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b=QcYMbdgF6Mz5fC/5+OIcISEIML/hLi20/XfclgLIvxREhH1W52SIRO0kfNwWvcVcu 9b07HTep+Jt9Jzxft+/qdGiBxxRwaH7fxeIKfBIprufCOqtU2v7hIA4OtdQcJ7hL8p rqoijW2f+Nm8yBUlaWetRvwJ1/AQd3k88NnoNn+Z9GfzXoClvA9z8+yK1Gn0QRRq1Z dH/HyhrErjcCagjCJaTUvvKwRzbGZFSNMqIytlsF/ldBdF26D2oeObDcLObicnhmrS MuQF4jh3x4gjHFdx13S42I0loT5s0U95oHmEPnAr4uTHhiJbEVwuu/7pGmlNL9cNAm IJsI5HNzZuDug== Message-ID: <16e9bcf6-3e7d-44e9-8b07-00a694c8e493@kernel.org> Date: Fri, 3 Jul 2026 10:51:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 1/4] mm/page_owner: add print_mode filter Content-Language: en-US To: "zhen.ni" , Ye Liu , akpm@linux-foundation.org Cc: surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260625043101.338794-1-zhen.ni@easystack.cn> <20260625043101.338794-2-zhen.ni@easystack.cn> <49526116-ce35-4447-bd98-f5f0ca12d92a@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 82851100007 X-Stat-Signature: hermaskc64ygrjte16my7yp7bzh16mco X-HE-Tag: 1783068680-84994 X-HE-Meta: U2FsdGVkX181f46Xz529ta6Xzgsv26AJr6UVryrWH4QlTwPXGLDTN18GZTRM6MIsRpe0I2C7HmNDj3MleQq1YtxWhgtK1IZi/afNT1nKzXGmpuEbImEnvmxzd53JqMNi/cZByN/Lv/tejfwoJniP8xAvZIJ7acOPJZ911UIZAKPI+jCvh/cHHM7y/OmfSi80huitTFt1lJtmGUAwD+4vvSdlib02kJrbBifPWzLQmqYS7wOupiqX2lZtWTIRrHb2aYRpfWvJTHuLZNoXgi9jDugnvvK265Z8j/J3XMXZ+SDH6bw7B6XsZKPCHUoC7MYXuQWvS4SKTBY16xZg4x64kfClaL0dWgTHXTbx5Cgp3UtqT3xlNNgHNZ4uGk3GhrascFbaC1kZWSBfuCcqDyVP9CflWSzkGXsxLgwYIJdlU59dKC637qiegxKvMC6YVSeG98p0I8yw/9SUsJOFha7ZNkodEbRZbJY7R5B8LQd+N8UqM3xe8D4R4md/Ti4YDRitWKt44nvaMOFtcR9q/BUxzYjk+NIzgbuyYcMyjgn8h7U5Xm6aCvGh7e06vJRBZ70NwWnbfG/4ZtXsvT2h/DuTIsUMR0nz6ULQU1U9NPISS+7j92lFSqUrdpO4U8Vl9jGn5ZUrcIFf1dpQuGKF38DxVln794ujpz0nt0coXbDINxLdcU/LlgTCwrgzFMdcQU/KB0+f8UkulEroF/y1KhVS54ODcNr+UyvzTh+OQJJVU/YkLtG4AKaCIom2MVZ8kp3OltfgusVf9VvPJzG1FCpFgkmO2i+z4ULsqsJ/7L0fuY2XicJA1xc0BVbYYauKKB7qcew9ElLPICTp+zwFDS5VbhXFvRkGpGkDShejwl9ne1ge1u1WJkCMo3jerwMKkkb3PcHyXvyJ+SDV3pw3zWE0ArigBoWBWLNGFgxoGbj9iAxn7NzdAudpiXFaMAXt9lJUytC9tL1jO8uvPopgTSw LtFTYMO1 PGqEFXqAbeHRmPgzBPKnMc/rhUepzGeY6sy0kLkf285MjqtWvdfKNpApnlXzmIEShw6lppHzDSYs5i3GTQELJTCa2fzMqLHzatAf5RNHQhj5qg+hwsUpVEMFLaKJTI3VMyGgKfo5zlgOVyIx5mraiLh6PI9CiD/5eUHtc8FjyDHvzBWC/tb+xjkGCW59Bjp69O9fn7DPAm5A+UUcvE0Tlhf9kKvEe7kgg8RUv6Y0XhzCdGLshDlsW4xIdxD0w842ex96w3ds2FwBrGnJneNOThl6jhgRQSEqOd73spsstKSYev9QmkSF0uwcAA65cNeudUxdH1nmftzlYS/I= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/3/26 10:14, zhen.ni wrote: > > > 在 2026/6/29 17:30, Vlastimil Babka (SUSE) 写道: >> On 6/29/26 04:59, Ye Liu wrote: >>> >>> 在 2026/6/25 12:30, Zhen Ni 写道: >>> Hi , Zhen >>> The spinlock in struct page_owner_filter_state is unnecessary and adds significant overhead in the read path. >>> >>> 1. Per-fd isolation: the state is allocated per open() and stored in file->private_data. >>> There is no cross-fd contention possible. >>> 2. Hot path cost: the lock is taken for every single page in read_page_owner() and >>> print_page_owner(). A single read can traverse millions of pages, each paying >>> spin_lock_irqsave/irqrestore — including interrupt disable — just to read a mode >>> enum or check a nodemask. This is measurable overhead for no real benefit. >>> 3. No practical race: nobody writes filter config to an fd while simultaneously reading from it. >>> >>> Suggest dropping the lock entirely. >>> >>> Just my take though — happy to follow whatever the other reviewers prefer here. >> >> I agree. If someone is writing (updating filter) and reading (getting >> page_owner output) at the same time from multiple threads, they might get >> inconsistent results but that's getting what you ask for. Importantly it >> can't cause any crash, AFAICS. >> >> > > Hi Vlastimil, Ye, > > Thanks for the review. I understand your concerns about the spinlock > overhead in the read path. > > The spinlock does have its use case: it prevents race conditions when > multiple threads share the same file descriptor and call read() and > write() concurrently. While we recommend users use the page_owner_filter > tool, we cannot exclude the possibility that some users might directly > share the fd across threads. Yeah and they will then get inconsistent results even with the spinlock, we don't have to prevent that. > That said, I'm open to discussion on whether we need the spinlock. As > Vlastimil noted, the issue isn't severe enough to cause crashes. My v8 > version didn't have the spinlock - I added it in response to review > feedback. It was in response to sashiko. Those suggestions are sometimes useful, sometimes not. It's good to consider them, but critically, and not implement blindly. > So the question is really whether we want to protect multi-threaded fd We don't. > sharing or not. Because, the overhead is small in non-contended cases > (single-threaded usage) since there are no competing lock holders. The atomic operations are still expensive and unnecessary. Thanks. > Thanks, > Zhen