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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
