Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D0927C636D6 for ; Fri, 3 Feb 2023 15:07:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233703AbjBCPHL (ORCPT ); Fri, 3 Feb 2023 10:07:11 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47726 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233657AbjBCPHK (ORCPT ); Fri, 3 Feb 2023 10:07:10 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 286FEA0E99; Fri, 3 Feb 2023 07:07:09 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=DFUkqlIb1koR34QSR5KYPFYQ+oC2ej0KqthqG2wrzL8=; b=QTLBPiQNVGLCOGdfPgtGITnHy8 VdbEIQzfUpVRnoHj6uEvO9vNTNXB3Y5GAKE52fN3jq0ciwAKlSiiJniPLlZ9mW7lpssuUH3AHoauX tZVG9s5oncmxw8GO3wfZRHq52EYoApCHer2IBgXz0X8NmYslkeLdNfL7irqEC8W1Hl1CskyZtzQk3 di4ZfKsWGctoXkMvrHi/SoFhY8QeTD9OaEUlRQ6qGCXYvAaL8p8cvhaNwgS3XvjDjxRXDUWl9PdWk gclC2AyLWzE0WqfI2Atf8bmUX8wg49kdHjsRVo0OPzlYU1ARyDAyaa6dpiJLpDabruhiyI+kS6Ylf Qbl9VHoQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pNxeb-00EOZb-KF; Fri, 03 Feb 2023 15:07:05 +0000 Date: Fri, 3 Feb 2023 15:07:05 +0000 From: Matthew Wilcox To: Brian Foster Cc: linux-fsdevel@vger.kernel.org, linux-afs@lists.infradead.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org, Hugh Dickins , linux-kernel@vger.kernel.org, fstests@vger.kernel.org Subject: Re: [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file Message-ID: References: <20230202204428.3267832-1-willy@infradead.org> <20230202204428.3267832-2-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri, Feb 03, 2023 at 08:00:16AM -0500, Brian Foster wrote: > On Thu, Feb 02, 2023 at 08:44:23PM +0000, Matthew Wilcox (Oracle) wrote: > > POSIX requires that "If the file size is increased, the extended area > > shall appear as if it were zero-filled". It is possible to use mmap to > > write past EOF and that data will become visible instead of zeroes. > > This fixes the problem for the filesystems which simply call > > truncate_setsize(). More complex filesystems will need their own > > patches. > > > > Signed-off-by: Matthew Wilcox (Oracle) > > --- > > mm/truncate.c | 7 +++++-- > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > diff --git a/mm/truncate.c b/mm/truncate.c > > index 7b4ea4c4a46b..cebfc5415e9a 100644 > > --- a/mm/truncate.c > > +++ b/mm/truncate.c > > @@ -763,9 +763,12 @@ void truncate_setsize(struct inode *inode, loff_t newsize) > > loff_t oldsize = inode->i_size; > > > > i_size_write(inode, newsize); > > - if (newsize > oldsize) > > + if (newsize > oldsize) { > > pagecache_isize_extended(inode, oldsize, newsize); > > - truncate_pagecache(inode, newsize); > > + truncate_pagecache(inode, oldsize); > > + } else { > > + truncate_pagecache(inode, newsize); > > + } > > I don't think this alone quite addresses the problem. Looking at ext4 > for example, if the eof page is dirty and writeback occurs between the > i_size update (because writeback also zeroes the post-eof portion of the > page) and the truncate_setsize() call, we end up with pagecache > inconsistency because pagecache truncate doesn't dirty the page it > zeroes. > > So for example, with this series plus a nefariously placed > filemap_flush() in ext4_setattr(): > > # xfs_io -fc "truncate 1" -c "mmap 0 1k" -c "mwrite 0 10" -c "truncate 5" -c "mread -v 0 5" /mnt/file > 00000000: 58 00 00 00 00 X.... > # umount /mnt/; mount /mnt/ > # xfs_io -c "mmap 0 1k" -c "mread -v 0 5" /mnt/file > 00000000: 58 58 58 58 58 XXXXX Hm, so switch the order of i_size_write() and truncate_pagecache()? There could still be a store between old-EOF and new-EOF from another thread, which would then be visible, but I don't think you could prove that store should have been zeroed. Not from the thread doing the ftruncate() anyway -- I think the thread doing the store could prove it, but that thread is relying on undefined behaviour anyway.