Re: [PATCH 13/35] fallthru: ext2 fallthru support

[Valerie Aurora <vaurora@redhat.com>]

On Wed, Apr 21, 2010 at 11:34:52AM +0200, Miklos Szeredi wrote:
> 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.

I couldn't have put it better myself.

To expand slightly, if the broken apps are not few and easily fixed,
then we'll go back and make the kernel more complicated.  I'd like to
try the simplest version we think will work, first.

Thanks!

-VAL