Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings

Kefeng Wang
2024-06-25T03:16:48Z

On 2024/6/24 23:56, Ryan Roberts wrote:
> + Baolin Wang and Yin Fengwei, who maybe able to help with this.
>
>
> Hi Kefeng,
>
> Thanks for the report!
>
>
> On 24/06/2024 15:30, Kefeng Wang wrote:
>> Hi Ryan,
>>
>> A big regression on page-fault3("Separate file shared mapping page
>> fault") testcase from will-it-scale on arm64, no issue on x86,
>>
>> ./page_fault3_processes -t 128 -s 5
>
> I see that this program is mkstmp'ing a file at "/tmp/willitscale.XXXXXX". Based
> on your description, I'm inferring that /tmp is backed by ext4 with your large
> folio patches enabled?

Yes, mount /tmp by ext4, sorry to forget to mention that.

>
>>
>> 1) large folio disabled on ext4:
>>    92378735
>> 2) large folio  enabled on ext4 +  CONTPTE enabled
>>    16164943
>> 3) large folio  enabled on ext4 +  CONTPTE disabled
>>    80364074
>> 4) large folio  enabled on ext4 +  CONTPTE enabled + large folio mapping enabled
>> in finish_fault()[2]
>>    299656874
>>
>> We found *contpte_convert* consume lots of CPU(76%) in case 2),
>
> contpte_convert() is expensive and to be avoided; In this case I expect it is
> repainting the PTEs with the PTE_CONT bit added in, and to do that it needs to
> invalidate the tlb for the virtual range. The code is there to mop up user space
> patterns where each page in a range is temporarily made RO, then later changed
> back. In this case, we want to re-fold the contpte range once all pages have
> been serviced in RO mode.
>
> Of course this path is only intended as a fallback, and the more optimium
> approach is to set_ptes() the whole folio in one go where possible - kind of
> what you are doing below.
>
>> and disappeared
>> by following change[2], it is easy to understood the different between case 2)
>> and case 4) since case 2) always map one page
>> size, but always try to fold contpte mappings, which spend a lot of
>> time. Case 4) is a workaround, any other better suggestion?
>
> See below.
>
>>
>> Thanks.
>>
>> [1] https://github.com/antonblanchard/will-it-scale
>> [2] enable large folio mapping in finish_fault()
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 00728ea95583..5623a8ce3a1e 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4880,7 +4880,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>          * approach also applies to non-anonymous-shmem faults to avoid
>>          * inflating the RSS of the process.
>>          */
>> -       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
>> +       if (unlikely(userfaultfd_armed(vma))) {
>
> The change to make finish_fault() handle multiple pages in one go are new; added
> by Baolin Wang at [1]. That extra conditional that you have removed is there to
> prevent RSS reporting bloat. See discussion that starts at [2].
>
> Anyway, it was my vague understanding that the fault around mechanism
> (do_fault_around()) would ensure that (by default) 64K worth of pages get mapped
> together in a single set_ptes() call, via filemap_map_pages() ->
> filemap_map_folio_range(). Looking at the code, I guess fault around only
> applies to read faults. This test is doing a write fault.
>
> I guess we need to do a change a bit like what you have done, but also taking
> into account fault_around configuration?

Yes, the current changes is not enough, I hint some issue and still
debugging, so our direction is trying to map large folio for
do_shared_fault(), right?

>
> [1]
> https://lore.kernel.org/all/3a190892355989d42f59cf9f2f98b94694b0d24d.1718090413.git.baolin.wang@linux.alibaba.com/
> [2] https://lore.kernel.org/linux-mm/13939ade-a99a-4075-8a26-9be7576b7e03@arm.com/
>
>
>>                 nr_pages = 1;
>>         } else if (nr_pages > 1) {
>>                 pgoff_t idx = folio_page_idx(folio, page);
>>
>>
>> On 2024/2/15 18:32, 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 8310875133ff..401087e8a43d 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -1185,6 +1185,8 @@ extern void ptep_modify_prot_commit(struct
>>> vm_area_struct *vma,
>>>    * 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);
>>> @@ -1206,6 +1208,29 @@ 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)
>>>   {
>>> @@ -1286,6 +1311,7 @@ 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 50e0173dc5ee..16788f07716d 100644
>>> --- a/arch/arm64/mm/contpte.c
>>> +++ b/arch/arm64/mm/contpte.c
>>> @@ -73,6 +73,70 @@ 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_start, folio_end;
>>> +    unsigned long cont_start, cont_end;
>>> +    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_start = addr - (page - &folio->page) * PAGE_SIZE;
>>> +    folio_end = folio_start + folio_nr_pages(folio) * PAGE_SIZE;
>>> +    cont_start = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>> +    cont_end = cont_start + CONT_PTE_SIZE;
>>> +
>>> +    if (folio_start > cont_start || folio_end < cont_end)
>>> +        return;
>>> +
>>> +    pfn = ALIGN_DOWN(pte_pfn(pte), CONT_PTES);
>>> +    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)
>>>   {
>
>