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Date: Tue, 25 Jun 2024 11:16:37 +0800
From: Kefeng Wang <wangkefeng.wang@...wei.com>
To: Ryan Roberts <ryan.roberts@....com>, Catalin Marinas
<catalin.marinas@....com>, Will Deacon <will@...nel.org>, Ard Biesheuvel
<ardb@...nel.org>, Marc Zyngier <maz@...nel.org>, James Morse
<james.morse@....com>, Andrey Ryabinin <ryabinin.a.a@...il.com>, Andrew
Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>,
Mark Rutland <mark.rutland@....com>, David Hildenbrand <david@...hat.com>,
John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>, Barry Song
<21cnbao@...il.com>, Alistair Popple <apopple@...dia.com>, Yang Shi
<shy828301@...il.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
<dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, Baolin Wang
<baolin.wang@...ux.alibaba.com>, "Yin, Fengwei" <fengwei.yin@...el.com>
CC: <linux-arm-kernel@...ts.infradead.org>, <x86@...nel.org>,
<linuxppc-dev@...ts.ozlabs.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings
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@....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)
>>> {
>
>
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