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Message-ID: <f0a50c66-ff21-caf6-1c73-04149d88be8f@arm.com>
Date: Tue, 18 Jul 2023 11:19:08 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>,
Yin Fengwei <fengwei.yin@...el.com>,
David Hildenbrand <david@...hat.com>,
Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
"Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH v1 3/3] mm: Batch-zap large anonymous folio PTE mappings
On 17/07/2023 17:15, Zi Yan wrote:
> On 17 Jul 2023, at 11:55, Ryan Roberts wrote:
>
>> On 17/07/2023 16:25, Zi Yan wrote:
>>> On 17 Jul 2023, at 10:31, Ryan Roberts wrote:
>>>
>>>> This allows batching the rmap removal with folio_remove_rmap_range(),
>>>> which means we avoid spuriously adding a partially unmapped folio to the
>>>> deferrred split queue in the common case, which reduces split queue lock
>>>> contention.
>>>>
>>>> Previously each page was removed from the rmap individually with
>>>> page_remove_rmap(). If the first page belonged to a large folio, this
>>>> would cause page_remove_rmap() to conclude that the folio was now
>>>> partially mapped and add the folio to the deferred split queue. But
>>>> subsequent calls would cause the folio to become fully unmapped, meaning
>>>> there is no value to adding it to the split queue.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>>> ---
>>>> mm/memory.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>> 1 file changed, 119 insertions(+)
>>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 01f39e8144ef..6facb8c8807a 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1391,6 +1391,95 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>>>> pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
>>>> }
>>>>
>>>> +static inline unsigned long page_addr(struct page *page,
>>>> + struct page *anchor, unsigned long anchor_addr)
>>>> +{
>>>> + unsigned long offset;
>>>> + unsigned long addr;
>>>> +
>>>> + offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
>>>> + addr = anchor_addr + offset;
>>>> +
>>>> + if (anchor > page) {
>>>> + if (addr > anchor_addr)
>>>> + return 0;
>>>> + } else {
>>>> + if (addr < anchor_addr)
>>>> + return ULONG_MAX;
>>>> + }
>>>> +
>>>> + return addr;
>>>> +}
>>>> +
>>>> +static int calc_anon_folio_map_pgcount(struct folio *folio,
>>>> + struct page *page, pte_t *pte,
>>>> + unsigned long addr, unsigned long end)
>>>> +{
>>>> + pte_t ptent;
>>>> + int floops;
>>>> + int i;
>>>> + unsigned long pfn;
>>>> +
>>>> + end = min(page_addr(&folio->page + folio_nr_pages(folio), page, addr),
>>>> + end);
>>>> + floops = (end - addr) >> PAGE_SHIFT;
>>>> + pfn = page_to_pfn(page);
>>>> + pfn++;
>>>> + pte++;
>>>> +
>>>> + for (i = 1; i < floops; i++) {
>>>> + ptent = ptep_get(pte);
>>>> +
>>>> + if (!pte_present(ptent) ||
>>>> + pte_pfn(ptent) != pfn) {
>>>> + return i;
>>>> + }
>>>> +
>>>> + pfn++;
>>>> + pte++;
>>>> + }
>>>> +
>>>> + return floops;
>>>> +}
>>>> +
>>>> +static unsigned long zap_anon_pte_range(struct mmu_gather *tlb,
>>>> + struct vm_area_struct *vma,
>>>> + struct page *page, pte_t *pte,
>>>> + unsigned long addr, unsigned long end,
>>>> + bool *full_out)
>>>> +{
>>>> + struct folio *folio = page_folio(page);
>>>> + struct mm_struct *mm = tlb->mm;
>>>> + pte_t ptent;
>>>> + int pgcount;
>>>> + int i;
>>>> + bool full;
>>>> +
>>>> + pgcount = calc_anon_folio_map_pgcount(folio, page, pte, addr, end);
>>>> +
