Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:38634 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725939AbeKUP15 (ORCPT ); Wed, 21 Nov 2018 10:27:57 -0500 Date: Wed, 21 Nov 2018 04:54:41 +0000 From: Al Viro To: Eiichi Tsukata Cc: andi@firstfloor.org, Chris Mason , Josef Bacik , David Sterba , Theodore Ts'o , Andreas Dilger , Jaegeuk Kim , Chao Yu , Miklos Szeredi , Bob Peterson , Andreas Gruenbacher , linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org, cluster-devel@redhat.com, linux-unionfs@vger.kernel.org Subject: Re: [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write Message-ID: <20181121045440.GM32577@ZenIV.linux.org.uk> References: <20181121024400.4346-1-devel@etsukata.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181121024400.4346-1-devel@etsukata.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Nov 21, 2018 at 11:43:56AM +0900, Eiichi Tsukata wrote: > Some file systems (including ext4, xfs, ramfs ...) have the following > problem as I've described in the commit message of the 1/4 patch. > > The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek") > removed almost all locks in llseek() including SEEK_END. It based on the > idea that write() updates size atomically. But in fact, write() can be > divided into two or more parts in generic_perform_write() when pos > straddles over the PAGE_SIZE, which results in updating size multiple > times in one write(). It means that llseek() can see the size being > updated during write(). And? Who has ever promised anything that insane? write(2) can take an arbitrary amount of time; another process doing lseek() on independently opened descriptor is *not* going to wait for that (e.g. page-in of the buffer being written, which just happens to be mmapped from a file on NFS over RFC1149 link).