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[2003:cb:c706:6b00:bf49:f14b:380d:f871]) by smtp.gmail.com with ESMTPSA id g9-20020a05600c308900b003fe15c466f3sm235178wmn.0.2023.07.28.14.02.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Jul 2023 14:02:47 -0700 (PDT) Message-ID: <69a5f457-63b6-2d4f-e5c0-4b3de1e6c9f1@redhat.com> Date: Fri, 28 Jul 2023 23:02:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 To: Linus Torvalds Cc: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Andrew Morton , liubo , Matthew Wilcox , Hugh Dickins , Jason Gunthorpe , John Hubbard References: <20230727212845.135673-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout 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-Stat-Signature: gksebi9wnafr4hqndu69jsxoxmmb9pwd X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 232CC100029 X-Rspam-User: X-HE-Tag: 1690578171-192376 X-HE-Meta: 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 rtsKflfb McnbFPJdQGUwzkb3irdzq951YqwqGBOSD7zbKDX3wb0JpI4uU3+laVwHWXiIz4kto3MBuv4g4c1Ts4WRRnKjb6NpbI3wU0+2GLkiWZwCZSBryDzW8/Uu7J5cZZ6ayUDgQRbDxV+2jBfw3DWngnCym3GfNgg6oASwRRgKxTo7Ir/LHSFGiyC0unmEiftLFLXTqnwkXfCpSCX8qjb7nAvFF8zUMfEBTu9TffI28pSiOtWFvzbmJqdrpFwhE4cblaJ0RKAaq4zhT5dCy5A7vkCSWlPEc8qrwpZL9o14npjwfnq5cODqPM5INbD5t4o4LroMp3ng+rdvS3iD73aFNMjtzBQ/Vz7ZFQom1NE8cbHTMBPb9mZKR4LbgmaWGnrAMgaZX5XQWD/H4HSIy8yNMIoS+iBG2tffPMa6dlWtib1sjCAf21Er1zzNKs77sJX2Q3wjGY2yuhI0wKrkSDrI= 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 28.07.23 22:50, Linus Torvalds wrote: > On Fri, 28 Jul 2023 at 13:33, David Hildenbrand wrote: >> >> So would you rather favor a FOLL_NUMA that has to be passed from the >> outside by selected callers or a FOLL_NUMA that is set on the GUP path >> unconditionally (but left clear for follow_page())? > > I'd rather see the FOLL_NUMA that has to be set by odd cases, and that > is never set by any sane user. Thanks! > > And it should not be called FOLL_NUMA. It should be called something > else. Because *not* having it doesn't disable following pages across > NUMA boundaries, and the name is actively misleading. > > It sounds like what KVM actually wants is a "Do NOT follow NUMA pages, > I'll force a page fault". > > And the fact that KVM wants a fault for NUMA pages shouldn't mean that > others - who clearly cannot care - get that insane behavior by > default. For KVM it represents actual CPU access. To map these pages into the VM MMU we have to look them up from the process -- in the context of the faulting CPU. So it makes a lot of sense for KVM. (which is also where autonuma gets heavily used) > > The name should reflect that, instead of being the misleading mess of > FOLL_FORCE and bad naming that it is now. > > So maybe it can be called "FOLL_HONOR_NUMA_FAULT" or something, to > make it clear that it's the *opposite* of FOLL_FORCE, and that it > honors the NUMA faulting that nobody should care about. Naming sounds much better to me. > > Then the KVM code can have a big comment about *why* it sets that bit. Yes. > > Hmm? Can we please aim for something that is understandable and > documented? No odd implicit rules. No "force NUMA fault even when it > makes no sense". No tie-in with FOLL_FORCE. I mean, I messed all that FOLL_NUMA handling up because I was very confused. So I'm all for better documentation. Can we get a simple revert in first (without that FOLL_FORCE special casing and ideally with a better name) to handle stable backports, and I'll follow-up with more documentation and letting GUP callers pass in that flag instead? That would help a lot. Then we also have more time to let that "move it to GUP callers" mature a bit in -next, to see if we find any surprises? -- Cheers, David / dhildenb