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a=rsa-sha256; c=relaxed/relaxed; d=stgolabs.net; s=dreamhost; t=1684010629; bh=ehonwRQjo3E8b31rGbigHFuhtd753Uy5ZgwZqWj0E64=; h=Date:From:To:Cc:Subject:Content-Type; b=XFjqC33YUbOOBqR7qMq7te0o24fAFLDk4JyP8NZf5wSN6fDijoI8lYte2LgAqTtsm ORmfxbJ7H9H9o4697eYugNgd5kTGXlsUNxKh3wLhBXfZ38aFrS74aQsIpQZJ0zAm2h WzEird8Jw23f2537sXsQrrJ3gXNR1xNOKo0MmoHfbcUBeTSnVj/tF6v2AZnouqxBy9 9zfB7HRBfiV+4i/FgAgjAZdbTeg6SauPLQlruKJFg2XfeXweBiEWntQ4K4Z430Rrht /BOWSJmTmUo3SENfyvsZ4e5KtZ2vjJmDeDkDNCn4TAmk5TTGqzBEXPBDZGlmDCXKZF Ax0tmpET39d0A== Date: Sat, 13 May 2023 13:10:56 -0700 From: Davidlohr Bueso To: Matthew Wilcox Cc: Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart , =?utf-8?B?QW5kcsOvwr/CvQ==?= Almeida , linux-kernel@vger.kernel.org, Jason Gunthorpe , linux-mm@kvack.org Subject: Re: Futexes & Folios Message-ID: <4b5rvoi6w6nhiihmog6xjftoveohd47oooodekqedo5qhflmrt@jemw5ss5xps3> Mail-Followup-To: Matthew Wilcox , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart , =?utf-8?B?QW5kcsOvwr/CvQ==?= Almeida , linux-kernel@vger.kernel.org, Jason Gunthorpe , linux-mm@kvack.org References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20230407 X-Stat-Signature: pynx8egcdefk6j7obmhbpq59be7ue9fb X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 722FD4000A X-Rspam-User: X-HE-Tag: 1684010632-318935 X-HE-Meta: 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 +y8GQgb2 LsxLvQsTeB3KywN+fR+bQ5bmXTobUApIj55fI5jub1uMByH4dJg3KeCEU9xwglPqFoq486u1mVnkFP0rsLHGyIZQShf47pZXLRm5kv9yHB0JWj3mw1NDh1V9KXi2DlrSCASAfTnIQ3+1Ot1eW0dRWcySzcqQB4keXxYoflcITe6uGrYOzGGP3yJqnUSVtokCKsdSoZ1N0VBlg51wma+J0BjMKhXehIvyq/4+J5Ge0TfRsxjZCUFpJL8+K6jNZGmBgOrABO2j6uULik/g= X-Bogosity: Ham, tests=bogofilter, spamicity=0.025478, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, 13 May 2023, Matthew Wilcox wrote: >Here's a patch which converts the futex code from using pages to using >folios. I do have some questions. > > - I assume we really do want to base the key on the page offset of the > futex in the file. That is, it's not good enough to base the key > on the folio's index because the folio can be split after setting up > the key, and if it is we'd then fail to find the futex later. > - In hugetlbfs, it doesn't matter whether the pgoff is based on base > pages or huge page size -- as long as it's consistent between > invocations, everything will work As tglx already mentioned, yes to both. >(I have ideas about a get_user_folio() but those are not represented in >this patch) Btw, regarding some of the concerns about the gup overhead for futexes in the physr discussion at lsfmm; 'perf bench futex hash' would be a good way of measuring any impact at a micro level and going from there. This is only necessary of course for shared futexes, fast/private ones just use the address space. Thanks, Davidlohr