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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6e805caf3sm4259252f8f.22.2025.06.25.03.49.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 03:49:57 -0700 (PDT) Message-ID: Date: Wed, 25 Jun 2025 12:49:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation To: Lance Yang Cc: 21cnbao@gmail.com, akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, chrisl@kernel.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, v-songbaohua@oppo.com, x86@kernel.org, ying.huang@intel.com, zhengtangquan@oppo.com, Lance Yang References: <2c19a6cf-0b42-477b-a672-ed8c1edd4267@redhat.com> <20250624162503.78957-1-ioworker0@gmail.com> <27d174e0-c209-4851-825a-0baeb56df86f@redhat.com> <938c4726-b93e-46df-bceb-65c7574714a6@linux.dev> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250625_035002_463146_7765E161 X-CRM114-Status: GOOD ( 23.52 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 25.06.25 12:47, Lance Yang wrote: > > > On 2025/6/25 18:00, David Hildenbrand wrote: >> On 24.06.25 18:25, Lance Yang wrote: >>> On 2025/6/24 23:34, David Hildenbrand wrote: >>>> On 24.06.25 17:26, Lance Yang wrote: >>>>> On 2025/6/24 20:55, David Hildenbrand wrote: >>>>>> On 14.02.25 10:30, Barry Song wrote: >>>>>>> From: Barry Song >>>>> [...] >>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>>> index 89e51a7a9509..8786704bd466 100644 >>>>>>> --- a/mm/rmap.c >>>>>>> +++ b/mm/rmap.c >>>>>>> @@ -1781,6 +1781,25 @@ void folio_remove_rmap_pud(struct folio >>>>>>> *folio, >>>>>>> struct page *page, >>>>>>>     #endif >>>>>>>     } >>>>>>> +/* We support batch unmapping of PTEs for lazyfree large folios */ >>>>>>> +static inline bool can_batch_unmap_folio_ptes(unsigned long addr, >>>>>>> +            struct folio *folio, pte_t *ptep) >>>>>>> +{ >>>>>>> +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY | >>>>>>> FPB_IGNORE_SOFT_DIRTY; >>>>>>> +    int max_nr = folio_nr_pages(folio); >>>>>> >>>>>> Let's assume we have the first page of a folio mapped at the last page >>>>>> table entry in our page table. >>>>> >>>>> Good point. I'm curious if it is something we've seen in practice ;) >>>> >>>> I challenge you to write a reproducer :P I assume it might be doable >>>> through simple mremap(). >>>> >>>>> >>>>>> >>>>>> What prevents folio_pte_batch() from reading outside the page table? >>>>> >>>>> Assuming such a scenario is possible, to prevent any chance of an >>>>> out-of-bounds read, how about this change: >>>>> >>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>> index fb63d9256f09..9aeae811a38b 100644 >>>>> --- a/mm/rmap.c >>>>> +++ b/mm/rmap.c >>>>> @@ -1852,6 +1852,25 @@ static inline bool >>>>> can_batch_unmap_folio_ptes(unsigned long addr, >>>>>        const fpb_t fpb_flags = FPB_IGNORE_DIRTY | >>>>> FPB_IGNORE_SOFT_DIRTY; >>>>>        int max_nr = folio_nr_pages(folio); >>>>>        pte_t pte = ptep_get(ptep); >>>>> +    unsigned long end_addr; >>>>> + >>>>> +    /* >>>>> +     * To batch unmap, the entire folio's PTEs must be contiguous >>>>> +     * and mapped within the same PTE page table, which corresponds to >>>>> +     * a single PMD entry. Before calling folio_pte_batch(), which >>>>> does >>>>> +     * not perform boundary checks itself, we must verify that the >>>>> +     * address range covered by the folio does not cross a PMD >>>>> boundary. >>>>> +     */ >>>>> +    end_addr = addr + (max_nr * PAGE_SIZE) - 1; >>>>> + >>>>> +    /* >>>>> +     * A fast way to check for a PMD boundary cross is to align both >>>>> +     * the start and end addresses to the PMD boundary and see if they >>>>> +     * are different. If they are, the range spans across at least two >>>>> +     * different PMD-managed regions. >>>>> +     */ >>>>> +    if ((addr & PMD_MASK) != (end_addr & PMD_MASK)) >>>>> +        return false; >>>> >>>> You should not be messing with max_nr = folio_nr_pages(folio) here at >>>> all. folio_pte_batch() takes care of that. >>>> >>>> Also, way too many comments ;) >>>> >>>> You may only batch within a single VMA and within a single page table. >>>> >>>> So simply align the addr up to the next PMD, and make sure it does not >>>> exceed the vma end. >>>> >>>> ALIGN and friends can help avoiding excessive comments. >>> >>> Thanks! How about this updated version based on your suggestion: >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index fb63d9256f09..241d55a92a47 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1847,12 +1847,25 @@ void folio_remove_rmap_pud(struct folio >>> *folio, struct page *page, >>>   /* We support batch unmapping of PTEs for lazyfree large folios */ >>>   static inline bool