From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f51.google.com (mail-pa0-f51.google.com [209.85.220.51]) by kanga.kvack.org (Postfix) with ESMTP id 2A24C6B0031 for ; Fri, 27 Dec 2013 14:20:46 -0500 (EST) Received: by mail-pa0-f51.google.com with SMTP id fa1so9628068pad.10 for ; Fri, 27 Dec 2013 11:20:45 -0800 (PST) Received: from mga09.intel.com (mga09.intel.com. [134.134.136.24]) by mx.google.com with ESMTP id m8si17801253pbq.179.2013.12.27.11.20.44 for ; Fri, 27 Dec 2013 11:20:44 -0800 (PST) Date: Fri, 27 Dec 2013 14:20:41 -0500 From: Matthew Wilcox Subject: Re: [PATCH] remap_file_pages needs to check for cache coherency Message-ID: <20131227192041.GD4945@linux.intel.com> References: <20131227180018.GC4945@linux.intel.com> <20131227.134814.345379118522548543.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131227.134814.345379118522548543.davem@davemloft.net> Sender: owner-linux-mm@kvack.org List-ID: To: David Miller Cc: linux-mm@kvack.org, sparclinux@vger.kernel.org, linux-parisc@vger.kernel.org, linux-mips@linux-mips.org On Fri, Dec 27, 2013 at 01:48:14PM -0500, David Miller wrote: > From: Matthew Wilcox > Date: Fri, 27 Dec 2013 13:00:18 -0500 > > > It seems to me that while (for example) on SPARC, it's not possible to > > create a non-coherent mapping with mmap(), after we've done an mmap, > > we can then use remap_file_pages() to create a mapping that no longer > > aliases in the D-cache. > > > > I have only compile-tested this patch. I don't have any SPARC hardware, > > and my PA-RISC hardware hasn't been turned on in six years ... I noticed > > this while wandering around looking at some other stuff. > > I suppose this is needed, but only in the case where the mapping is > shared and writable, right? I don't see you testing those conditions, > but with them I'd be OK with this change. VM_SHARED is checked a few lines above; too far to be visible in the original context diff: if (!vma || !(vma->vm_flags & VM_SHARED)) goto out; if (!vma->vm_ops || !vma->vm_ops->remap_pages) goto out; if (start < vma->vm_start || start + size > vma->vm_end) goto out; +#ifdef __ARCH_FORCE_SHMLBA + /* Is the mapping cache-coherent? */ + if ((pgoff ^ linear_page_index(vma, start)) & + ((SHMLBA-1) >> PAGE_SHIFT)) + goto out; +#endif I don't understand why we need to check for writable here. We don't seem to check VM_WRITE in arch_get_unmapped_area(), so I don't see why we should be checking it here. Put it another way; if I mmap() a file with PROT_READ only, should I be able to see stale data after another thread has written to it? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org