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[2003:cb:c706:6b00:bf49:f14b:380d:f871]) by smtp.gmail.com with ESMTPSA id o12-20020adfcf0c000000b00301a351a8d6sm5544158wrj.84.2023.07.28.12.52.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Jul 2023 12:52:12 -0700 (PDT) Message-ID: Date: Fri, 28 Jul 2023 21:52:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout To: Peter Xu , Linus Torvalds Cc: 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 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-Stat-Signature: 5hdhijho36nt4u1keh8ema3jq9kii3iq X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: C6B1E180016 X-HE-Tag: 1690573936-197030 X-HE-Meta: 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 JtcBuqjO HMdEt317XP0bvQDH1ohHXHpNOf4tEpSrdTkgKKMdJJyi7mszebYw0SaIRy4vuxwWR24AH2g9cxzCGMJqlDvm4/kNzUfaSN1z+OAD8TB74Vg7pCe0xsvub0LChNEv+UhMOjLliE0XtZ1gpxDT5pCDAivrYTCRRPX/nGsEfb28STjlvpmUJfAAsQpjigMvMha70wRXHi7Nqqi8ZskUuEo2dMcJtnt/Fu9uQa432SzjS0UpvFtp+sULtVGbb66Z7qyw3wsFGaJCQ99Ydh2uo/MIw1cXvOsE1nnrsp0i6vBDJXPVVCdv3c6VWBvblV/8cYzReFoeGeYZczfAgc/AVJMUqAZhpW186aXYvLgGD3rsbyJgHL/jTSSnyWz/g6cJ2OlW1MU6waJVDamksuDtxwg+1ZBgXiqG3Ch69zoTvBIjiRKS9MX8= 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 21:39, Peter Xu wrote: > Hi, Linus, > > On Fri, Jul 28, 2023 at 09:18:45AM -0700, Linus Torvalds wrote: >> The original reason for FOLL_NUMA simply does not exist any more. We >> know exactly when a page is marked for NUMA faulting, and we should >> simply *ignore* it for GUP and follow_page(). >> >> I think we should treat a NUMA-faulting page as just being present >> (and not NUMA-fault it). > > But then does it means that any gup-only user will have numa balancing > completely disabled? Since as long as the page will only be accessed by > GUP, the numa balancing will never trigger anyway.. I think KVM is > manipulating guest pages just like that. Not sure whether it means it'll > void the whole numa effort there. > > If we allow that GUP from happening (taking protnone as present) I assume > it'll also stop any further numa balancing on this very page to trigger > too, because even if some page fault handler triggered on this protnone > page later that is not GUP anymore, when it wants to migrate the page to > the other numa node it'll see someone is holding a reference on it already, > and then we should give up the balancing. > > So to me FOLL_NUMA (or any identifier like it.. just to describe the > caller's need; some may really just want to fetch the pfn/page) still makes > sense. But maybe I totally misunderstood above.. Yes, I agree, took me a bit longer to realize (being a KVM developer :) ... I'm really ready for the weekend). So if this series is not acceptable then better revert that commit -- or let callers like KVM specify FOLL_NUMA. -- Cheers, David / dhildenb