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[2003:cb:c739:8e00:7a46:1b8c:8b13:d3d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432b05305a4sm214906925e9.5.2024.11.12.07.19.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Nov 2024 07:19:07 -0800 (PST) Message-ID: Date: Tue, 12 Nov 2024 16:19:07 +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: EDd59VMep1lcfdbw07XT_loo_e6jzJ5peh8b7vVw72E_1731424749 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: A52C114002D X-Rspamd-Server: rspam01 X-Stat-Signature: qyyec9hoc5n5btgq7nx8kidwkzozcqzs X-HE-Tag: 1731424722-812118 X-HE-Meta: U2FsdGVkX19xHWR2O33BBGXPuf5ugQYkb+It/8pYnXjW5szyMaoz/xHzMA4y39tc7UELHkwueK3C/Qi1j/Da8segpRUIoa+h/i9SL0Dy/hn6VHA80wVvnwX8yt/BJ2StSWuDfVkgQiBuqkZjp9/VwMLVD45sOaZ+qdrdcnfQoTr9WnOE1bKmIYKVkWhrmWoPVXY/6HORYyBphey8GVcVOH4xtCvyGPRbKr6jT7y9iWphG8Wcm/VeYVfiIx7H5ij17v5uzClPFMGGl2QO81re/cGkXqmN1NKpNYx29e18mIXMeT2E5DXvnmmy6YNh/JyAxECz2NIw+Qnb5sWNMwH61WBqGs1oMjudTNM96fDb72HihmqtwCdd//qZ1v2y/Zd2B16SfrbHryw5/RnDmTApnkA6K4mTfjE9ztcP8NjYgHHO8i1W/7UvU0LithqaY2C+RSGOhKJYHQFzpIeZdwaS1koBDOgPitXFoZxUj/qMk5GK9HlMdOha3i+KBttZX9NTg0HBzwnfSafSdTzVcih/jrX+YawAN6nodteD5oNPqbxnycC0dqPMXnPTratHb8fI+jEqnaGc89mD4McSYxhANOEu+8BfoMUxh4PYAHWCc0XfA3oCfIfApBger/pLyQmAwOYFaOD6I3n7dBCTZHRzksbVL4hFsNCfEWuo0b1ieBd9cEJRKWy5y0AInBA8nzlHdlQHxkUDuoNIytkFE7c9QgFHF0SUYjKmJdp94ajxJ8klZSN4eKHJLZm3SLwjThOjZzO539yEgWx5TInyoU+Rw5iL9VSllmAltWF7cNECDMmja1IwreZZ8i1/WLb5SPzy+oe9yREWrBsLN8o94XZ1sgCIOd61mIklCuXJjjYod5Z2eBayUzsahmRG2PSa/H4imykgNuGkPyYGLrpJT0F/nrSKCx18MJHYVHK334AkgyEBKtWUrLpJLELunuKo5OwrvayjSN3U2JzrOqaaZCG J3se06Ts CCK2xVf91ZeF+z1dr8ursFH1rZ0dHTauhjvWx4DO2XOtD9u4V42nxZmRhcmHG9zRvHe5p85SkG0HDA6XWxBj8x/Rj4120IMVS2psxLKeOpmxztstWDej+pOlqRYOObQmuOpyfLbBh655X/UUI0lKWFBa5kdggZitt79tPAGunhe9mNjqWkcAnfX/c4uzysjPoKMBCY41BljXblZViDFEW7adEhfwntP5sULqtESL3OK3yHhSQDs+vyPp/8Px0nAxIK9E507scG4Y2+dzTUD3nOs+nSZxHiTD9kmESeDc9L4xplHrs+ZLtZOZY0QNkn9DfZHCEY6R2Q8LKHwwx5XOLbj1+0EPID9/sSzXdpDWi/nMHLglbSs4R1DpEM6Cd3m2vCJfqnbyfd9Fb9FGM+ybAoW46hc+l7RlIFaPIMAMCo3hNCdm7GaW67Q07xexGMl8gdbBR+wBU0/ebNrTFJCOAvwKRmEXSs0U2Q54p3lv3rZQ8e2E= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000419, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> 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". :) "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". 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. > >> >> Maybe pr_info_once() when someone uses MADV_HUGEPAGE with such backends >> to tell the sysadmin that something stupid is happening ... > > It's not a flawed setup; it's just that this new feature doesn’t work > well with the existing settings, and updating those settings to > accommodate it isn't always feasible. I don't agree. But it really is Willy's take. The code, as it stands is confusing and nobody will be able to figure out how MADV_HUGEPAGE comes into play here and why we suddenly exceed "max/config" simply because "cur" is larger than max. For example, in the code ra->size = start - index; /* old async_size */ ra->size += req_count; ra->size = get_next_ra_size(ra, max_pages); What happens if ra->size was at max, then we add "req_count" and suddenly we exceed "max" and say "well, sure that's fine now". Even if MADV_HUGEPAGE was never involved? Maybe it cannot happen, but it sure is confusing. Not to mention that "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." sounds very odd :( -- Cheers, David / dhildenb