Re: [PATCH 12/32] Unionfs: documentation updates

["Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu>]

On Mon, Sep 03, 2007 at 08:59:02AM +0200, Jan Engelhardt wrote:
> 
> On Sep 2 2007 22:20, Josef 'Jeff' Sipek wrote:
> >+
> >+While rebuilding Unionfs's objects, we also purge any page mappings and
> >+truncate inode pages (see fs/Unionfs/dentry.c:purge_inode_data).  This is to
> 
> fs/unionfs/dentry.c
> 
> >+Unionfs maintains the following important invariant regarding mtime's,
> >+ctime's, and atime's: the upper inode object's times are the max() of all of
> 
> utimes, ctimes and atimes.
> 
> >+2. Lockdep (a debugging feature) isn't aware of stacking, and so it
> >+   incorrectly complains about locking problems.  The problem boils down to
> >+   this: Lockdep considers all objects of a certain type to be in the same
> >+   class, for example, all inodes.  Lockdep doesn't like to see a lock held
> >+   on two inodes within the same task, and warns that it could lead to a
> >+   deadlock.  However, stackable file systems do precisely that: they lock
> >+   an upper object, and then a lower object, in a strict order to avoid
> >+   locking problems; in addition, Unionfs, as a fan-out file system, may
> >+   have to lock several lower inodes.  We are currently looking into Lockdep
> >+   to see how to make it aware of stackable file systems.  In the mean time,
> 
> meantime
> 
> >@@ -86,5 +86,12 @@ command:
> > 
> > # mount -t unionfs -o remount,incgen none MOUNTPOINT
> > 
> >+Note that the older way of incrementing the generation number using an
> >+ioctl, is no longer supported in Unionfs 2.0.  Ioctls in general are not
> 
> 2.1?

Thanks.

Jeff.

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