementally where users move one at a time over to a new scheme.
> 
> Agree. Calling a pXd_entry() about folded one seems meaningless.
> But seems enough to check mm_pXd_folded() before calling pXd_entry()
> in each walk_pXd_range().

Needs some double-checking that callers can handle it (like effective_prot()
would currently not), but certainly something to look into!

-- 
Cheers,

David