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[2003:cb:c714:c00:b08b:a871:ce99:dfde]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3680db0484dsm12077535f8f.110.2024.07.17.07.25.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 17 Jul 2024 07:25:41 -0700 (PDT) Message-ID: <7efc4b0b-4e12-4b29-815f-0854182ce135@redhat.com> Date: Wed, 17 Jul 2024 16:25:40 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/4] Control folio sizes used for page cache memory To: Ryan Roberts , Andrew Morton , Hugh Dickins , Jonathan Corbet , "Matthew Wilcox (Oracle)" , Barry Song , Lance Yang , Baolin Wang , Gavin Shan , Pankaj Raghav , Daniel Gomez Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20240717071257.4141363-1-ryan.roberts@arm.com> <99b33a29-e97a-4932-8d7a-85bc01885d18@arm.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: <99b33a29-e97a-4932-8d7a-85bc01885d18@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 17.07.24 12:45, Ryan Roberts wrote: > On 17/07/2024 11:31, David Hildenbrand wrote: >> On 17.07.24 09:12, Ryan Roberts wrote: >>> Hi All, >>> >>> This series is an RFC that adds sysfs and kernel cmdline controls to configure >>> the set of allowed large folio sizes that can be used when allocating >>> file-memory for the page cache. As part of the control mechanism, it provides >>> for a special-case "preferred folio size for executable mappings" marker. >>> >>> I'm trying to solve 2 separate problems with this series: >>> >>> 1. Reduce pressure in iTLB and improve performance on arm64: This is a modified >>> approach for the change at [1]. Instead of hardcoding the preferred executable >>> folio size into the arch, user space can now select it. This decouples the arch >>> code and also makes the mechanism more generic; it can be bypassed (the default) >>> or any folio size can be set. For my use case, 64K is preferred, but I've also >>> heard from Willy of a use case where putting all text into 2M PMD-sized folios >>> is preferred. This approach avoids the need for synchonous MADV_COLLAPSE (and >>> therefore faulting in all text ahead of time) to achieve that. >>> >>> 2. Reduce memory fragmentation in systems under high memory pressure (e.g. >>> Android): The theory goes that if all folios are 64K, then failure to allocate a >>> 64K folio should become unlikely. But if the page cache is allocating lots of >>> different orders, with most allocations having an order below 64K (as is the >>> case today) then ability to allocate 64K folios diminishes. By providing control >>> over the allowed set of folio sizes, we can tune to avoid crucial 64K folio >>> allocation failure. Additionally I've heard (second hand) of the need to disable >>> large folios in the page cache entirely due to latency concerns in some >>> settings. These controls allow all of this without kernel changes. >>> >>> The value of (1) is clear and the performance improvements are documented in >>> patch 2. I don't yet have any data demonstrating the theory for (2) since I >>> can't reproduce the setup that Barry had at [2]. But my view is that by adding >>> these controls we will enable the community to explore further, in the same way >>> that the anon mTHP controls helped harden the understanding for anonymous >>> memory. >>> >>> --- >> >> How would this interact with other requirements we get from the filesystem (for >> example, because of the device) [1]. >> >> Assuming a device has a filesystem has a min order of X, but we disable anything >>> = X, how would we combine that configuration/information? > > Currently order-0 is implicitly the "always-on" fallback order. My thinking was > that with [1], the specified min order just becomes that "always-on" fallback order. > > Today: > > orders = file_orders_always() | BIT(0); > > Tomorrow: > > orders = (file_orders_always() & ~(BIT(min_order) - 1)) | BIT(min_order); > > That does mean that in this case, a user-disabled order could still be used. So > the controls are really hints rather than definitive commands. Okay, because that's a difference to order-0, which is -- as you note -- always-on (not even a toggle). Staring at patch #1, you use the name "file_enable". That might indeed cause some confusion. Thinking out loud, I wonder if a different terminology could better express the semantics. Hm ... but maybe it only would have to be documented. Thanks for the details. -- Cheers, David / dhildenb