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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 5 Sep 2019 18:06:33 +0100 Received: from d06av22.portsmouth.uk.ibm.com (d06av22.portsmouth.uk.ibm.com [9.149.105.58]) by b06avi18626390.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x85H68Tu33030630 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 5 Sep 2019 17:06:08 GMT Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 104054C050; Thu, 5 Sep 2019 17:06:32 +0000 (GMT) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0017A4C044; Thu, 5 Sep 2019 17:06:30 +0000 (GMT) Received: from thinkpad (unknown [9.152.96.45]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 5 Sep 2019 17:06:30 +0000 (GMT) Date: Thu, 5 Sep 2019 19:06:29 +0200 From: Gerald Schaefer To: Anshuman Khandual Cc: linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Greg Kroah-Hartman , Thomas Gleixner , Mike Rapoport , Jason Gunthorpe , Dan Williams , Peter Zijlstra , Michal Hocko , Mark Rutland , Mark Brown , Steven Price , Ard Biesheuvel , Masahiro Yamada , Kees Cook , Tetsuo Handa , Matthew Wilcox , Sri Krishna chowdary , Dave Hansen , Russell King - ARM Linux , Michael Ellerman , Paul Mackerras , Martin Schwidefsky , Heiko Carstens , "David S. Miller" , Vineet Gupta , James Hogan , Paul Burton , Ralf Baechle , linux-snps-arc@lists.infradead.org, linux-mips@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture page table helpers In-Reply-To: <20e3044d-2af5-b27b-7653-cec53bdec941@arm.com> References: <1567497706-8649-1-git-send-email-anshuman.khandual@arm.com> <1567497706-8649-2-git-send-email-anshuman.khandual@arm.com> <20190904221618.1b624a98@thinkpad> <20e3044d-2af5-b27b-7653-cec53bdec941@arm.com> X-Mailer: Claws Mail 3.17.3 (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-cbid: 19090517-0012-0000-0000-000003471F7D X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19090517-0013-0000-0000-000021817691 Message-Id: <20190905190629.523bdb87@thinkpad> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-09-05_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1909050164 Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Thu, 5 Sep 2019 14:48:14 +0530 Anshuman Khandual wrote: > > [...] > >> + > >> +#if !defined(__PAGETABLE_PMD_FOLDED) && !defined(__ARCH_HAS_4LEVEL_HACK) > >> +static void pud_clear_tests(pud_t *pudp) > >> +{ > >> + memset(pudp, RANDOM_NZVALUE, sizeof(pud_t)); > >> + pud_clear(pudp); > >> + WARN_ON(!pud_none(READ_ONCE(*pudp))); > >> +} > > > > For pgd/p4d/pud_clear(), we only clear if the page table level is present > > and not folded. The memset() here overwrites the table type bits, so > > pud_clear() will not clear anything on s390 and the pud_none() check will > > fail. > > Would it be possible to OR a (larger) random value into the table, so that > > the lower 12 bits would be preserved? > > So the suggestion is instead of doing memset() on entry with RANDOM_NZVALUE, > it should OR a large random value preserving lower 12 bits. Hmm, this should > still do the trick for other platforms, they just need non zero value. So on > s390, the lower 12 bits on the page table entry already has valid value while > entering this function which would make sure that pud_clear() really does > clear the entry ? Yes, in theory the table entry on s390 would have the type set in the last 4 bits, so preserving those would be enough. If it does not conflict with others, I would still suggest preserving all 12 bits since those would contain arch-specific flags in general, just to be sure. For s390, the pte/pmd tests would also work with the memset, but for consistency I think the same logic should be used in all pxd_clear_tests. However, there is another issue on s390 which will make this only work for pud_clear_tests(), and not for the p4d/pgd_tests. The problem is that mm_alloc() will only give you a 3-level page table initially on s390. This means that pudp == p4dp == pgdp, and so the p4d/pgd_tests will both see the pud level (of course this also affects other tests). Not sure yet how to fix this, i.e. how to initialize/update the page table to 5 levels. We can handle 5 level page tables, and it would be good if all levels could be tested, but using mm_alloc() to establish the page tables might not work on s390. One option could be to provide an arch-hook or weak function to allocate/initialize the mm. IIUC, the (dummy) mm is really only needed to provide an mm->pgd as starting point, right? Regards, Gerald