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[2003:cb:c741:de00:bf0f:cd46:dc1c:2de9]) by smtp.gmail.com with ESMTPSA id s8-20020a05600c45c800b0040e527602c8sm46523956wmo.9.2024.01.23.04.19.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Jan 2024 04:19:36 -0800 (PST) Message-ID: <31a0661e-fa69-419c-9936-98bfe168d5a7@redhat.com> Date: Tue, 23 Jan 2024 13:19:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 09/11] mm/memory: optimize fork() with PTE-mapped THP To: Ryan Roberts , linux-kernel@vger.kernel.org References: <20240122194200.381241-1-david@redhat.com> <20240122194200.381241-10-david@redhat.com> <63be0c3c-bf34-4cbb-b47b-7c9be0e65058@arm.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: <63be0c3c-bf34-4cbb-b47b-7c9be0e65058@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Catalin Marinas , linux-mm@kvack.org, sparclinux@vger.kernel.org, Alexander Gordeev , Will Deacon , linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, Russell King , Matthew Wilcox , "Aneesh Kumar K.V" , "Naveen N. Rao" , Gerald Schaefer , Christian Borntraeger , Albert Ou , Vasily Gorbik , Heiko Carstens , Nicholas Piggin , Paul Walmsley , linux-arm-kernel@lists.infradead.org, Dinh Nguyen , Palmer Dabbelt , Sven Schnelle , Andrew Morton , linuxppc-dev@lists.ozlabs.org, "David S. Miller" Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" [...] > > I wrote some documentation for this (based on Matthew's docs for set_ptes() in > my version. Perhaps it makes sense to add it here, given this is overridable by > the arch. > > /** > * wrprotect_ptes - Write protect a consecutive set of pages. > * @mm: Address space that the pages are mapped into. > * @addr: Address of first page to write protect. > * @ptep: Page table pointer for the first entry. > * @nr: Number of pages to write protect. > * > * May be overridden by the architecture, else implemented as a loop over > * ptep_set_wrprotect(). > * > * Context: The caller holds the page table lock. The PTEs are all in the same > * PMD. > */ > I could have sworn I had a documentation at some point. Let me add some, thanks. [...] >> + >> + /* >> + * If we likely have to copy, just don't bother with batching. Make >> + * sure that the common "small folio" case stays as fast as possible >> + * by keeping the batching logic separate. >> + */ >> + if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) { >> + nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr); >> + if (folio_test_anon(folio)) { >> + folio_ref_add(folio, nr); >> + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, >> + nr, src_vma))) { > > What happens if its not the first page of the batch that fails here? Aren't you > signalling that you need a prealloc'ed page for the wrong pte? Shouldn't you > still batch copy all the way up to the failing page first? Perhaps it all comes > out in the wash and these events are so infrequent that we don't care about the > lost batching opportunity? I assume you mean the weird corner case that some folio pages in the range have PAE set, others don't -- and the folio maybe pinned. In that case, we fallback to individual pages, and might have preallocated a page although we wouldn't have to preallocate one for processing the next page (that doesn't have PAE set). It should all work, although not optimized to the extreme, and as it's a corner case, we don't particularly care. Hopefully, in the future we'll only have a single PAE flag per folio. Or am I missing something? > >> + folio_ref_sub(folio, nr); >> + return -EAGAIN; >> + } >> + rss[MM_ANONPAGES] += nr; >> + VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio); >> + } else { >> + folio_ref_add(folio, nr); > > Perhaps hoist this out to immediately after folio_pte_batch() since you're > calling it on both branches? Makes sense, thanks. -- Cheers, David / dhildenb