From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qv1-xf42.google.com ([2607:f8b0:4864:20::f42]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kGVsM-0002nY-TE for linux-um@lists.infradead.org; Thu, 10 Sep 2020 23:21:12 +0000 Received: by mail-qv1-xf42.google.com with SMTP id db4so4256574qvb.4 for ; Thu, 10 Sep 2020 16:21:10 -0700 (PDT) Date: Thu, 10 Sep 2020 20:21:06 -0300 From: Jason Gunthorpe Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding Message-ID: <20200910232106.GR87483@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> <20200910130233.GK87483@ziepe.ca> <20200910195749.795232d1@thinkpad> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200910195749.795232d1@thinkpad> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-um" Errors-To: linux-um-bounces+geert=linux-m68k.org@lists.infradead.org To: Gerald Schaefer 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 , Heiko Carstens , Arnd Bergmann , John Hubbard , Jeff Dike , linux-um , Borislav Petkov , Andy Lutomirski , Thomas Gleixner , linux-arm , linux-mm , linux-power , LKML , Michael Ellerman , Andrew Morton , Linus Torvalds , Mike Rapoport On Thu, Sep 10, 2020 at 07:57:49PM +0200, Gerald Schaefer wrote: > On Thu, 10 Sep 2020 10:02:33 -0300 > Jason Gunthorpe wrote: > > > 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) > > > > Hmm, IIUC, all architectures with static folding will simply return > the passed-in p4d pointer for pud_offset(p4d, addr), for 3-level > pagetables. It is probably moot now, but since other arch's don't crash they also return pud_addr_end() == end so the loop only does one iteration. ie pud == pud_offset(p4d, addr) for all iterations as the pud++ never happens. Which is what this addr_end patch does for s390.. Jason _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um