lkml.org 
[lkml]   [2010]   [Apr]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 13/35] fallthru: ext2 fallthru support
From
Date
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


\
 
 \ /
  Last update: 2010-04-21 11:37    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site