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Thu, 10 Sep 2020 09:39:29 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7BD21AE053; Thu, 10 Sep 2020 09:39:27 +0000 (GMT) Received: from oc3871087118.ibm.com (unknown [9.145.67.15]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTPS; Thu, 10 Sep 2020 09:39:27 +0000 (GMT) Date: Thu, 10 Sep 2020 11:39:25 +0200 From: Alexander Gordeev To: Jason Gunthorpe Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding Message-ID: <20200910093925.GB29166@oc3871087118.ibm.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200909180324.GI87483@ziepe.ca> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-09-10_01:2020-09-10, 2020-09-10 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 mlxscore=0 suspectscore=0 clxscore=1011 adultscore=0 spamscore=0 phishscore=0 mlxlogscore=999 bulkscore=0 lowpriorityscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009100085 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Zijlstra , Dave Hansen , Dave Hansen , Paul Mackerras , linux-sparc , 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 , John Hubbard , Jeff Dike , linux-um , Borislav Petkov , Andy Lutomirski , Thomas Gleixner , linux-arm , linux-mm , linux-power , LKML , Andrew Morton , Linus Torvalds , Mike Rapoport Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, Sep 09, 2020 at 03:03:24PM -0300, Jason Gunthorpe wrote: > On Wed, Sep 09, 2020 at 07:25:34PM +0200, Gerald Schaefer wrote: > > I actually had to draw myself a picture to get some hold of > > this, or rather a walk-through with a certain pud-crossing > > range in a folded 3-level scenario. Not sure if I would have > > understood my explanation above w/o that, but I hope you can > > make some sense out of it. Or draw yourself a picture :-) > > What I don't understand is how does anything work with S390 today? > > If the fix is only to change pxx_addr_end() then than generic code > like mm/pagewalk.c will iterate over a *different list* of page table > entries. > > It's choice of entries to look at is entirely driven by pxx_addr_end(). > > Which suggest to me that mm/pagewalk.c also doesn't work properly > today on S390 and this issue is not really about stack variables? > > Fundamentally if pXX_offset() and pXX_addr_end() must be consistent > together, if pXX_offset() is folded then pXX_addr_end() must cause a > single iteration of that level. Your observation is correct. Another way to describe the problem is existing pXd_addr_end helpers could be applied to mismatching levels on s390 (e.g p4d_addr_end applied to pud or pgd_addr_end applied to p4d). As you noticed, all *_pXd_range iterators could be called with address ranges that exceed single pXd table. However, when it happens with pointers to real page tables (passed to *_pXd_range iterators) we still operate on valid tables, which just (lucky for us) happened to be folded. Thus we still reference correct table entries. 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. As Gerald mentioned, it is very difficult to explain in a clear way. Hopefully, one could make sense ot of it. > Jason