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

Ryan Roberts
2023-11-22T08:36:07Z

On 22/11/2023 06:01, Alistair Popple wrote:
>
> Ryan Roberts <ryan.roberts@arm.com> writes:
>
>> 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. That works for me, although sadly I don't have any better ideas
> for names atm.

Thanks - I'll make this change for v3 and go with contpte_convert().

>
> - Alistair
>
>> Thanks,
>> Ryan
>>
>>>
>>> Which IMHO is more immediately understandable.
>>>
>>> - Alistair
>>>
>