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-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
