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[2003:cb:c708:1500:d584:7ad8:d3f7:5539]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-381fc0f5f91sm9180316f8f.62.2024.11.13.00.28.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Nov 2024 00:28:34 -0800 (PST) Message-ID: <88211032-80e1-4067-a74c-c9dcea1abff8@redhat.com> Date: Wed, 13 Nov 2024 09:28:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/readahead: Fix large folio support in async readahead To: Yafang Shao Cc: willy@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, stable@vger.kernel.org References: <20241108141710.9721-1-laoar.shao@gmail.com> <85cfc467-320f-4388-b027-2cbad85dfbed@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: T9b0LkjAGG26ytAHebAfvihuvbmty-ZkZ01yDPGRzXw_1731486517 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B80691C0004 X-Stat-Signature: 6x6n3wu5e9ecrjjmi1ztzuoixbsm31u6 X-Rspam-User: X-HE-Tag: 1731486467-150827 X-HE-Meta: U2FsdGVkX18uODqBjRx+O5mATch65K+oP/XV+qbX87/lJoIYMpeyHMeVpoELhFEalCm8sCsVD1NIlLOFjIRSBA1AU27JvHkafpdwW74ROK9TQmpdVjaAVOwLGjaDVd/aYhJN9o/BOLac0FspJqUYXI5f5LQddFrbZZ/C6+kZlqVcegpXYvty2mXKpOk6iRt6sRHEvbyPsBzZHQrOpPMreVv4htTCnTkB9N0uWR+iOh1unATZcqEFTQ2W9JJ6+wJejh0WtHpd5tXH8hC+L2NwnJNgEfdJMFDL+Q78/TmciamjZdUdIhhDnTcAPB5kYxYStSNY14+QblKuqpxrMc4StQT4gGEsJQUhsWTciPzCjry3W4qTtW0IN88azVB8xmmVHS+bu18jk6U9bMX04rOZmgA0zOTDWYkwIGVUXuA2pYvLtpbOlJcIqxI7iKoqzPlTSf+gZdwc/YxzgbgbxVICSSkqeFZbaj7095W0XQm/0GZ49hKoI1vFklyFkoaAJhP9MiUnqW0o3f6mgsuYAeAXxygH4dQwsdLGoh1vGD9yurs5D3HrcvlPyKl+s+w3YveOW3l7Uas3Tf3mm7pbjMWRKbO7O0LjkkLp4y9cuyff5OTVEX1b5qh2RtIra59u4gqQlt0TMwbDTSzsCpVWTVKuBa3zutEFH7rNrSO6FKWZR9TfoHV13xu6gfBy48UNTHyMfs0Zkl2TFP6DH+Vg59XbftcTeUwjw+8919h1yCNBw722x5PVLCg5lWkOFFoSRfgF8FeUUTtSVrXjrSnvpq6AxmzWX1hfFQvTy7wIAA2ciiNw+HjBv43ZWzMVjS3Q9CxHGIbVHDBeAk4dHBwX2ZOPVIY6JGQ1LRYsg2czEcsDA+GiKjvtzoByc5DmWUVBT1ASbZ2jCtn9PxZ4QgDnT4SbwfXc4kr5qp9VZtgjjImwqTCzMsFBESrLI3yyIDgHNCmGrty4EAKg1ahETnJVxIi hdHCl0HO 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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 13.11.24 03:16, Yafang Shao wrote: > On Tue, Nov 12, 2024 at 11:19 PM David Hildenbrand wrote: >> >>>> Sorry, but this code is getting quite confusing, especially with such >>>> misleading "large folio" comments. >>>> >>>> Even without MADV_HUGEPAGE we will be allocating large folios, as >>>> emphasized by Willy [1]. So the only thing MADV_HUGEPAGE controls is >>>> *which* large folios we allocate. .. as Willy says [2]: "We were only >>>> intending to breach the 'max' for the MADV_HUGE case, not for all cases." >>>> >>>> I have no idea how *anybody* should derive from the code here that we >>>> treat MADV_HUGEPAGE in a special way. >>>> >>>> Simply completely confusing. >>>> >>>> My interpretation of "I don't know if we should try to defend a stupid >>>> sysadmin against the consequences of their misconfiguration like this" >>>> means" would be "drop this patch and don't change anything". >>> >>> Without this change, large folios won’t function as expected. >>> Currently, to support MADV_HUGEPAGE, you’d need to set readahead_kb to >>> 2MB, 4MB, or more. However, many applications run without >> > MADV_HUGEPAGE, and a larger readahead_kb might not be optimal for> them. >> >> Someone configured: "Don't readahead more than 128KiB" >> >> And then we complain why we "don't readahead more than 128KiB". > > That is just bikeshielding. It's called "reading the documentation and trying to make sense of a patch". ;) > > So, what’s your suggestion? Simply setting readahead_kb to 2MB? That > would almost certainly cause issues elsewhere. I'm not 100% sure. I'm trying to make sense of it all. And I assume there is a relevant difference now between "readahead 2M using all 4k pages" and "readahead 2M using a single large folio". I agree that likely readahead using many 4k pages is a worse idea than just using a single large folio ... if we manage to allocate one. And it's all not that clear in the code ... FWIW, I looked at "read_ahead_kb" values on my Fedora40 notebook and they are all set to 128KiB. I'm not so sure if they really should be that small ... or if large folio readahead code should just be able to exceed it. >> "mm/filemap: Support VM_HUGEPAGE for file mappings" talks about "even if >> we have no history of readahead being successful". >> >> So not about exceeding the configured limit, but exceeding the >> "readahead history". >> >> So I consider VM_HUGEPAGE the sign here to "ignore readahead history" >> and not to "violate the config". > > MADV_HUGEPAGE is definitely a new addition to readahead, and its > behavior isn’t yet defined in the documentation. All we need to do is > clarify its behavior there. The documentation isn’t set in stone—we > can update it as long as it doesn’t disrupt existing applications. If Willy thinks this is the way to go, then we should document that MADV_HUGEPAGE may ignore the parameter, agreed. I still don't understand your one comment: "It's worth noting that if read_ahead_kb is set to a larger value that isn't aligned with huge page sizes (e.g., 4MB + 128KB), it may still fail to map to hugepages." Do you mean that MADV_HUGEPAGE+read_ahead_kb<=4M will give you 2M pages, but MADV_HUGEPAGE+read_ahead_kb>4M won't? Or is this the case without MADV_HUGEPAGE? If MADV_HUGEPAGE ignores read_ahead_kb completely, it's easy to document. > >> >> But that's just my opinion. >> >>> >>>> >>>> No changes to API, no confusing code. >>> >>> New features like large folios can often create confusion with >>> existing rules or APIs, correct? >> >> We should not try making it even more confusing, if possible. > > A quick tip for you: the readahead size already exceeds readahead_kb > even without MADV_HUGEPAGE. You might want to spend some time tracing > that behavior. Care to save me some time and point me at what you mean? > > In summary, it’s really the readahead code itself that’s causing the > confusion—not MADV_HUGEPAGE. Let me dig into the code in more detail if can make sense of it all. -- Cheers, David / dhildenb