From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.110.172]:39878 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726237AbgCGA4u (ORCPT ); Fri, 6 Mar 2020 19:56:50 -0500 Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers References: <61250cdc-f80b-2e50-5168-2ec67ec6f1e6@arm.com> From: Anshuman Khandual Message-ID: Date: Sat, 7 Mar 2020 06:26:40 +0530 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-s390-owner@vger.kernel.org List-ID: To: Qian Cai Cc: linux-mm@kvack.org, Andrew Morton , Mike Rapoport , Vineet Gupta , Catalin Marinas , Will Deacon , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , "Kirill A . Shutemov" , Paul Walmsley , Palmer Dabbelt , linux-snps-arc@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Christophe Leroy On 03/07/2020 06:04 AM, Qian Cai wrote: > > >> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual wrote: >> >> Hmm, set_pte_at() function is not preferred here for these tests. The idea >> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >> of 'static' tests. set_pte_at() is platform provided and could/might trigger >> these flushes or some other platform specific synchronization stuff. Just > > Why is that important for this debugging option? Primarily reason is to avoid TLB/cache flush instructions on the system during these tests that only involve transforming different page table level entries through helpers. Unless really necessary, why should it emit any TLB/cache flush instructions ? > >> wondering is there specific reason with respect to the soft lock up problem >> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? > > Looks at the s390 version of set_pte_at(), it has this comment, > vmaddr); > > /* > * Certain architectures need to do special things when PTEs > * within a page table are directly modified. Thus, the following > * hook is made available. > */ > > I can only guess that powerpc could be the same here. This comment is present in multiple platforms while defining set_pte_at(). Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() does as compared to WRITE_ONCE() that avoids the soft lock up, just trying to understand.