Re: [EXT4 set 5][PATCH 1/1] expand inode i_extra_isize to support features in larger inode

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 Jul 2007 16:32:47 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> > +	brelse(bh);
> > +	up_write(&EXT4_I(inode)->xattr_sem);
> > +	return error;
> > +}
> > +
> 
> We're doing GFP_KERNEL memory allocations while holding xattr_sem.  This
> can cause the VM to reenter the filesystem, perhaps taking i_mutex and/or
> i_truncate_sem and/or journal_start() (I forget whether this still
> happens).  Have we checked whether this can occur and if so, whether we are
> OK from a lock ranking POV?  Bear in mind that journalled-data mode is more
> complex in this regard.

I notice that everyone carefully avoided addressing this ;)

Oh well, hopefully people are testing with lockdep enabled.  As long
as the fs is put under extreme memory pressure, most bugs should be reported.

Except lockdep doesn't know about journal_start(), which has ranking
requirements similar to a semaphore.  Nor does it know about lock_page().
We already have hard-to-hit but deadlockable bugs in this area.