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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6e80ff461sm4398628f8f.67.2025.06.25.03.59.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 03:59:22 -0700 (PDT) Message-ID: <146b4cb1-aa1e-4519-9e03-f98cfb1135d2@redhat.com> Date: Wed, 25 Jun 2025 12:59:20 +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: Barry Song <21cnbao@gmail.com> Cc: Lance Yang , 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 References: <2c19a6cf-0b42-477b-a672-ed8c1edd4267@redhat.com> <20250624162503.78957-1-ioworker0@gmail.com> <27d174e0-c209-4851-825a-0baeb56df86f@redhat.com> <8a157228-0b7e-479d-a224-ec85b458ea75@redhat.com> 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_035926_954233_85718535 X-CRM114-Status: GOOD ( 19.35 ) 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 12:49, Barry Song wrote: > On Wed, Jun 25, 2025 at 10:43 PM David Hildenbrand wrote: >> >> On 25.06.25 12:38, Barry Song wrote: >>>>> 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? >>>> >>>> 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); >>> >>> Is this pmd_addr_end()? >>> >> >> Yes, that could be reused as well here. >> >>>> + 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? >>>> >>>> >>>> Can someone enlighten me why that is required? >>> >>> It's probably not a strict requirement — I thought cases where the >>> count is greater than 1 but less than nr_pages might not provide much >>> practical benefit, except perhaps in very rare edge cases, since >>> madv_free() already calls split_folio(). >> >> Okay, but it makes the code more complicated. If there is no reason to >> prevent the batching, we should drop it. > > It's not necessarily more complex, since page_vma_mapped_walk() still > has to check each PTE individually and can't skip ahead based on nr. > With nr_pages batched, we can exit the loop early in one go. I said "complicated", not "complex". The code is more complicated than necessary. -- Cheers, David / dhildenb