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


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