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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id i1sm6457907qkd.58.2020.09.10.06.02.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2020 06:02:34 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kGMDh-004Ksi-Dp; Thu, 10 Sep 2020 10:02:33 -0300 Date: Thu, 10 Sep 2020 10:02:33 -0300 From: Jason Gunthorpe To: Alexander Gordeev Cc: Gerald Schaefer , Dave Hansen , John Hubbard , LKML , linux-mm , linux-arch , Andrew Morton , Linus Torvalds , 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 Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding Message-ID: <20200910130233.GK87483@ziepe.ca> References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com> <20200907180058.64880-2-gerald.schaefer@linux.ibm.com> <0dbc6ec8-45ea-0853-4856-2bc1e661a5a5@intel.com> <20200909142904.00b72921@thinkpad> <20200909192534.442f8984@thinkpad> <20200909180324.GI87483@ziepe.ca> <20200910093925.GB29166@oc3871087118.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200910093925.GB29166@oc3871087118.ibm.com> X-Rspamd-Queue-Id: 418F9180B31D1 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 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 Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote: > As Gerald mentioned, it is very difficult to explain in a clear way. > Hopefully, one could make sense ot of it. I would say the page table API requires this invariant: pud = pud_offset(p4d, addr); do { WARN_ON(pud != pud_offset(p4d, addr); next = pud_addr_end(addr, end); } while (pud++, addr = next, addr != end); ie pud++ is supposed to be a shortcut for pud_offset(p4d, next) While S390 does not follow this. Fixing addr_end brings it into alignment by preventing pud++ from happening. The only currently known side effect is that gup_fast crashes, but it sure is an unexpected thing. This suggests another fix, which is to say that pud++ is undefined and pud_offset() must always be called, but I think that would cause worse codegen on all other archs. Jason