Re: [PATCH v5 25/25] arm64/mm: Automatically fold contpte mappings

Ryan Roberts
2024-02-13T18:05:15Z

On 13/02/2024 17:44, Mark Rutland wrote:
> On Fri, Feb 02, 2024 at 08:07:56AM +0000, Ryan Roberts wrote:
>> There are situations where a change to a single PTE could cause the
>> contpte block in which it resides to become foldable (i.e. could be
>> repainted with the contiguous bit). Such situations arise, for example,
>> when user space temporarily changes protections, via mprotect, for
>> individual pages, such can be the case for certain garbage collectors.
>>
>> We would like to detect when such a PTE change occurs. However this can
>> be expensive due to the amount of checking required. Therefore only
>> perform the checks when an indiviual PTE is modified via mprotect
>> (ptep_modify_prot_commit() -> set_pte_at() -> set_ptes(nr=1)) and only
>> when we are setting the final PTE in a contpte-aligned block.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>> arch/arm64/include/asm/pgtable.h | 26 +++++++++++++
>> arch/arm64/mm/contpte.c | 64 ++++++++++++++++++++++++++++++++
>> 2 files changed, 90 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index cdc310880a3b..d3357fe4eb89 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -1192,6 +1192,8 @@ void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte);
>> * where it is possible and makes sense to do so. The PTE_CONT bit is considered
>> * a private implementation detail of the public ptep API (see below).
>> */
>> +extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>> + pte_t *ptep, pte_t pte);
>> extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>> pte_t *ptep, pte_t pte);
>> extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
>> @@ -1213,6 +1215,29 @@ extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
>> unsigned long addr, pte_t *ptep,
>> pte_t entry, int dirty);
>>
>> +static __always_inline void contpte_try_fold(struct mm_struct *mm,
>> + unsigned long addr, pte_t *ptep, pte_t pte)
>> +{
>> + /*
>> + * Only bother trying if both the virtual and physical addresses are
>> + * aligned and correspond to the last entry in a contig range. The core
>> + * code mostly modifies ranges from low to high, so this is the likely
>> + * the last modification in the contig range, so a good time to fold.
>> + * We can't fold special mappings, because there is no associated folio.
>> + */
>> +
>> + const unsigned long contmask = CONT_PTES - 1;
>> + bool valign = ((addr >> PAGE_SHIFT) & contmask) == contmask;
>> +
>> + if (unlikely(valign)) {
>> + bool palign = (pte_pfn(pte) & contmask) == contmask;
>> +
>> + if (unlikely(palign &&
>> + pte_valid(pte) && !pte_cont(pte) && !pte_special(pte)))
>> + __contpte_try_fold(mm, addr, ptep, pte);
>> + }
>> +}
>> +
>> static __always_inline void contpte_try_unfold(struct mm_struct *mm,
>> unsigned long addr, pte_t *ptep, pte_t pte)
>> {
>> @@ -1287,6 +1312,7 @@ static __always_inline void set_ptes(struct mm_struct *mm, unsigned long addr,
>> if (likely(nr == 1)) {
>> contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> __set_ptes(mm, addr, ptep, pte, 1);
>> + contpte_try_fold(mm, addr, ptep, pte);
>> } else {
>> contpte_set_ptes(mm, addr, ptep, pte, nr);
>> }
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index 80346108450b..2c7dafd0552a 100644
>> --- a/arch/arm64/mm/contpte.c
>> +++ b/arch/arm64/mm/contpte.c
>> @@ -67,6 +67,70 @@ static void contpte_convert(struct mm_struct *mm, unsigned long addr,
>> __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.
>> + */
>> +
>> + unsigned long folio_saddr, folio_eaddr;
>> + unsigned long cont_saddr, cont_eaddr;
>> + pte_t expected_pte, subpte;
>> + struct folio *folio;
>> + struct page *page;
>> + unsigned long pfn;
>> + pte_t *orig_ptep;
>> + pgprot_t prot;
>> +
>> + int i;
>> +
>> + if (!mm_is_user(mm))
>> + return;
>> +
>> + page = pte_page(pte);
>> + folio = page_folio(page);
>> + folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
>> + folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
>> + cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>> + cont_eaddr = cont_saddr + CONT_PTE_SIZE;
>
> I assume that the 's' in *_sddar is for "start", and the 'e' in *_eaddr is for
> "end". Could we use "start" and "end" directly, e.g. folio_start, folio_end?

ACK; will fix.

>
>> +
>> + if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
>> + return;
>> +
>> + pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
>
> IIUC this should be the same as:
>
> pfn = ALIGN_DOWN(pte_pfn(pte), NR_CONT_PTES);
>
> ... which would align with the way we generate 'cont_saddr' above.

ACK; will fix.

>
> Otherwise, this looks good to me.

Great thanks!

I'll get these changes done and rebase onto mm-unstable once David's zap
batching series is in, retest and re-post (hopefully in the next couple of days!)

>
> Mark.
>
>> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>> + expected_pte = pfn_pte(pfn, prot);
>> + orig_ptep = ptep;
>> + ptep = contpte_align_down(ptep);
>> +
>> + for (i = 0; i < CONT_PTES; i++) {
>> + subpte = pte_mkold(pte_mkclean(__ptep_get(ptep)));
>> + if (!pte_same(subpte, expected_pte))
>> + return;
>> + expected_pte = pte_advance_pfn(expected_pte, 1);
>> + ptep++;
>> + }
>> +
>> + pte = pte_mkcont(pte);
>> + contpte_convert(mm, addr, orig_ptep, pte);
>> +}
>> +EXPORT_SYMBOL(__contpte_try_fold);
>> +
>> void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>> pte_t *ptep, pte_t pte)
>> {
>> --
>> 2.25.1
>>