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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45382366f88sm14780565e9.30.2025.06.25.03.00.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 03:00:40 -0700 (PDT) Message-ID: <27d174e0-c209-4851-825a-0baeb56df86f@redhat.com> Date: Wed, 25 Jun 2025 12:00:39 +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 References: <2c19a6cf-0b42-477b-a672-ed8c1edd4267@redhat.com> <20250624162503.78957-1-ioworker0@gmail.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250625_030048_555714_6C2BFE5A X-CRM114-Status: GOOD ( 31.15 ) 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 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? 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? Can someone enlighten me why that is required? -- Cheers, David / dhildenb