can_batch_unmap_folio_ptes(unsigned long addr, >>> -            struct folio *folio, pte_t *ptep) >>> +                          struct folio *folio, pte_t *ptep, >>> +                          struct vm_area_struct *vma) >>>   { >>>       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>> +    unsigned long next_pmd, vma_end, end_addr; >>>       int max_nr = folio_nr_pages(folio); >>>       pte_t pte = ptep_get(ptep); >>> +    /* >>> +     * Limit the batch scan within a single VMA and within a single >>> +     * page table. >>> +     */ >>> +    vma_end = vma->vm_end; >>> +    next_pmd = ALIGN(addr + 1, PMD_SIZE); >>> +    end_addr = addr + (unsigned long)max_nr * PAGE_SIZE; >>> + >>> +    if (end_addr > min(next_pmd, vma_end)) >>> +        return false; >> >> May I suggest that we clean all that up as we fix it? > > Yeah, that looks much better. Thanks for the suggestion! > >> >> Maybe something like this: >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 3b74bb19c11dd..11fbddc6ad8d6 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -1845,23 +1845,38 @@ void folio_remove_rmap_pud(struct folio *folio, >> struct page *page, >>  #endif >>  } >> >> -/* We support batch unmapping of PTEs for lazyfree large folios */ >> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr, >> -                       struct folio *folio, pte_t *ptep) >> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >> +               struct page_vma_mapped_walk *pvmw, enum ttu_flags flags, >> +               pte_t pte) >>  { >>         const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >> -       int max_nr = folio_nr_pages(folio); >> -       pte_t pte = ptep_get(ptep); >> +       struct vm_area_struct *vma = pvmw->vma; >> +       unsigned long end_addr, addr = pvmw->address; >> +       unsigned int max_nr; >> + >> +       if (flags & TTU_HWPOISON) >> +               return 1; >> +       if (!folio_test_large(folio)) >> +               return 1; >> + >> +       /* We may only batch within a single VMA and a single page >> table. */ >> +       end_addr = min_t(unsigned long, ALIGN(addr + 1, PMD_SIZE), vma- >> >vm_end); >> +       max_nr = (end_addr - addr) >> PAGE_SHIFT; >> >> +       /* We only support lazyfree batching for now ... */ >>         if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >> -               return false; >> +               return 1; >>         if (pte_unused(pte)) >> -               return false; >> -       if (pte_pfn(pte) != folio_pfn(folio)) >> -               return false; >> +               return 1; >> +       /* ... where we must be able to batch the whole folio. */ >> +       if (pte_pfn(pte) != folio_pfn(folio) || max_nr != >> folio_nr_pages(folio)) >> +               return 1; >> +       max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, >> fpb_flags, >> +                                NULL, NULL, NULL); >> >> -       return folio_pte_batch(folio, addr, ptep, pte, max_nr, >> fpb_flags, NULL, >> -                              NULL, NULL) == max_nr; >> +       if (max_nr != folio_nr_pages(folio)) >> +               return 1; >> +       return max_nr; >>  } >> >>  /* >> @@ -2024,9 +2039,7 @@ static bool try_to_unmap_one(struct folio *folio, >> struct vm_area_struct *vma, >>                         if (pte_dirty(pteval)) >>                                 folio_mark_dirty(folio); >>                 } else if (likely(pte_present(pteval))) { >> -                       if (folio_test_large(folio) && !(flags & >> TTU_HWPOISON) && >> -                           can_batch_unmap_folio_ptes(address, folio, >> pvmw.pte)) >> -                               nr_pages = folio_nr_pages(folio); >> +                       nr_pages = folio_unmap_pte_batch(folio, &pvmw, >> flags, pteval); >>                         end_addr = address + nr_pages * PAGE_SIZE; >>                         flush_cache_range(vma, address, end_addr); >> >> >> Note that I don't quite understand why we have to batch the whole thing >> or fallback to >> individual pages. Why can't we perform other batches that span only some >> PTEs? What's special >> about 1 PTE vs. 2 PTEs vs. all PTEs? > > That's a good point about the "all-or-nothing" batching logic ;) > > It seems the "all-or-nothing" approach is specific to the lazyfree use > case, which needs to unmap the entire folio for reclamation. If that's > not possible, it falls back to the single-page slow path. > > Also, supporting partial batches would be useful, but not common case > I guess, so let's leave it as is ;p We can literally make this code less complicated if we just support it? :) I mean, it's dropping 3 if conditions from the code I shared. -- Cheers, David / dhildenb