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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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