>>>> + for (i = 0; i < pgcount;) {
>>>> + ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>> + tlb_remove_tlb_entry(tlb, pte, addr);
>>>> + full = __tlb_remove_page(tlb, page, 0);
>>>> +
>>>> + if (unlikely(page_mapcount(page) < 1))
>>>> + print_bad_pte(vma, addr, ptent, page);
>>>> +
>>>> + i++;
>>>> + page++;
>>>> + pte++;
>>>> + addr += PAGE_SIZE;
>>>> +
>>>> + if (unlikely(full))
>>>> + break;
>>>> + }
>>>> +
>>>> + folio_remove_rmap_range(folio, page - i, i, vma);
>>>> +
>>>> + *full_out = full;
>>>> + return i;
>>>> +}
>>>> +
>>>> static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>> struct vm_area_struct *vma, pmd_t *pmd,
>>>> unsigned long addr, unsigned long end,
>>>> @@ -1428,6 +1517,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>> page = vm_normal_page(vma, addr, ptent);
>>>> if (unlikely(!should_zap_page(details, page)))
>>>> continue;
>>>> +
>>>> + /*
>>>> + * Batch zap large anonymous folio mappings. This allows
>>>> + * batching the rmap removal, which means we avoid
>>>> + * spuriously adding a partially unmapped folio to the
>>>> + * deferrred split queue in the common case, which
>>>> + * reduces split queue lock contention. Require the VMA
>>>> + * to be anonymous to ensure that none of the PTEs in
>>>> + * the range require zap_install_uffd_wp_if_needed().
>>>> + */
>>>> + if (page && PageAnon(page) && vma_is_anonymous(vma)) {
>>>> + bool full;
>>>> + int pgcount;
>>>> +
>>>> + pgcount = zap_anon_pte_range(tlb, vma,
>>>> + page, pte, addr, end, &full);
>>>
>>> Are you trying to zap as many ptes as possible if all these ptes are
>>> within a folio?
>>
>> Yes.
>>
>>> If so, why not calculate end before calling zap_anon_pte_range()?
>>> That would make zap_anon_pte_range() simpler.
>>
>> I'm not sure I follow. That's currently done in calc_anon_folio_map_pgcount(). I
>> could move it to here, but I'm not sure that makes things simpler, just puts
>> more code in here and less in there?
>
> Otherwise your zap_anon_pte_range() is really zap_anon_pte_in_folio_range() or
> some other more descriptive name. When I first look at the name, I thought
> PTEs will be zapped until the end. But that is not the case when I look at the
> code. And future users can easily be confused too and use it in a wrong way.
OK I see your point. OK let me pull the page count calculation into here and
pass it to zap_anon_pte_range(). Then I think we can keep the name as is?
>
> BTW, page_addr() needs a better name and is easily confused with existing
> page_address().
Yeah... I'll try to think of something for v2.
>
>>
>>> Also check if page is part of
>>> a large folio first to make sure you can batch.
>>
>> Yeah that's fair. I'd be inclined to put that in zap_anon_pte_range() to short
>> circuit calc_anon_folio_map_pgcount(). But ultimately zap_anon_pte_range() would
>> still zap the single pte.
>>
>>
>>>
>>>> +
>>>> + rss[mm_counter(page)] -= pgcount;
>>>> + pgcount--;
>>>> + pte += pgcount;
>>>> + addr += pgcount << PAGE_SHIFT;
>>>> +
>>>> + if (unlikely(full)) {
>>>> + force_flush = 1;
>>>> + addr += PAGE_SIZE;
>>>> + break;
>>>> + }
>>>> + continue;
>>>> + }
>>>> +
>>>> ptent = ptep_get_and_clear_full(mm, addr, pte,
>>>> tlb->fullmm);
>>>> tlb_remove_tlb_entry(tlb, pte, addr);
>>>> --
>>>> 2.25.1
>>>
>>>
>>> --
>>> Best Regards,
>>> Yan, Zi
>
>
> --
> Best Regards,
> Yan, Zi
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