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[2003:cb:c715:2a00:ac0a:9a14:a490:dc02]) by smtp.gmail.com with ESMTPSA id d2-20020a5d6442000000b002c70ce264bfsm2289924wrw.76.2023.05.18.06.56.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 18 May 2023 06:56:10 -0700 (PDT) Message-ID: <8b898768-a3c0-198d-a70c-9aa96b7f4a29@redhat.com> Date: Thu, 18 May 2023 15:56:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.11.0 Subject: Re: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm To: Sumit Garg , Christoph Hellwig Cc: Xiaoming Ding , Jens Wiklander , Matthias Brugger , AngeloGioacchino Del Regno , op-tee@lists.trustedfirmware.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, fei.xu@mediatek.com, srv_heupstream@mediatek.com, linux-mm@kvack.org References: <20230517031856.19660-1-xiaoming.ding@mediatek.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 6566B40010 X-Stat-Signature: mqq3xssq6f3hqk1c1sqmuw935xgi78m4 X-HE-Tag: 1684418176-276141 X-HE-Meta: U2FsdGVkX1+jQg91iNhA53kKuzCM4Y3QSighmGoXEnspmx1cwxm9FlelzQDlz3mGlGvRZloRVttLSA7LCSoB8qWyjGsGdfj1tRClsGo3AoRBEIKz+QC7nq7BexajJb+ht0E37dS/7raYSNc1J5j/Y4UsJ5klsrgAdAAIp3RBD4vN4iFf/OBUzuGExGDFwnCQBAkSJBItpec0Vqd8IhudKacJ6EwjF5341gt77jbpeGBcUbkvQ7OzWcCFhlVljBF2HDGFeKPO3bmMxwD4Fhz6kvuIDcdcwxk3GoIfxlrzGR0TXMVvS41gXEXu0g2ICJFjvw5oukjQjnz32E+X8zGVdpt93C6YChDpe4w+8wTKv1sLlzR7dOuexJmsgxZt/zxEpR8VJvffZYgKnhZrLQ6mMruSNGcvgneTfvTQUJPAbopPiA7txOldt4QZt/ectCdjoLA5B+MSj64aFw3frmXJzrkDVZNGG5TgNJkm1y7C+196yi6KRgWYpb9xvFBqI10dhjA8bMciuRXbDOnojZrB2kVM2uet1Oshj0ZRrVZPNgrATnRjl4ApkfVs7/PTMdVUMvx8aYZuU7UNIuLHTKpH0HyHJujLcEoAmtPo9smgl8kIwNJYSfPY4zocooSE4Xd2w4SlSjFNmx3q68DzN4SzYBOcZXKl8d7QQYYRjgFmGej/t6XEdVxuzNKt2+SXxPal0VGie0KY+TeeHyAMGrDhw3IqAHdcUNoLlm1DuI2jK8R2NVLM16vPL/Upj84hE/dOmO1EjOopPpwMO/Jj110l2G15Ct6vS65pJZfEFJKUBnU59iNYTGDDEX+VCRKw9HCDv/Ooo6OwviiGa9Sm0yRW9VNkWSHh5Hm93efqyVsJY2zZLW/pp4vB7WIIhz8im4latFIX9JVcbISi3l5Kjvq2UygKH2znG/4dNceDbZqRdhRspLWENkgUUZjaOG0MZf0/25x45roKDnQSH9V5u5d JTt3he5u 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 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 18.05.23 08:08, Sumit Garg wrote: > On Thu, 18 May 2023 at 09:51, Christoph Hellwig wrote: >> >> On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand wrote: >>> In general: if user space controls it -> possibly forever -> long-term. Even >>> if in most cases it's a short delay: there is no trusting on user space. >>> >>> For example, iouring fixed buffers keep pages pinned until user space >>> decides to unregistered the buffers -> long-term. >>> >>> Short-term is, for example, something like O_DIRECT where we pin -> DMA -> >>> unpin in essentially one operation. >> >> Btw, one thing that's been on my mind is that I think we got the >> polarity on FOLL_LONGTERM wrong. Instead of opting into the long term >> behavior it really should be the default, with a FOLL_EPHEMERAL flag >> to opt out of it. And every users of this flag is required to have >> a comment explaining the life time rules for the pin.. > > It does look like a better approach to me given the very nature of > user space pages. Yeah, there is a lot of historical baggage. For example, FOLL_GET should be inaccessible to kernel modules completely at one point, to be only used by selected core-mm pieces. Maybe we should even disallow passing in FOLL_LONGTERM as a flag and only provide functions like pin_user_pages() vs. pin_user_pages_longterm(). Then, discussions about conditional flag-setting are no more :) ... or even use pin_user_pages_shortterm() vs. pin_user_pages() ... to make the default be longterm. -- Thanks, David / dhildenb