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Tue, 12 Nov 2024 06:07:18 -0800 (PST) Received: from [192.168.1.116] ([96.43.243.2]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-29546c92853sm3381006fac.18.2024.11.12.06.07.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Nov 2024 06:07:18 -0800 (PST) Message-ID: <1671bb73-2e51-4ecc-b33d-d0b483348cda@kernel.dk> Date: Tue, 12 Nov 2024 07:07:16 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 03/16] mm: add PG_uncached page flag To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org, willy@infradead.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org References: <20241111234842.2024180-1-axboe@kernel.dk> <20241111234842.2024180-4-axboe@kernel.dk> Content-Language: en-US From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 0AA1CC002E X-Stat-Signature: nn46ridu5hwk555k99jawuxkx78swcw4 X-Rspam-User: X-HE-Tag: 1731420387-539172 X-HE-Meta: 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 AIUkfHq4 t1wcNDQ59v9jS4tqHnf5mQdLDKV41ZiEwolsQ0UXT8Q4mXsVDjyGIWqP7g7g5mhYdE7yHCQvCzb4XgIw5+zYplJZqEfTOVaOEPdxsat2xcoWJxUezcCHjAXEdT+cLRDIUtiKkTokiIleHNQjoK8wD1fYUOaTkUUjEnTmC2RdpnL98YvebCtG19+UpIUtzahzhw/vboCKzU5UM4HbHhOoCJP8+5Y49lUV4pn6NrB8kbl3agkdgM1LCAG3211VFL5jtJUfArLB05GZKqLmIbpEpxXIw4EO7LDjAreo4IUAIZL36uOqCof0tNVNpVbduetZZM+NndOQTXcewMKF3Vam8n58KfXyiSfQdXqONmlEzWzoFP2dOQI3VmRJhDwzO2WwB5TrBrmYlLnltqQBf8tK73gN/bA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 11/12/24 2:12 AM, Kirill A. Shutemov wrote: > On Mon, Nov 11, 2024 at 04:37:30PM -0700, Jens Axboe wrote: >> Add a page flag that file IO can use to indicate that the IO being done >> is uncached, as in it should not persist in the page cache after the IO >> has been completed. > > I have not found a way to avoid using a new bit. I am unsure if we have > enough bits on 32-bit systems with all possible features enabled. I think it should be OK, at least the kernel test bot reports build success on all the archs it tests, which has a lot of 32-bit archs. I have to say I didn't check on numbering and if the mm subsystem has a BUILD_BUG_ON() for bits exceeding the allowable value for unsigned long on the host, but I'm assuming it does? > In the worst-case scenario, we may need to make the feature 64-bit only. > I believe it should be acceptable as long as userspace is prepared for the > possibility that RWF_UNCACHED may fail. It is not going to be supported by > all filesystems anyway. Right, I would not even see that as a big issue. 32-bit would just see -EOPNOTSUPP for any fs, even ones that support it on 64-bit archs. -- Jens Axboe