Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

[Vlastimil Babka]

On 8/26/21 10:02, Mike Rapoport wrote:
> On Mon, Aug 23, 2021 at 04:50:10PM -0700, Dave Hansen wrote:
>> On 8/23/21 6:25 AM, Mike Rapoport wrote:
>> >  void ___pte_free_tlb(struct mmu_gather *tlb, struct page *pte)
>> >  {
>> > +	enable_pgtable_write(page_address(pte));
>> >  	pgtable_pte_page_dtor(pte);
>> >  	paravirt_release_pte(page_to_pfn(pte));
>> >  	paravirt_tlb_remove_table(tlb, pte);
>> > @@ -69,6 +73,7 @@ void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
>> >  #ifdef CONFIG_X86_PAE
>> >  	tlb->need_flush_all = 1;
>> >  #endif
>> > +	enable_pgtable_write(pmd);
>> >  	pgtable_pmd_page_dtor(page);
>> >  	paravirt_tlb_remove_table(tlb, page);
>> >  }
>> 
>> I'm also cringing a bit at hacking this into the page allocator.   A
>> *lot* of what you're trying to do with getting large allocations out and
>> splitting them up is done very well today by the slab allocators.  It
>> might take some rearrangement of 'struct page' metadata to be more slab
>> friendly, but it does seem like a close enough fit to warrant investigating.
> 
> I thought more about using slab, but it seems to me the least suitable
> option. The usecases at hand (page tables, secretmem, SEV/TDX) allocate in
> page granularity and some of them use struct page metadata, so even its
> rearrangement won't help. And adding support for 2M slabs to SLUB would be
> quite intrusive.

Agree, and there would be unnecessary memory overhead too, SLUB would be happy
to cache a 2MB block on each CPU, etc.

> I think that better options are moving such cache deeper into buddy or
> using e.g. genalloc instead of a list to deal with higher order allocations. 
> 
> The choice between these two will mostly depend of the API selection, i.e.
> a GFP flag or a dedicated alloc/free.

Implementing on top of buddy seem still like the better option to me.