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Tue, 8 Sep 2020 07:22:44 +0200 (CEST) Subject: Re: [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding To: Mike Rapoport , Gerald Schaefer Cc: Peter Zijlstra , Dave Hansen , linux-mm , Paul Mackerras , linux-sparc , Alexander Gordeev , Claudio Imbrenda , Will Deacon , linux-arch , linux-s390 , Vasily Gorbik , Christian Borntraeger , Richard Weinberger , linux-x86 , Russell King , Jason Gunthorpe , Ingo Molnar , Catalin Marinas , Andrey Ryabinin , Heiko Carstens , Arnd Bergmann , John Hubbard , Jeff Dike , linux-um , Borislav Petkov , Andy Lutomirski , Thomas Gleixner , linux-arm , linux-power , LKML , Andrew Morton , Linus Torvalds References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com> <20200907201256.GC1976319@kernel.org> From: Christophe Leroy Message-ID: <9bde9857-fdfd-e384-ea27-a14e5a06f1e6@csgroup.eu> Date: Tue, 8 Sep 2020 07:22:39 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <20200907201256.GC1976319@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr X-Rspamd-Queue-Id: 7741B180AD81D X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 Content-Transfer-Encoding: quoted-printable 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: Le 07/09/2020 =C3=A0 22:12, Mike Rapoport a =C3=A9crit=C2=A0: > On Mon, Sep 07, 2020 at 08:00:55PM +0200, Gerald Schaefer wrote: >> This is v2 of an RFC previously discussed here: >> https://lore.kernel.org/lkml/20200828140314.8556-1-gerald.schaefer@lin= ux.ibm.com/ >> >> Patch 1 is a fix for a regression in gup_fast on s390, after our conve= rsion >> to common gup_fast code. It will introduce special helper functions >> pXd_addr_end_folded(), which have to be used in places where pagetable= walk >> is done w/o lock and with READ_ONCE, so currently only in gup_fast. >> >> Patch 2 is an attempt to make that more generic, i.e. change pXd_addr_= end() >> themselves by adding an extra pXd value parameter. That was suggested = by >> Jason during v1 discussion, because he is already thinking of some oth= er >> places where he might want to switch to the READ_ONCE logic for pageta= ble >> walks. In general, that would be the cleanest / safest solution, but t= here >> is some impact on other architectures and common code, hence the new a= nd >> greatly enlarged recipient list. >> >> Patch 3 is a "nice to have" add-on, which makes pXd_addr_end() inline >> functions instead of #defines, so that we get some type checking for t= he >> new pXd value parameter. >> >> Not sure about Fixes/stable tags for the generic solution. Only patch = 1 >> fixes a real bug on s390, and has Fixes/stable tags. Patches 2 + 3 mig= ht >> still be nice to have in stable, to ease future backports, but I guess >> "nice to have" does not really qualify for stable backports. >=20 > I also think that adding pXd parameter to pXd_addr_end() is a cleaner > way and with this patch 1 is not really required. I would even merge > patches 2 and 3 into a single patch and use only it as the fix. Why not merging patches 2 and 3, but I would keep patch 1 separate but=20 after the generic changes, so that we first do the generic changes, then=20 we do the specific S390 use of it. Christophe