From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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: <2d950d8c-4b23-741e-591f-e22e857c0755@arm.com> Date: Tue, 10 Mar 2020 08:01:51 +0530 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Christophe Leroy , 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 On 03/07/2020 12:35 PM, Christophe Leroy wrote: > > > Le 07/03/2020 à 01:56, Anshuman Khandual a écrit : >> >> >> 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 ? > > What's the problem with thoses flushes ? > >> >>> >>>> 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. >> > > > Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess. > > To properly set the page table entries, set_pte_at() has to be used: > - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't. > - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't. > > set_pte_at() also does a few other mandatory things, like calling pte_mkpte() > > So, the WRITE_ONCE() must definitely become a set_pte_at() Sure, will do. These are part of the clear tests that populates a given entry with a non zero value before clearing and testing it with pxx_none(). In that context, WRITE_ONCE() seemed sufficient. But pte_clear() might be closely tied with proper page table entry update and hence a preceding set_pte_at() will be better. There are still more WRITE_ONCE() for other page table levels during these clear tests. set_pmd_at() and set_pud_at() are defined on platforms that support (and enable) THP and PUD based THP respectively. Hence they could not be used for clear tests as remaining helpers pmd_clear(), pud_clear(), p4d_clear() and pgd_clear() still need to be validated with or without THP support and enablement. We should just leave all other WRITE_ONCE() instances unchanged. Please correct me if I am missing something here. > > Christophe >