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[209.85.167.47]) by smtp.gmail.com with ESMTPSA id h13sm580819lji.134.2020.09.11.09.45.50 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Sep 2020 09:45:50 -0700 (PDT) Received: by mail-lf1-f47.google.com with SMTP id b12so6484633lfp.9 for ; Fri, 11 Sep 2020 09:45:50 -0700 (PDT) X-Received: by 2002:ac2:5594:: with SMTP id v20mr585120lfg.344.1599842749970; Fri, 11 Sep 2020 09:45:49 -0700 (PDT) MIME-Version: 1.0 References: <0dbc6ec8-45ea-0853-4856-2bc1e661a5a5@intel.com> <20200909142904.00b72921@thinkpad> <20200909192534.442f8984@thinkpad> <20200909180324.GI87483@ziepe.ca> <20200910093925.GB29166@oc3871087118.ibm.com> <20200910181319.GO87483@ziepe.ca> <0c9bcb54-914b-e582-dd6d-3861267b6c94@nvidia.com> <20200910221116.GQ87483@ziepe.ca> <20200911121955.GA10250@oc3871087118.ibm.com> In-Reply-To: <20200911121955.GA10250@oc3871087118.ibm.com> From: Linus Torvalds Date: Fri, 11 Sep 2020 09:45:33 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding To: Alexander Gordeev Cc: Jason Gunthorpe , John Hubbard , Gerald Schaefer , Dave Hansen , LKML , linux-mm , linux-arch , Andrew Morton , Russell King , Mike Rapoport , Catalin Marinas , Will Deacon , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Jeff Dike , Richard Weinberger , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Arnd Bergmann , Andrey Ryabinin , linux-x86 , linux-arm , linux-power , linux-sparc , linux-um , linux-s390 , Vasily Gorbik , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: A6E0118229835 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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 Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev wrote: > > What if the entry is still pud_present, but got remapped after > READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere? That can't happen. The GUP walk doesn't hold any locks, but it *is* done with interrupts disabled, and anybody who is modifying the page tables needs to do the TLB flush, and/or RCU-free them. The interrupt disable means that on architectures where the TLB flush involves an IPI, it will be delayed until afterwards, but it also acts as a big RCU read lock hammer. So the page tables can get modified under us, but the old pages won't be released and re-used. Linus