From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding 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> <20200910181319.GO87483@ziepe.ca> From: John Hubbard Message-ID: <0c9bcb54-914b-e582-dd6d-3861267b6c94@nvidia.com> Date: Thu, 10 Sep 2020 14:22:37 -0700 MIME-Version: 1.0 In-Reply-To: <20200910181319.GO87483@ziepe.ca> Content-Language: en-US List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-um" Errors-To: linux-um-bounces+geert=linux-m68k.org@lists.infradead.org To: Jason Gunthorpe , Linus Torvalds Cc: Peter Zijlstra , Benjamin Herrenschmidt , Dave Hansen , Dave Hansen , Paul Mackerras , linux-sparc , Alexander Gordeev , Claudio Imbrenda , Will Deacon , linux-arch , linux-s390 , Vasily Gorbik , Richard Weinberger , linux-x86 , Russell King , Christian Borntraeger , Ingo Molnar , Catalin Marinas , Andrey Ryabinin , Gerald Schaefer , Heiko Carstens , Arnd Bergmann , Jeff Dike , linux-um , Borislav Petkov , Andy Lutomirski , Thomas Gleixner , linux-arm , linux-mm , LKML , Michael Ellerman , Andrew Morton , linux-power , Mike Rapoport On 9/10/20 11:13 AM, Jason Gunthorpe wrote: > On Thu, Sep 10, 2020 at 10:35:38AM -0700, Linus Torvalds wrote: >> On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev >> wrote: >>> >>> It is only gup_fast case that exposes the issue. It hits because >>> pointers to stack copies are passed to gup_pXd_range iterators, not >>> pointers to real page tables itself. >> >> Can we possibly change fast-gup to not do the stack copies? >> >> I'd actually rather do something like that, than the "addr_end" thing. > >> As you say, none of the other page table walking code does what the >> GUP code does, and I don't think it's required. > > As I understand it, the requirement is because fast-gup walks without > the page table spinlock, or mmap_sem held so it must READ_ONCE the > *pXX. > > It then checks that it is a valid page table pointer, then calls > pXX_offset(). > > The arch implementation of pXX_offset() derefs again the passed pXX > pointer. So it defeats the READ_ONCE and the 2nd load could observe > something that is no longer a page table pointer and crash. Just to be clear, though, that makes it sound a little wilder and reckless than it really is, right? Because actually, the page tables cannot be freed while gup_fast is walking them, due to either IPI blocking during the walk, or the moral equivalent (MMU_GATHER_RCU_TABLE_FREE) for non-IPI architectures. So the pages tables can *change* underneath gup_fast, and for example pages can be unmapped. But they remain valid page tables, it's just that their contents are unstable. Even if pXd_none()==true. Or am I way off here, and it really is possible (aside from the current s390 situation) to observe something that "is no longer a page table"? thanks, -- John Hubbard NVIDIA _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um