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[2003:cb:c708:1500:d584:7ad8:d3f7:5539]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432d552d0a8sm19092625e9.39.2024.11.13.02.24.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Nov 2024 02:24:02 -0800 (PST) Message-ID: Date: Wed, 13 Nov 2024 11:24:01 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> <88211032-80e1-4067-a74c-c9dcea1abff8@redhat.com> From: David Hildenbrand Content-Language: en-US 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit >> >> 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 ... > > It depends on the use case. For our hardop servers, we set it to 4MB, > as they prioritize throughput over latency. However, for our > Kubernetes servers, we keep it at 128KB since those services are more > latency-sensitive, and increasing it could lead to more frequent > latency spikes. Thanks for sharing. > >> 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'll submit an additional patch to update the documentation for MADV_HUGEPAGE. > >> >> 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? > > Typically, users set read_ahead_kb to aligned sizes, such as 128KB, > 256KB, 512KB, 1MB, 2MB, 4MB, or 8MB. With this patch, MADV_HUGEPAGE > functions well for all these settings. However, if read_ahead_kb is > set to a non-hugepage-aligned size (e.g., 4MB + 128KB), MADV_HUGEPAGE > won’t work. This is because the initial readahead size for > MADV_HUGEPAGE is set to 4MB, as established in commit 4687fdbb805a: > > ra->size = HPAGE_PMD_NR; > if (!(vmf->vma->vm_flags & VM_RAND_READ)) > ra->size *= 2; > > However, as Willy noted, non-aligned settings are quite stupid, so we > should disregard them. Right. What I've been wondering, to make this code easier to understand, if there should be some kind of ra->size_fixed=true parameter that tells readahead code to simply not mess with the ra->size until something changes. (below) [...] >>> 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? > > I reached this conclusion by tracing ra->size in each > page_cache_ra_order() call, but I’m not fully equipped to provide all > the details ;） I've been staring at the readhead code for 30 minutes and I am still lost. Either I'm too stupid for the code or the code is too complicated. If we'd have something like ra->start += ra->size; /* * If someone like VM_HUGEPAGE fixed the size, just don't mess with it. */ if (!ra->size_fixed) ra->size = get_next_ra_size(ra, max_pages); ra->async_size = ra->size; It would be a lot clearer at least to me -- and we'd likely be able to get rid of the "unaligned readhahead" oddity. If we'd grep for who sets "size_fixed", we could even figure out how all of this belongs together. -- Cheers, David / dhildenb