From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id E438E6B0044 for ; Fri, 16 Jan 2009 00:33:58 -0500 (EST) Date: Thu, 15 Jan 2009 22:33:38 -0700 From: Matthew Wilcox Subject: Re: [PATCH] Remove needless flush_dcache_page call Message-ID: <20090116053338.GC31013@parisc-linux.org> References: <20090116052804.GA18737@barrios-desktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090116052804.GA18737@barrios-desktop> Sender: owner-linux-mm@kvack.org To: MinChan Kim Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, npiggin@suse.de, akpm@linux-foundation.org List-ID: On Fri, Jan 16, 2009 at 02:28:04PM +0900, MinChan Kim wrote: > Now, Anyone don't maintain cramfs. > I don't know who is maintain romfs. so I send this patch to linux-mm, > lkml, linux-dev. > > I am not sure my thought is right. > > When readpage is called, page with argument in readpage is just new > allocated because kernel can't find that page in page cache. > > At this time, any user process can't map the page to their address space. > so, I think D-cache aliasing probelm never occur. > > It make sense ? Sorry, no. You have to call fluch_dcache_page() in two situations -- when the kernel is going to read some data that userspace wrote, *and* when userspace is going to read some data that the kernel wrote. From a quick look at the patch, this seems to be the second case. The kernel wrote data to a pagecache page, and userspace should be able to read it. To understand why this is necessary, consider a processor which is virtually indexed and has a writeback cache. The kernel writes to a page, then a user process reads from the same page through a different address. The cache doesn't find the data the kernel wrote because it has a different virtual index, so userspace reads stale data. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- 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