To: Jamie Lokier <jamie@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
In-reply-to: <20100421092235.GB13114@shareable.org> (message from Jamie Lokier
	on Wed, 21 Apr 2010 10:22:35 +0100)
Subject: Re: [PATCH 13/35] fallthru: ext2 fallthru support
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> <E1O3sLi-0008EA-QW@pomaz-ex.szeredi.hu> <20100420213450.GM11723@shareable.org> <20100421084211.GB22741@bolzano.suse.de> <20100421092235.GB13114@shareable.org>
Message-Id: <E1O4WKa-0003Pp-0y@pomaz-ex.szeredi.hu>
From: Miklos Szeredi <miklos@szeredi.hu>
Date: Wed, 21 Apr 2010 11:34:52 +0200
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2295
Lines: 51

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
--
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/