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

Andy Lutomirski
2021-08-24T03:34:37Z

On Mon, Aug 23, 2021, at 4:50 PM, 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 would expected this to have leveraged the pte_offset_map/unmap() code
> to enable/disable write access. Granted, it would enable write access
> even when only a read is needed, but that could be trivially fixed with
> having a variant like:
>
> pte_offset_map_write()
> pte_offset_unmap_write()

I would also like to see a discussion of how races in which multiple threads or CPUs access ptes in the same page at the same time.

>
> in addition to the existing (presumably read-only) versions:
>
> pte_offset_map()
> pte_offset_unmap()
>
> Although those only work for the leaf levels, it seems a shame not to to
> use them.
>
> 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.
>