From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 892A06B0044 for ; Fri, 16 Jan 2009 01:08:37 -0500 (EST) Received: by rv-out-0708.google.com with SMTP id f25so1334846rvb.26 for ; Thu, 15 Jan 2009 22:08:36 -0800 (PST) Date: Fri, 16 Jan 2009 15:08:30 +0900 From: MinChan Kim Subject: Re: [PATCH] Remove needless flush_dcache_page call Message-ID: <20090116060830.GB6515@barrios-desktop> References: <20090116052804.GA18737@barrios-desktop> <20090116053338.GC31013@parisc-linux.org> <20090116055119.GA6515@barrios-desktop> <20090116055729.GF31013@parisc-linux.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090116055729.GF31013@parisc-linux.org> Sender: owner-linux-mm@kvack.org To: Matthew Wilcox 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 Thu, Jan 15, 2009 at 10:57:30PM -0700, Matthew Wilcox wrote: > On Fri, Jan 16, 2009 at 02:51:19PM +0900, MinChan Kim wrote: > > On Thu, Jan 15, 2009 at 10:33:38PM -0700, Matthew Wilcox wrote: > > > 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. > > > > I see. :) > > > > Thanks for quick reponse and good explaination. > > Hmm,.. one more question. > > > > I can't find flush_dcache_page call in mpage_readpage which is > > generic read function. In case of ext fs, it use mpage_readpage > > with readpage. > > > > who and where call flush_dcache_page in mpage_readpage call path? > > Most I/O devices will do DMA to the page in question and thus the kernel > hasn't written to it and the CPU won't have the data in cache. For the > few devices which can't do DMA, it's the responsibility of the device > driver to call flush_dcache_page() (or some other flushing primitive). Hmm.. Now I am confusing. If devicer driver or with DMA makes sure cache consistency, Why filesystem code have to handle it ? > See the gdth driver for an example: > > address = kmap_atomic(sg_page(sl), KM_BIO_SRC_IRQ) + sl->offset; > memcpy(address, buffer, cpnow); > flush_dcache_page(sg_page(sl)); > kunmap_atomic(address, KM_BIO_SRC_IRQ); > > -- > 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." -- Kinds regards, MinChan Kim -- 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