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[2003:d8:2f3c:2f00:8afc:23fe:3813:776c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538a3b2fd7sm22569255e9.17.2025.06.26.07.39.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Jun 2025 07:39:06 -0700 (PDT) Message-ID: <6fbcf806-eb3c-4bcd-8daf-8d87fd759d2b@redhat.com> Date: Thu, 26 Jun 2025 16:39:04 +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: 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: <20250626092905.31305-1-ioworker0@gmail.com> <20250626124445.77865-1-ioworker0@gmail.com> <1a55f9f3-f5b1-4761-97ba-423756c707fe@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 26.06.25 15:52, Lance Yang wrote: > > > On 2025/6/26 21:16, David Hildenbrand wrote: >> On 26.06.25 14:44, Lance Yang wrote: >>> >>> On 2025/6/26 17:29, Lance Yang wrote: >>>> Before I send out the real patch, I'd like to get some quick feedback to >>>> ensure I've understood the discussion correctly ;) >>>> >>>> Does this look like the right direction? >>>> >>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>> index fb63d9256f09..5ebffe2137e4 100644 >>>> --- a/mm/rmap.c >>>> +++ b/mm/rmap.c >>>> @@ -1845,23 +1845,37 @@ 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); >>>> +    unsigned long end_addr, addr = pvmw->address; >>>> +    struct vm_area_struct *vma = pvmw->vma; >>>> +    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 = pmd_addr_end(addr, 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; >>>> +    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr; >>>>    } >>>>    /* >>>> @@ -2024,9 +2038,7 @@ 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); >>>> @@ -2206,13 +2218,16 @@ static bool try_to_unmap_one(struct folio >>>> *folio, struct vm_area_struct *vma, >>>>                hugetlb_remove_rmap(folio); >>>>            } else { >>>>                folio_remove_rmap_ptes(folio, subpage, nr_pages, vma); >>>> -            folio_ref_sub(folio, nr_pages - 1); >>>>            } >>>>            if (vma->vm_flags & VM_LOCKED) >>>>                mlock_drain_local(); >>>> -        folio_put(folio); >>>> -        /* We have already batched the entire folio */ >>>> -        if (nr_pages > 1) >>>> +        folio_put_refs(folio, nr_pages); >>>> + >>>> +        /* >>>> +         * If we are sure that we batched the entire folio and cleared >>>> +         * all PTEs, we can just optimize and stop right here. >>>> +         */ >>>> +        if (nr_pages == folio_nr_pages(folio)) >>>>                goto walk_done; >>>>            continue; >>>>    walk_abort: >>>> -- >>> >>> Oops ... Through testing on my machine, I found that the logic doesn't >>> behave as expected because I messed up the meaning of max_nr (the >>> available >>> scan room in the page table) with folio_nr_pages(folio) :( >>> >>> With the following change: >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 5ebffe2137e4..b1407348e14e 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1850,9 +1850,9 @@ static inline unsigned int >>> folio_unmap_pte_batch(struct folio *folio, >>>               enum ttu_flags flags, pte_t pte) >>>   { >>>       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>> +    unsigned int max_nr, nr_pages = folio_nr_pages(folio); >>>       unsigned long end_addr, addr = pvmw->address; >>>       struct vm_area_struct *vma = pvmw->vma; >>> -    unsigned int max_nr; >>>       if (flags & TTU_HWPOISON) >>>           return 1; >>> @@ -1870,12 +1870,13 @@ static inline unsigned int >>> folio_unmap_pte_batch(struct folio *folio, >>>           return 1; >>>       /* ... where we must be able to batch the whole folio. */ >> >> Why is that still required? :) > > Sorry ... I was still stuck in the "all-or-nothing" mindset ... > > So, IIUC, you mean we should completely remove the "max_nr < nr_pages" > check and just let folio_pte_batch handle whatever partial batch it > safely can. > >> >>> -    if (pte_pfn(pte) != folio_pfn(folio) || max_nr != >>> folio_nr_pages(folio)) >>> +    if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages) >>>           return 1; >>> -    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, >>> fpb_flags, >>> -                 NULL, NULL, NULL); >>> -    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr; >>> +    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages, >>> +                 fpb_flags, NULL, NULL, NULL); >>> + >>> +    return (max_nr != nr_pages) ? 1 : max_nr; >> >> Why is that still required? :) > > Then simply return the number of PTEs that consecutively map to the > large folio. Right? Yes. Any part of the large folio. Just return folio_pte_batch() ... -- Cheers, David / dhildenb