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Message-ID: <C8DCA632-EA87-4CC1-A740-F26E49F67649@nvidia.com>
Date: Mon, 17 Jul 2023 12:15:27 -0400
From: Zi Yan <ziy@...dia.com>
To: Ryan Roberts <ryan.roberts@....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 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.
BTW, page_addr() needs a better name and is easily confused with existing
page_address().
>
>> 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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