From: Jan Kara Subject: Re: [PATCH 3/4] fs: Avoid data corruption with blocksize < pagesize Date: Wed, 18 Mar 2009 19:57:37 +0100 Message-ID: <20090318185737.GB1129@duck.suse.cz> References: <1237311235-13623-1-git-send-email-jack@suse.cz> <1237311235-13623-2-git-send-email-jack@suse.cz> <1237311235-13623-3-git-send-email-jack@suse.cz> <1237311235-13623-4-git-send-email-jack@suse.cz> <20090318120022.GB14622@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: LKML , linux-ext4@vger.kernel.org To: Nick Piggin Return-path: Received: from cantor.suse.de ([195.135.220.2]:36473 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755885AbZCRS5l (ORCPT ); Wed, 18 Mar 2009 14:57:41 -0400 Content-Disposition: inline In-Reply-To: <20090318120022.GB14622@wotan.suse.de> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed 18-03-09 13:00:23, Nick Piggin wrote: > On Tue, Mar 17, 2009 at 06:33:54PM +0100, Jan Kara wrote: > > Assume the following situation: > > Filesystem with blocksize < pagesize - suppose blocksize = 1024, > > pagesize = 4096. File 'f' has first four blocks already allocated. > > (line with "state:" contains the state of buffers in the page - m = mapped, > > u = uptodate, d = dirty) > > > > process 1: process 2: > > > > write to 'f' bytes 0 - 1024 > > state: |mud,-,-,-|, page dirty > > write to 'f' bytes 1024 - 4096: > > __block_prepare_write() maps blocks > > state: |mud,m,m,m|, page dirty > > we fail to copy data -> copied = 0 > > block_write_end() does nothing > > page gets unlocked > > writepage() is called on the page > > block_write_full_page() writes buffers with garbage > > > > This patch fixes the problem by skipping !uptodate buffers in > > block_write_full_page(). > > > > CC: Nick Piggin > > Signed-off-by: Jan Kara > > --- > > fs/buffer.c | 7 ++++++- > > 1 files changed, 6 insertions(+), 1 deletions(-) > > > > diff --git a/fs/buffer.c b/fs/buffer.c > > index 9f69741..22c0144 100644 > > --- a/fs/buffer.c > > +++ b/fs/buffer.c > > @@ -1774,7 +1774,12 @@ static int __block_write_full_page(struct inode *inode, struct page *page, > > } while (bh != head); > > > > do { > > - if (!buffer_mapped(bh)) > > + /* > > + * Parallel write could have already mapped the buffers but > > + * it then had to restart before copying in new data. We > > + * must avoid writing garbage so just skip the buffer. > > + */ > > + if (!buffer_mapped(bh) || !buffer_uptodate(bh)) > > continue; > > I don't quite see how this can happen. Further down in this loop, > we do a test_clear_buffer_dirty(), which should exclude this I > think? And marking the buffer dirty if it is not uptodate should > be a bug. OK, I spoke too soon. Now I reproduced the corruption under UML even with this patch. So it may be something different... Honza -- Jan Kara SUSE Labs, CR