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[2003:cb:c708:2700:bdf6:739b:9f9d:862f]) by smtp.gmail.com with ESMTPSA id b16-20020a05600c4e1000b0040fafc8bb3asm204081wmq.9.2024.01.30.02.43.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Jan 2024 02:43:22 -0800 (PST) Message-ID: Date: Tue, 30 Jan 2024 11:43:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops in willneed range To: Mike Snitzer , Dave Chinner Cc: Ming Lei , Matthew Wilcox , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Don Dutile , Raghavendra K T , Alexander Viro , Christian Brauner , linux-block@vger.kernel.org References: <20240128142522.1524741-1-ming.lei@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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: B305AA0007 X-Stat-Signature: q7jomp763yeecoya4o3iuhwpgjqnfdxq X-Rspam-User: X-HE-Tag: 1706611407-82914 X-HE-Meta: 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 byTBekdH 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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 29.01.24 23:46, Mike Snitzer wrote: > On Mon, Jan 29 2024 at 5:12P -0500, > Dave Chinner wrote: > >> On Mon, Jan 29, 2024 at 12:19:02PM -0500, Mike Snitzer wrote: >>> While I'm sure this legacy application would love to not have to >>> change its code at all, I think we can all agree that we need to just >>> focus on how best to advise applications that have mixed workloads >>> accomplish efficient mmap+read of both sequential and random. >>> >>> To that end, I heard Dave clearly suggest 2 things: >>> >>> 1) update MADV/FADV_SEQUENTIAL to set file->f_ra.ra_pages to >>> bdi->io_pages, not bdi->ra_pages * 2 >>> >>> 2) Have the application first issue MADV_SEQUENTIAL to convey that for >>> the following MADV_WILLNEED is for sequential file load (so it is >>> desirable to use larger ra_pages) >>> >>> This overrides the default of bdi->ra_pages and _should_ provide the >>> required per-file duality of control for readahead, correct? >> >> I just discovered MADV_POPULATE_READ - see my reply to Ming >> up-thread about that. The applicaiton should use that instead of >> MADV_WILLNEED because it gives cache population guarantees that >> WILLNEED doesn't. Then we can look at optimising the performance of >> MADV_POPULATE_READ (if needed) as there is constrained scope we can >> optimise within in ways that we cannot do with WILLNEED. > > Nice find! Given commit 4ca9b3859dac ("mm/madvise: introduce > MADV_POPULATE_(READ|WRITE) to prefault page tables"), I've cc'd David > Hildenbrand just so he's in the loop. Thanks for CCing me. MADV_POPULATE_READ is indeed different; it doesn't give hints (not "might be a good idea to read some pages" like MADV_WILLNEED documents), it forces swapin/read/.../. In a sense, MADV_POPULATE_READ is similar to simply reading one byte from each PTE, triggering page faults. However, without actually reading from the target pages. MADV_POPULATE_READ has a conceptual benefit: we know exactly how much memory user space wants to have populated (which range). In contrast, page faults contain no such hints and we have to guess based on historical behavior. One could use that range information to *not* do any faultaround/readahead when we come via MADV_POPULATE_READ, and really only popoulate the range of interest. Further, one can use that range information to allocate larger folios, without having to guess where placement of a large folio is reasonable, and which size we should use. > > FYI, I proactively raised feedback and questions to the reporter of > this issue: > > CONTEXT: madvise(WILLNEED) doesn't convey the nature of the access, > sequential vs random, just the range that may be accessed. Indeed. The "problem" with MADV_SEQUENTIAL/MADV_RANDOM is that it will fragment/split VMAs. So applying it to smaller chunks (like one would do with MADV_WILLNEED) is likely not a good option. -- Cheers, David / dhildenb