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[195.135.220.15]) by mx.google.com with ESMTPS id g23si3272283ejm.69.2019.04.23.19.31.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 23 Apr 2019 19:31:59 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning dave@stgolabs.net does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning dave@stgolabs.net does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=dave@stgolabs.net X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 0466CAF1F; Wed, 24 Apr 2019 02:31:59 +0000 (UTC) Date: Tue, 23 Apr 2019 19:31:52 -0700 From: Davidlohr Bueso To: Daniel Jordan , Christophe Leroy , akpm@linux-foundation.org, Alexey Kardashevskiy , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Paul Mackerras , Christoph Lameter , linuxppc-dev@lists.ozlabs.org, jgg@mellanox.com Subject: Re: [PATCH 5/6] powerpc/mmu: drop mmap_sem now that locked_vm is atomic Message-ID: <20190424023152.vrnyx4r4oapt7vdy@linux-r8p5> Mail-Followup-To: Daniel Jordan , Christophe Leroy , akpm@linux-foundation.org, Alexey Kardashevskiy , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Paul Mackerras , Christoph Lameter , linuxppc-dev@lists.ozlabs.org, jgg@mellanox.com References: <20190402204158.27582-1-daniel.m.jordan@oracle.com> <20190402204158.27582-6-daniel.m.jordan@oracle.com> <964bd5b0-f1e5-7bf0-5c58-18e75c550841@c-s.fr> <20190403164002.hued52o4mga4yprw@ca-dmjordan1.us.oracle.com> <20190424021544.ygqa4hvwbyb6nuxp@linux-r8p5> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: <20190424021544.ygqa4hvwbyb6nuxp@linux-r8p5> User-Agent: NeoMutt/20180323 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: On Tue, 23 Apr 2019, Bueso wrote: >On Wed, 03 Apr 2019, Daniel Jordan wrote: > >>On Wed, Apr 03, 2019 at 06:58:45AM +0200, Christophe Leroy wrote: >>>Le 02/04/2019 =E0 22:41, Daniel Jordan a =E9crit=A0: >>>> With locked_vm now an atomic, there is no need to take mmap_sem as >>>> writer. Delete and refactor accordingly. >>> >>>Could you please detail the change ? >> >>Ok, I'll be more specific in the next version, using some of your languag= e in >>fact. :) >> >>>It looks like this is not the only >>>change. I'm wondering what the consequences are. >>> >>>Before we did: >>>- lock >>>- calculate future value >>>- check the future value is acceptable >>>- update value if future value acceptable >>>- return error if future value non acceptable >>>- unlock >>> >>>Now we do: >>>- atomic update with future (possibly too high) value >>>- check the new value is acceptable >>>- atomic update back with older value if new value not acceptable and re= turn >>>error >>> >>>So if a concurrent action wants to increase locked_vm with an acceptable >>>step while another one has temporarily set it too high, it will now fail. >>> >>>I think we should keep the previous approach and do a cmpxchg after >>>validating the new value. > >Wouldn't the cmpxchg alternative also be exposed the locked_vm changing be= tween >validating the new value and the cmpxchg() and we'd bogusly fail even when= there >is still just because the value changed (I'm assuming we don't hold any lo= cks, >otherwise all this is pointless). > > current_locked =3D atomic_read(&mm->locked_vm); > new_locked =3D current_locked + npages; > if (new_locked < lock_limit) > if (cmpxchg(&mm->locked_vm, current_locked, new_locked) =3D=3D curren= t_locked) Err, this being !=3D of course.