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Wed, 9 Sep 2020 12:29:08 GMT Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DD7B64204F; Wed, 9 Sep 2020 12:29:06 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7957C42045; Wed, 9 Sep 2020 12:29:05 +0000 (GMT) Received: from thinkpad (unknown [9.171.79.102]) by d06av24.portsmouth.uk.ibm.com (Postfix) with SMTP; Wed, 9 Sep 2020 12:29:05 +0000 (GMT) Date: Wed, 9 Sep 2020 14:29:04 +0200 From: Gerald Schaefer To: Dave Hansen Cc: Jason Gunthorpe , 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 , Alexander Gordeev , 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: <20200909142904.00b72921@thinkpad> In-Reply-To: <0dbc6ec8-45ea-0853-4856-2bc1e661a5a5@intel.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> X-Mailer: Claws Mail 3.17.6 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-09-09_06:2020-09-09,2020-09-09 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 priorityscore=1501 suspectscore=0 mlxscore=0 malwarescore=0 spamscore=0 phishscore=0 bulkscore=0 adultscore=0 mlxlogscore=999 lowpriorityscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009090103 X-Rspamd-Queue-Id: 18FA81826B6AE X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 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 Tue, 8 Sep 2020 07:30:50 -0700 Dave Hansen wrote: > On 9/7/20 11:00 AM, Gerald Schaefer wrote: > > Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast > > code") introduced a subtle but severe bug on s390 with gup_fast, due to > > dynamic page table folding. > > Would it be fair to say that the "fake" page table entries s390 > allocates on the stack are what's causing the trouble here? That might > be a nice thing to open up with here. "Dynamic page table folding" > really means nothing to me. Sorry, I guess my previous reply does not really explain "what the heck is dynamic page table folding?". On s390, we can have different number of page table levels for different processes / mms. We always start with 3 levels, and update dynamically on process demand to 4 or 5 levels, hence the dynamic folding. Still, the PxD_SIZE/SHIFT is defined statically, so that e.g. pXd_addr_end() will not reflect this dynamic behavior. For the various pagetable walkers using pXd_addr_end() (w/o READ_ONCE logic) this is no problem. With static folding, iteration over the folded levels will always happen at pgd level (top-level folding). For s390, we stay at the respective level and iterate there (dynamic middle-level folding), only return to pgd level if there really were 5 levels. This only works well as long there are real pagetable pointers involved, that can also be used for iteration. For gup_fast, or any other future pagetable walkers using the READ_ONCE logic w/o lock, that is not true. There are pointers involved to local pXd values on the stack, because of the READ_ONCE logic, and our middle-level iteration will suddenly iterate over such stack pointers instead of pagetable pointers. This will be addressed by making the pXd_addr_end() dynamic, for which we need to see the pXd value in order to determine its level / type.