From: Miklos Szeredi Subject: Re: [PATCH 13/35] fallthru: ext2 fallthru support Date: Wed, 21 Apr 2010 11:34:52 +0200 Message-ID: References: <1271372682-21225-14-git-send-email-vaurora@redhat.com> <20100419124025.GG10776@bolzano.suse.de> <1271682168.14748.718.camel@macbook.infradead.org> <20100419132344.GI10776@bolzano.suse.de> <20100419133028.GA3631@shareable.org> <20100419141248.GK10776@bolzano.suse.de> <20100419142315.GA2688@shell> <20100420213450.GM11723@shareable.org> <20100421084211.GB22741@bolzano.suse.de> <20100421092235.GB13114@shareable.org> Cc: jblunck@suse.de, miklos@szeredi.hu, vaurora@redhat.com, dwmw2@infradead.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, tytso@mit.edu, linux-ext4@vger.kernel.org To: Jamie Lokier Return-path: Received: from fxip-0047f.externet.hu ([88.209.222.127]:59822 "EHLO pomaz-ex.szeredi.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753683Ab0DUJfV (ORCPT ); Wed, 21 Apr 2010 05:35:21 -0400 In-reply-to: <20100421092235.GB13114@shareable.org> (message from Jamie Lokier on Wed, 21 Apr 2010 10:22:35 +0100) Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, 21 Apr 2010, Jamie Lokier wrote: > Hmm. I smell potential confusion for some otherwise POSIX-friendly > userspaces. > > When I open /path/to/foo, call fstat (st_dev=2, st_ino=5678), and then > keep the file open, then later do a readdir which includes foo > (dir.st_dev=1, d_ino=1234), I'm going to immediately assume a rename > or unlink happened, close the file, abort streaming from it, refresh > the GUI windows, refresh application caches for that name entry, etc. > > Because in the POSIX world I think open files have stable inode > numbers (as long as they are open), and I don't think that an open > file can have it's name's d_ino not match the inode number unless it's > a mount point, which my program would know about. > > This plays into inotify, where you have to know if you are monitoring > every directory that contains a link to a file, to know if you need to > monitor the file itself directly instead. > > Now I think it's fair enough that a union mount doesn't play all the > traditional rules :-) C'est la vie. > > This mismatch of (dir.st_dev,d_ino) and st_ino strongly resembles a > file-bind-mount. Like bind mounts, it's quite annoying for programs > that like to assume they've seen all of a file's links when they've > seen i_nlink of them. > > Bind mounts can be detected by looking in /proc/mounts. st_dev > changing doesn't work because it can be a binding of the same > filesystem. > > How would I go about detecting when a union mount's directory entry > has similar behaviour, without calling stat() on each entry? Is it > just a matter of recognising a particular filesystem name in > /proc/mounts, or something more? Detecting mount points is best done by comparing st_dev for the parent directory with st_dev of the child. This is much simpler than parsing /proc/mounts and will work for bind mounts as well as union mounts. I think there's no question that union mounts might break apps (POSIX or not). But I think there's hope that they are few and can easily be fixed. Thanks, Miklos