From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7CA61C10F00 for ; Sat, 7 Mar 2020 07:05:16 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1B2AB206D5 for ; Sat, 7 Mar 2020 07:05:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=c-s.fr header.i=@c-s.fr header.b="hFwvKFlt" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1B2AB206D5 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=c-s.fr Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 737226B0003; Sat, 7 Mar 2020 02:05:15 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6E7446B0006; Sat, 7 Mar 2020 02:05:15 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5AF2C6B0007; Sat, 7 Mar 2020 02:05:15 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0060.hostedemail.com [216.40.44.60]) by kanga.kvack.org (Postfix) with ESMTP id 4414A6B0003 for ; Sat, 7 Mar 2020 02:05:15 -0500 (EST) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id E5F294DA5 for ; Sat, 7 Mar 2020 07:05:14 +0000 (UTC) X-FDA: 76567679748.29.brass22_67e9228e49d55 X-HE-Tag: brass22_67e9228e49d55 X-Filterd-Recvd-Size: 5903 Received: from pegase1.c-s.fr (pegase1.c-s.fr [93.17.236.30]) by imf33.hostedemail.com (Postfix) with ESMTP for ; Sat, 7 Mar 2020 07:05:13 +0000 (UTC) Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 48ZFqW22V5zB09Zy; Sat, 7 Mar 2020 08:05:11 +0100 (CET) Authentication-Results: localhost; dkim=pass reason="1024-bit key; insecure key" header.d=c-s.fr header.i=@c-s.fr header.b=hFwvKFlt; dkim-adsp=pass; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id JH70JFMo2Nhm; Sat, 7 Mar 2020 08:05:11 +0100 (CET) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 48ZFqW06j2zB09Zx; Sat, 7 Mar 2020 08:05:11 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=c-s.fr; s=mail; t=1583564711; bh=KLykoiwakmvXlj7iOgiNeBA+k4S6on8FDV27weKg6MI=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=hFwvKFlt5tItEoeChiMcVbq8Z6WEJiAIueL914VqsjAkNiTW/tX4E4mkXgZPRPVQp 1TSwGC1wl0uEf1SM3tvrvXA0/srRG2BvZ2uOnFi+rWlXqmPEgF9Of2NNwJRqkfVxFW 8RUJEw8G1tWI+NWaS36EliXDmOzscoTf3q3gSbXQ= Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id D550D8B784; Sat, 7 Mar 2020 08:05:11 +0100 (CET) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id OhKGFoGl11OV; Sat, 7 Mar 2020 08:05:11 +0100 (CET) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 4A0DC8B776; Sat, 7 Mar 2020 08:05:09 +0100 (CET) Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers To: Anshuman Khandual , 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 References: <61250cdc-f80b-2e50-5168-2ec67ec6f1e6@arm.com> From: Christophe Leroy Message-ID: Date: Sat, 7 Mar 2020 08:05:09 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: quoted-printable 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: Le 07/03/2020 =C3=A0 01:56, Anshuman Khandual a =C3=A9crit=C2=A0: >=20 >=20 > 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 thes= e 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? >=20 > 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 ? >=20 >> >>> wondering is there specific reason with respect to the soft lock up p= roblem >>> making it necessary to use set_pte_at() rather than a simple WRITE_ON= CE() ? >> >> 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. >=20 > This comment is present in multiple platforms while defining set_pte_at= (). > Is not 'barrier()' here alone good enough ? Else what exactly set_pte_a= t() > does as compared to WRITE_ONCE() that avoids the soft lock up, just try= ing > to understand. >=20 Argh ! I didn't realise that you were writing directly into the page=20 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=20 four times. set_pte_at() does it, WRITE_ONCE() doesn't. - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be=20 preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't. set_pte_at() also does a few other mandatory things, like calling=20 pte_mkpte() So, the WRITE_ONCE() must definitely become a set_pte_at() Christophe