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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538237ac2csm15646965e9.38.2025.06.25.03.43.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 03:43:14 -0700 (PDT) Message-ID: <8a157228-0b7e-479d-a224-ec85b458ea75@redhat.com> Date: Wed, 25 Jun 2025 12:43:12 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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. > > if (folio_test_large(folio)) { > bool any_young, any_dirty; > nr = madvise_folio_pte_batch(addr, end, folio, pte, > ptent, > &any_young, &any_dirty); > > > if (nr < folio_nr_pages(folio)) { > ... > err = split_folio(folio); > ... > } > } > > Another reason is that when we extend this to non-lazyfree anonymous > folios [1], things get complicated: checking anon_exclusive and updating > folio_try_share_anon_rmap_pte with the number of PTEs becomes tricky if > a folio is partially exclusive and partially shared. Right, but that's just another limitation on top how much we can batch, right? -- Cheers, David / dhildenb