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

Baolin Wang
2024-06-25T07:24:13Z

On 2024/6/25 11:16, Kefeng Wang wrote:
>
>
> 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?

For the writable mmap() of tmpfs, we will use mTHP interface to control
the size of folio to allocate, as discussed in previous meeting [1], so
I don't think fault_around configuration will be helpful for tmpfs.

For other filesystems, like ext4, I did not found the logic to determin
what size of folio to allocate in writable mmap() path (Kefeng, please
correct me if I missed something). If there is a control like mTHP, we
can rely on that instead of 'fault_around'?

[1]
https://lore.kernel.org/all/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com/

> 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?

I think this is the right direction to do. I add this
'!vma_is_anon_shmem(vma)' conditon to gradually implement support for
large folio mapping buidling, especially for writable mmap() support in
tmpfs.

>> [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)
>>>>    {
>>
>>