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[91.12.110.230]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3749efaf8b8sm6161217f8f.91.2024.08.31.03.07.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 31 Aug 2024 03:07:23 -0700 (PDT) Message-ID: Date: Sat, 31 Aug 2024 12:07:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Barry Song , Chuanhua Han , Baolin Wang , Ryan Roberts , Zi Yan , Chris Li , Kairui Song , Kalesh Singh , Suren Baghdasaryan References: <20240831092339.66085-1-21cnbao@gmail.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-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-Queue-Id: 8214640015 X-Rspamd-Server: rspam01 X-Stat-Signature: mqyk5bxs5o3r8se1ii9bfhwxkj96metu X-HE-Tag: 1725099079-784822 X-HE-Meta: U2FsdGVkX19eNPb1AJNxBJlt4qrqsvJhW6a4F8/o+dMWVPQUa/gsVs+JCfL8c27DeQtpbPnMIRfUx4DqPs71SQ6s7AyXaEBrBbbWoVA2TJ/2gt6KtkuaGdZm/ULMqI6tBDT0yIVO133HTRzzqRigadpFQrD0InK6W8/ISDYUp2E3CtwlkqzTJv1i0r2bJR2/xpzUipIUMvvLnox/zgrKckx0cD3Ly2U0RnQb7NNF03BW3jX3j7YpArD4UoKVkau8HAg8EbCwQOj8PZnFR8LASp4yFK6sGpma/zagnNpM/0XR9X5uKzpGk2v8NvDAFxCEv/Cu2+l0RhLiKuhO9XfQ7mhpE98gKM6v+i6qRRnDuDvEehgDS+/mYch4LFDxXl2dAlhoncvcK2DNrwpPIVlSv7gDGSceCBfpsDxAiOG7iIZ/jbOtLRMeeYMTBMA+DSw9KpaTNJTEu+nwGB1oXD0sxadDiAWbUzQCDDSn3Mq0vnsrtCCTo3N5tCPaBCS/d9avHMUHdv5HRHn7WEovOn/ke2NIMnZ7cPLT31FEZ57FbTim1uEgltuu4mQXy+6FhdAWtET8PC5fkXjUsQnW+HnEG7C9T+S6VO3VjdYPJeZLFtrEE/zpe4gukOxFt6l76WDxRIcZPJpJB/4T+fXJUkS97ho6IrLb5TADoqaVLe6hN8uXyfnVThoTJXy+aPpM8uVlxAF6Ck7/OxVbX0kRfGzxvsxk5TesM4fQ5Phkm3zsaJLNl88OeNwFvNud2GOzj2ieNoyeFVEFd0Og5VeA9dB+lXisl5Ty5Ti1jfzTYuqOQ3mKUrFNeRqwFhWXFgXg6feSC0DZpwJr3UyoF9vpEc1fTkD0sN+kbrmDRUggsnWURxT1zBYsyuzK55Nvl/QXQkb2d5nuyR00r3Bx6DQHd6tYwVHbIITk2VIj4H9Mpt8s7WJMUkYZY0iOKS6vqM48Et7S8m1wAtZ6Eb1FnrhdoFM hKZc7K5N 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 31.08.24 11:55, Barry Song wrote: > On Sat, Aug 31, 2024 at 9:44 PM David Hildenbrand wrote: >> >> On 31.08.24 11:23, Barry Song wrote: >>> From: Barry Song >>> >>> On a physical phone, it's sometimes observed that deferred_split >>> mTHPs account for over 15% of the total mTHPs. Profiling by Chuanhua >>> indicates that the majority of these originate from the typical fork >>> scenario. >>> When the child process either execs or exits, the parent process should >>> ideally be able to reuse the entire mTHP. However, the current kernel >>> lacks this capability and instead places the mTHP into split_deferred, >>> performing a CoW (Copy-on-Write) on just a single subpage of the mTHP. >>> >>> main() >>> { >>> #define SIZE 1024 * 1024UL >>> void *p = malloc(SIZE); >>> memset(p, 0x11, SIZE); >>> if (fork() == 0) >>> exec(....); >>> /* >>> * this will trigger cow one subpage from >>> * mTHP and put mTHP into split_deferred >>> * list >>> */ >>> *(int *)(p + 10) = 10; >>> printf("done\n"); >>> while(1); >>> } >>> >>> This leads to two significant issues: >>> >>> * Memory Waste: Before the mTHP is fully split by the shrinker, >>> it wastes memory. In extreme cases, such as with a 64KB mTHP, >>> the memory usage could be 64KB + 60KB until the last subpage >>> is written, at which point the mTHP is freed. >>> >>> * Fragmentation and Performance Loss: It destroys large folios >>> (negating the performance benefits of CONT-PTE) and fragments memory. >>> >>> To address this, we should aim to reuse the entire mTHP in such cases. >>> >>> Hi David, >>> >>> I’ve renamed wp_page_reuse() to wp_folio_reuse() and added an >>> entirely_reuse argument because I’m not sure if there are still cases >>> where we reuse a subpage within an mTHP. For now, I’m setting >>> entirely_reuse to true only for the newly supported case, while all >>> other cases still get false. Please let me know if this is incorrect—if >>> we don’t reuse subpages at all, we could remove the argument. >> >> See [1] I sent out this week, that is able to reuse even without >> scanning page tables. If we find the the folio is exclusive we could try >> processing surrounding PTEs that map the same folio. >> >> [1] https://lkml.kernel.org/r/20240829165627.2256514-1-david@redhat.com > > Great! It looks like I missed your patch again. Since you've implemented this > in a better way, I’d prefer to use your patchset. I wouldn't say better, just more universally. And while taking care of properly sync'ing the mapcount vs. refcount :P > > I’m curious about how you're handling ptep_set_access_flags_nr() or similar > things because I couldn’t find the related code in your patch 10/17: > > [PATCH v1 10/17] mm: COW reuse support for PTE-mapped THP with CONFIG_MM_ID > > Am I missing something? The idea is to keep individual write faults as fast as possible. So the patch set keeps it simple and only reuses a single PTE at a time, setting that one PAE and mapping it writable. As the patch states, it might be reasonable to optimize some cases, maybe also only on some architectures. For example to fault-around and map the other ones writable as well. It might not always be desirable though, especially not for larger folios. -- Cheers, David / dhildenb