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

[Alistair Popple <apopple@nvidia.com>]

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.

 - Alistair

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


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