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[91.12.110.230]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-374b9d54f98sm2702991f8f.69.2024.08.31.03.29.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 31 Aug 2024 03:29:12 -0700 (PDT) Message-ID: <36933711-ae0f-468c-93bd-d6a67d974c9d@redhat.com> Date: Sat, 31 Aug 2024 12:29:11 +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-Rspamd-Queue-Id: B89C9180009 X-Stat-Signature: 6f3qsezjjbzhbz15eprdtrp491cp93wo X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1725100157-8075 X-HE-Meta: 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 wFUPejYU 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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 12:21, Barry Song wrote: > On Sat, Aug 31, 2024 at 10:07 PM David Hildenbrand wrote: >> >> 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. > > I got your point, thanks! as anyway the mTHP has been exclusive, > so the following nr-1 minor page faults will set their particular PTE > to writable one by one. Yes, assuming you would get these page faults, and assuming you would get them in the near future. > >> >> 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. > > as anyway, the mTHP has been entirely exclusive, setting all PTEs > directly to writable should help reduce nr - 1 minor page faults and > ideally help reduce CONTPTE unfold and fold? Yes, doing that on CONTPTE granularity would very likely make sense. For anything bigger than that, I am not sure. Assuming we have a 1M folio mapped by PTEs. Trying to fault-around in aligned CONTPTE granularity likely makes sense. Bigger than that, I am not convinced. > > What is the downside to doing that? I also don't think mapping them > all together will waste memory? No, it's all about increasing the latency of individual write faults. -- Cheers, David / dhildenb