[Devel] [netns] sysfs: issues porting shadow directories on top of 2.6.21-mm2

[Benjamin Thery]

Hi Eric,

For the past few weeks, I've been trying to port your netns patchset 
on top of 2.6.21-mm2. It took me a lot more time than I first expected 
to have something working.

I started the port based on your latest public git repository tag: 
"netns/v2.6.21-rc6-netns17".

I met a few difficulties during the port the worst being porting the 
shadow directories patches on top of Greg's sysfs patches.

Greg modified a lot of things in sysfs and I had to "rewrite"/adapt 
most of your "sysfs: Implement sysfs manged shadow directory support" 
patch. My knowledge of sysfs approaching zero, the result isn't that 
great.

Any chance you've updated the patchset for a recent version of the -mm 
kernel?

Here are some issues I have with the sysfs part of the netns patchset:

* The first thing I'm not sure to understand in your patch is how 
shadow dirs and there "real" counterpart are supposed to be linked 
(via dentry and via sysfs_dirent).

Is it something like:

/sys/class/net/          ("real" net class)
/sys/class/net-shadow1/
/sys/class/net-shadow2/

or:

/sys/class/net/
/sys/class/net/net-shadow1/
/sys/class/net/net-shadow2/

In add_shadow_dir(), it seems the shadow dentry parent is "class" :
   shadow = d_alloc(dir->d_parent, &dir->d_name);
and the shadow sysfs_dirent parent is the real "net":
   sysfs_make_dirent(dir->d_fsdata, ....);

On 2.6.21-mm2, if I attach the dentry to "class" (dir->d_parent) as 
you did initially, then  the shadow directory lookup "fails": it 
always returns the same shadow dir, whatever network namespace is 
current. Indeed, sysfs_shadow_follow_link() is never called with a 
SYSFS_DIR dentry, but always directly with a SYSFS_SHADOW one, and the 
tag comparison is never done.

In add_shadow_dir(), I changed the d_alloc() call and passed dir 
instead of dir->d_parent, and it "solved" the issue: 
sysfs_shadow_follow_link() is called with the SYSFS_DIR dentry, and 
the shadow dir lookup is done.


* I also have some issues with symlinks.

Indeed, the way symlinks are "resolved" have changed. Symlinks paths 
aren't resolved anymore using kobject linking but uses sysfs_dirent 
instead. So I had to use a dirty hack to skip shadow directories in 
fs/sysfs/symlink.c: fill_object_path()/object_path_length().



Regards.
Benjamin

-- 
B e n j a m i n   T h e r y  - BULL/DT/Open Software R&D

    http://www.bull.com
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