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=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham 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 6ACE1C43331 for ; Fri, 27 Mar 2020 14:29:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4720A206F2 for ; Fri, 27 Mar 2020 14:29:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727505AbgC0O3l (ORCPT ); Fri, 27 Mar 2020 10:29:41 -0400 Received: from foss.arm.com ([217.140.110.172]:45436 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726739AbgC0O3l (ORCPT ); Fri, 27 Mar 2020 10:29:41 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 27B011FB; Fri, 27 Mar 2020 07:29:41 -0700 (PDT) Received: from C02TD0UTHF1T.local (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B719B3F71F; Fri, 27 Mar 2020 07:29:39 -0700 (PDT) Date: Fri, 27 Mar 2020 14:29:37 +0000 From: Mark Rutland To: Li Wang Cc: Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64: mmu: no write cache for O_SYNC flag Message-ID: <20200327142937.GB94838@C02TD0UTHF1T.local> References: <20200326163625.30714-1-li.wang@windriver.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200326163625.30714-1-li.wang@windriver.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 26, 2020 at 09:36:25AM -0700, Li Wang wrote: > reproduce steps: > 1. > disable CONFIG_STRICT_DEVMEM in linux kernel > 2. > Process A gets a Physical Address of global variable by > "/proc/self/pagemap". > 3. > Process B writes a value to the same Physical Address by mmap(): > fd=open("/dev/mem",O_SYNC); > Virtual Address=mmap(fd); Is this just to demonstrate the behaviour, or is this meant to be indicative of a real use-case? I'm struggling to see the latter. > problem symptom: > after Process B write a value to the Physical Address, > Process A of the value of global variable does not change. > They both W/R the same Physical Address. If Process A is not using the same attributes as process B, there is no guarantee of coherency. How did process A map this memory? > technical reason: > Process B writing the Physical Address is by the Virtual Address, > and the Virtual Address comes from "/dev/mem" and mmap(). > In arm64 arch, the Virtual Address has write cache. > So, maybe the value is not written into Physical Address. I don't think that's true. I think what's happening here is: * Process A has a Normal WBWA Cacheable mapping. * Process B as a Normal Non-cacheable mapping. * Process B's write does not snoop any caches, and goes straight to memory. * Process A reads a value from cache, which does not include process B's write. That's a natural result of using mismatched attributes, and is consistent with the O_SYNC flag meaning that the write "is transferred to the underlying hardware". > > fix reason: > giving write cache flag in arm64 is in phys_mem_access_prot(): > ===== > arch/arm64/mm/mmu.c > phys_mem_access_prot() > { > if (!pfn_valid(pfn)) > return pgprot_noncached(vma_prot); > else if (file->f_flags & O_SYNC) > return pgprot_writecombine(vma_prot); > return vma_prot; > } > ==== > the other arch and the share function drivers/char/mem.c of phys_mem_access_prot() > does not add write cache flag. > So, removing the flag to fix the issue This will change behaviour that other software may be relying upon, and as above I do not believe this actually solves the problem you describe. Thanks, Mark. > > Signed-off-by: Li Wang > Cc: Catalin Marinas > Cc: Will Deacon > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-kernel@vger.kernel.org > --- > arch/arm64/mm/mmu.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 128f70852bf3..d7083965ca17 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -81,8 +81,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn, > { > if (!pfn_valid(pfn)) > return pgprot_noncached(vma_prot); > - else if (file->f_flags & O_SYNC) > - return pgprot_writecombine(vma_prot); > return vma_prot; > } > EXPORT_SYMBOL(phys_mem_access_prot); > -- > 2.24.1 >