Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761939AbXEPMAg (ORCPT ); Wed, 16 May 2007 08:00:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758806AbXEPMAZ (ORCPT ); Wed, 16 May 2007 08:00:25 -0400 Received: from smtp102.mail.mud.yahoo.com ([209.191.85.212]:40353 "HELO smtp102.mail.mud.yahoo.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1757862AbXEPMAY (ORCPT ); Wed, 16 May 2007 08:00:24 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com.au; h=Received:X-YMail-OSG:Message-ID:Date:From:User-Agent:X-Accept-Language:MIME-Version:To:CC:Subject:References:In-Reply-To:Content-Type:Content-Transfer-Encoding; b=ILAyxrI/iIHKn8KZ0HqHV+fB6hV0DPk8E56hXXNkJ4KAuAXYvBZc41wi4SFr8HIIuya7OZ4myG3JMdtCMTirE9i7aDlEXkbz48s36uE9X1h6WCTo2I5uB7PdaD05PClTHashNcxBR2LKo8nJO5JyrUv3LxtHuVDYLcWaBgS4Akk= ; X-YMail-OSG: e.BPOsEVM1mRdtZz.TPpayqrLavklJC9pql7htldA5VXM3CIFn6e3Jl4boVT6JTRMnjwpzptAQ-- Message-ID: <464AF224.30105@yahoo.com.au> Date: Wed, 16 May 2007 21:59:32 +1000 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20051007 Debian/1.7.12-1 X-Accept-Language: en MIME-Version: 1.0 To: David Howells CC: David Chinner , lkml , linux-mm , linux-fsdevel Subject: Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 References: <20070318233008.GA32597093@melbourne.sgi.com> <18993.1179310769@redhat.com> In-Reply-To: <18993.1179310769@redhat.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2328 Lines: 64 David Howells wrote: > David Chinner wrote: > > >>+ ret = block_prepare_write(page, 0, end, get_block); > > > As I understand the way prepare_write() works, this is incorrect. I think it is actually OK. > The start and end points passed to block_prepare_write() delimit the region of > the page that is going to be modified. This means that prepare_write() > doesn't need to fill it in if the page is not up to date. It does, however, > need to fill in the region before (if present) and the region after (if > present). Look at it like this: > > +---------------+ > | | > | | <-- Filled in by prepare_write() > | | > to-> |:::::::::::::::| > | | > | | <-- Filled in by caller > | | > offset->|:::::::::::::::| > | | > | | <-- Filled in by prepare_write() > | | > page-> +---------------+ > > However, page_mkwrite() isn't told which bit of the page is going to be > written to. This means it has to ask prepare_write() to make sure the whole > page is filled in. In other words, offset and to must be equal (in AFS I set > them both to 0). Dave is using prepare_write here to ensure blocks are allocated in the given range. The filesystem's ->nopage function must ensure it is uptodate before allowing it to be mapped. > With what you've got, if, say, 'offset' is 0 and 'to' is calculated at > PAGE_SIZE, then if the page is not up to date for any reason, then none of the > page will be updated before the page is written on by the faulting code. Consider that the code currently works OK today _without_ page_mkwrite. page_mkwrite is being added to do block allocation / reservation. > You probably get away with this in a blockdev-based filesystem because it's > unlikely that the page will cease to be up to date. > > However, if someone adds a syscall to punch holes in files, this may change... We have one. Strangely enough, it is done with madvise(MADV_REMOVE). -- SUSE Labs, Novell Inc. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/