Re: [PATCH 3/4] vfs: count unlinked inodes

[Steven Whitehouse <swhiteho@redhat.com>]

Hi,

On Mon, 2011-12-19 at 17:03 +0100, Miklos Szeredi wrote:
> On Mon, Dec 19, 2011 at 3:38 PM, Steven Whitehouse <swhiteho@redhat.com> wrote:
> > Hi,
> >
> > On Sat, 2011-12-17 at 07:36 +0000, Al Viro wrote:
> >> On Mon, Nov 21, 2011 at 12:11:32PM +0100, Miklos Szeredi wrote:
> >> > @@ -241,6 +242,11 @@ void __destroy_inode(struct inode *inode)
> >> >     BUG_ON(inode_has_buffers(inode));
> >> >     security_inode_free(inode);
> >> >     fsnotify_inode_delete(inode);
> >> > +   if (!inode->i_nlink) {
> >> > +           WARN_ON(atomic_long_read(&inode->i_sb->s_remove_count) == 0);
> >> > +           atomic_long_dec(&inode->i_sb->s_remove_count);
> >> > +   }
> >>
> >> Umm...  That relies on ->destroy_inode() doing nothing stupid; granted,
> >> all work on actual file removal should've been done in ->evice_inode()
> >> leaving only (RCU'd) freeing of in-core, but there are odd ones that
> >> do strange things in ->destroy_inode() and I'm not sure that it's not
> >> a Yet Another Remount Race(tm).  OTOH, it's clearly not worse than what
> >> we used to have; just something to keep in mind for future work.
> >>
> > GFS2 is one of those cases. The issue is that when we enter
> > ->evict_inode() with i_nlink 0, we do not know whether any other node
> > still has the inode open. If it does, then we do not deallocate it in
> > ->evict_inode() but instead just forget about it, just as if i_nlink was
> >> 0 leaving the remaining opener(s) to do the deallocation later,
> 
> And does GFS2 care about read-only remount races because of that?
> I.e. if an unlinked file is still open on another node, should we
> prevent remounting read-only until it the file is released and
> actually gone?
> 
> If that's not a requirement (and I don't see why it should be) then all is fine.
> 
> Thanks,
> Miklos

Ok. Good, we don't need to worry about that. We can support any mix of
read-write, and read-only nodes with the caveat that a cluster with only
one read-write node will have no other node to perform recovery for it,
should it fail. Also, since read-only nodes cannot deallocate inodes
(even if they are the last openers of a file) then they will simply
ignore such inodes, and wait for the next read-write node to perform an
allocation in that resource group, whereupon the deallocation will be
completed.

So remounting read-only is a purely local operation so far as GFS2 is
concerned,

Steve.