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

    Sorry, no: That does not work for bind mounts. Both layers can have
    the same st_dev. Nor does O_NOFOLLOW stop traversal in the middle of
    a path, there is no handy O_NOCROSSMOUNTS, and no st_mode flag or
    d_type to say it's a bind mount. Bind mounts are really a big pain
    for i_nlink+inotify name counting.

    Besides, calling stat() on every entry in a large directory to check
    st_ino can be orders of magnitude slower than readdir() on a large
    directory - especially with a cold cache. It is quicker, but much
    more complicated, to parse /proc/mounts and apply arcane rules to find
    the exceptions.

    Can a union mount overlap two parts of the same filesystem?

    > 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 agree, and union moint is a very useful feature that's worth
    breaking a few apps for :-)

    I'm curious if there's a clear way to go about it in this case, or
    if it'll involve a certain amount of pattern recognition in /proc/mounts.

    Basically I'm wondering if it's been thought about already.

    -- Jamie


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