Re: [PATCH v2 12/14] arm64/mm: Wire up PTE_CONT for user mappings

Ryan Roberts
2023-11-21T15:15:05Z

On 21/11/2023 11:22, Alistair Popple wrote:
>
> Ryan Roberts <ryan.roberts@arm.com> writes:
>
> [...]
>
>> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
>> + pte_t *ptep, pte_t pte, bool fold)
>> +{
>> + struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
>> + unsigned long start_addr;
>> + pte_t *start_ptep;
>> + int i;
>> +
>> + start_ptep = ptep = contpte_align_down(ptep);
>> + start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>> + pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
>> + pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
>> +
>> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
>> + pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
>> +
>> + if (pte_dirty(ptent))
>> + pte = pte_mkdirty(pte);
>> +
>> + if (pte_young(ptent))
>> + pte = pte_mkyoung(pte);
>> + }
>> +
>> + __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
>> +
>> + __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
>> +}
>> +
>> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>> + pte_t *ptep, pte_t pte)
>> +{
>> + /*
>> + * We have already checked that the virtual and pysical addresses are
>> + * correctly aligned for a contpte mapping in contpte_try_fold() so the
>> + * remaining checks are to ensure that the contpte range is fully
>> + * covered by a single folio, and ensure that all the ptes are valid
>> + * with contiguous PFNs and matching prots. We ignore the state of the
>> + * access and dirty bits for the purpose of deciding if its a contiguous
>> + * range; the folding process will generate a single contpte entry which
>> + * has a single access and dirty bit. Those 2 bits are the logical OR of
>> + * their respective bits in the constituent pte entries. In order to
>> + * ensure the contpte range is covered by a single folio, we must
>> + * recover the folio from the pfn, but special mappings don't have a
>> + * folio backing them. Fortunately contpte_try_fold() already checked
>> + * that the pte is not special - we never try to fold special mappings.
>> + * Note we can't use vm_normal_page() for this since we don't have the
>> + * vma.
>> + */
>> +
>> + struct page *page = pte_page(pte);
>> + struct folio *folio = page_folio(page);
>> + unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
>> + unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
>> + unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>> + unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
>> + unsigned long pfn;
>> + pgprot_t prot;
>> + pte_t subpte;
>> + pte_t *orig_ptep;
>> + int i;
>> +
>> + if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
>> + return;
>> +
>> + pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
>> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>> + orig_ptep = ptep;
>> + ptep = contpte_align_down(ptep);
>> +
>> + for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
>> + subpte = __ptep_get(ptep);
>> + subpte = pte_mkold(pte_mkclean(subpte));
>> +
>> + if (!pte_valid(subpte) ||
>> + pte_pfn(subpte) != pfn ||
>> + pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
>> + return;
>> + }
>> +
>> + contpte_fold(mm, addr, orig_ptep, pte, true);
>> +}
>> +EXPORT_SYMBOL(__contpte_try_fold);
>> +
>> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>> + pte_t *ptep, pte_t pte)
>> +{
>> + /*
>> + * We have already checked that the ptes are contiguous in
>> + * contpte_try_unfold(), so we can unfold unconditionally here.
>> + */
>> +
>> + contpte_fold(mm, addr, ptep, pte, false);
>
> I'm still working my way through the series but

Thanks for taking the time to review!

> calling a fold during an
> unfold stood out as it seemed wrong. Obviously further reading revealed
> the boolean flag that changes the functions meaning but I think it would
> be better to refactor that.

Yes that sounds reasonable.

>
> We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
> the pte calculation loop into a separate helper
> (eg. calculate_contpte_dirty_young() or some hopefully better name)
> called further up the stack. That has an added benefit of providing a
> spot to add the nice comment for young/dirty rules you provided in the
> patch description ;-)
>
> In other words we'd have something like:
>
> void __contpte_try_unfold() {
> pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
> pte = pte_mknoncont(pte);
> set_cont_ptes(mm, addr, ptep, pte);
> }

My concern with this approach is that calculate_contpte_dirty_young() has side
effects; it has to clear each PTE as it loops through it prevent a race between
our reading access/dirty and another thread causing access/dirty to be set. So
its not just a "calculation", its the teardown portion of the process too. I
guess its a taste thing, so happy for it to be argued the other way, but I would
prefer to keep it all together in one function.

How about renaming contpte_fold() to contpte_convert() or contpte_repaint()
(other suggestions welcome), and extracting the pte_mkcont()/pte_mknoncont()
part (so we can remove the bool param):

void __contpte_try_unfold() {
pte = pte_mknoncont(pte);
contpte_convert(mm, addr, ptep, pte);
}

Thanks,
Ryan

>
> Which IMHO is more immediately understandable.
>
> - Alistair
>