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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3861ecf4087sm13575665f8f.5.2024.12.09.06.46.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Dec 2024 06:46:15 -0800 (PST) Message-ID: Date: Mon, 9 Dec 2024 15:46:14 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: remove an avoidable load of page refcount in page_ref_add_unless To: Mateusz Guzik Cc: yuzhao@google.com, akpm@linux-foundation.org, willy@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20241207082931.1707465-1-mjguzik@gmail.com> <94d0dcbe-2001-4a9c-a767-b337b688b616@redhat.com> <606fbf9a-c9ba-4f08-a708-db38fe6065ce@redhat.com> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 4o7zeuykbflwGFeSfGvahDBd3f73VsnQcoek12BhU2s_1733755576 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 4A564C0019 X-Stat-Signature: g1zzxp45po4dqyi6rp1a93bo6sy65xnr X-HE-Tag: 1733755557-346382 X-HE-Meta: U2FsdGVkX18UQmTlZEViErFR+vAyBiVlyX3hLVlqYa9QHJ64gJ/VL+1uZsE+sKcQCrjSD7JjgMsX1abbWnaJR0YhjGcNu7LqOU8R+ODae/3loMwYHHbc0L7dRpK6nATwTw88s2/Kk9o5K4ghGu/U/93urcllRJqXZqZ9e4xJoctEV8tcaw2MOMOQQM27VX4ffeIdIoFeg8xONc7nFRGtrk5rhwL0BsTV5odw9i4dRqff7sp3O+LmLRId7pjqcpELQl3QyBymP9L8cqPiOROg/GBHJZuBTMXCzaSLlp9lofEh1lgSLJhb/Su/I4R/y5pSJI/bXTAVIcKhUrUk7CHdVsQXxtCBtoT1BWYPN9l2/I615kbVhgUSfzwl0ikO1oxFL+mupmNEz3zJY/S4z3ugGRqneedUPCpSysFb9G+xVBPZuI9KF49tuNviCQX80jptPCKZzTa/ytRJ4jW1tTrtV0RejYXyN5wAdI0LAOCfLI6xqq6LkzxndY2+FT3fqPBuZWjlfvttjX9FYqfpfs+UC1nX9uZvElMjQooypimmbtrcigtzXtYQd/59BmeYXVIWzBShUJzHsrvKCUT7svCoOMZrPHbn7I/YCjmhj1VGgVCugf8YYxAuzmzYpF+zbBkEFTzF63kx94ubPxz26Q0j9N64YdUbK/VceLrqD2ys5pf/Jf9Bo0tYNcJWjw52D541zUNGr798f2B/aRra49TgWOgz2PdKT60PWHMAAsIWUOxc50qWusuLieL6xbxr2igM1EJHQNmbHJJ6tKGVexVhCThEjUj44Lpb/GjXE6gG+R5hnr2dW2nnAah4jhVGsjI4Mp08JLo5NiYvGVpD6DhJQW7rqLEROtOoAzO17DjAT4bO9k7e6TzSnjjBUqxXLcHCLFJuzQ+TETjzKSUgwunC+RCshMkvnUjJjDskBoCVwsAjPMRY9lZFK+99CV8bEaGTvQAaQ/x8QZsx6PLLwFX npWUXOcX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000138, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09.12.24 15:30, Mateusz Guzik wrote: > On Mon, Dec 9, 2024 at 3:22 PM David Hildenbrand wrote: >> >> On 09.12.24 13:33, Mateusz Guzik wrote: >>> That is to say I think this thread just about exhausted the time >>> warranted by this patch. No hard feelz if it gets dropped, but then I >>> do strongly suggest adding a justification to the extra load. >> >> Maybe it's sufficient for now to simply do your change with a comment: >> >> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h >> index 8c236c651d1d6..1efc992ad5687 100644 >> --- a/include/linux/page_ref.h >> +++ b/include/linux/page_ref.h >> @@ -234,7 +234,13 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u) >> >> rcu_read_lock(); >> /* avoid writing to the vmemmap area being remapped */ >> - if (!page_is_fake_head(page) && page_ref_count(page) != u) >> + if (!page_is_fake_head(page)) >> + /* >> + * atomic_add_unless() will currently never modify the value >> + * if it already is u. If that ever changes, we'd have to have >> + * a separate check here, such that we won't be writing to >> + * write-protected vmemmap areas. >> + */ >> ret = atomic_add_unless(&page->_refcount, nr, u); >> rcu_read_unlock(); >> >> >> It would bail out during testing ... hopefully, such that we can detect any such change. >> > > Not my call to make, but looks good. ;) > > fwiw I don't need any credit and I would be more than happy if you > just submitted the thing as your own without me being mentioned. *No* > cc would also be appreciated. Likely Andrew can add the comment as a fixup. -- Cheers, David / dhildenb