From mboxrd@z Thu Jan 1 00:00:00 1970 From: Phillip Susi Subject: Re: unified page and buffer cache? Date: Fri, 7 May 2010 11:45:34 -0400 Message-ID: <4BE4359E.9020400@cfl.rr.com> References: <4BCC7C05.8000803@cfl.rr.com> <4BE417CB.7000806@cfl.rr.com> <20100507135329.GA17259@parisc-linux.org> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: linux-fsdevel@vger.kernel.org, Linux-kernel To: Matthew Wilcox Return-path: In-Reply-To: <20100507135329.GA17259@parisc-linux.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 5/7/2010 9:53 AM, Matthew Wilcox wrote: > The problem you're seeing is aliasing in the page cache, not a failed > unification of the buffer and page caches. Pages are addressed by > (mapping, offset). Each inode generally has its own mapping. Depending > on the file system, directories may be addressed by their own inode's > mapping, or by the block device's mapping. If there are two mappings that don't know about each other, then the caches don't seem very unified to me. If I write to the file and that data sits in the mapping for the inode, then I read the corresponding blocks though the block device, and it has a different mapping, then I read the old data off the disk instead of the new data in the cache. I thought that this exact problem had been fixed long ago. > Resolving aliasing would be horribly expensive, so it's unlikely to > happen. Back to the drawing board I guess. Maybe ext could be fixed to use an inode mapping for directories instead of relying on the block device mapping, then I could readahead() the directory instead of having to go to the block device at all.