Re: [PATCH] fix nr_unused accounting, and avoid recursing in iput with I_WILL_FREE set

[Andrew Morton <akpm@osdl.org>]

Chris Mason <mason@suse.com> wrote:
>
> On Tue, Oct 18, 2005 at 06:15:48PM -0700, Andrew Morton wrote:
> > > > Well according to my assertion (below), the inode in __sync_single_inode()
> > > > cannot have a zero refcount, so the whole if() statement is never executed.
> > > 
> > > generic_forget_inode->write_inode_now->__writeback_single_inode->
> > > __sync_single_inode
> > 
> > oshit.
> 
> When does this ever happen?  Just for private inodes released during
> put_super right?

I suppose so, yes.

> > 
> > > We do have I_WILL_FREE, but i_count will be zero.
> > 
> > yup.
> > 
> > > > 
> > > > The thinking behind that increment is that __sync_single_inode() has just
> > > > taken a dirty, zero-refcount inode and has cleaned it.  A dirty inode
> > > > cannot have previously been on inode_unused, hence we now are newly moving
> > > > it to inode_unused.
> > > 
> > > nr_unused doesn't seem to count the number of inodes on the unused list.
> > > It is actually counting the number of inodes whose i_count is 0.  See
> > > generic_forget_inode and invalidate_list to see what I mean.
> > 
> > hm, OK.  It'd be nice to make that more explicit.  Something like this?
> 
> Well, I can't quite convince myself it is wrong, but when 
> (!sb || (sb->s_flags & MS_ACTIVE), we're dropping the
> inode_lock with an inode with i_count == 0 and nr_unused hasn't been
> incremented.
> 
> So, if someone (sync_sb_inodes?) comes in and runs __iget,
> the counts end up wrong.  Then again, whoever ran __iget would also run
> iput and things would go horribly wrong anyway.

Nope, it's equivalent:

--- devel/fs/inode.c~generic_forget_inode-nr_unused-cleanup	2005-10-18 18:13:22.000000000 -0700
+++ devel-akpm/fs/inode.c	2005-10-18 18:13:57.000000000 -0700
@@ -1067,8 +1067,8 @@ static void generic_forget_inode(struct 
 	if (!hlist_unhashed(&inode->i_hash)) {
 		if (!(inode->i_state & (I_DIRTY|I_LOCK)))
 			list_move(&inode->i_list, &inode_unused);
-		inodes_stat.nr_unused++;
 		if (!sb || (sb->s_flags & MS_ACTIVE)) {
+			inodes_stat.nr_unused++;  /* One more 0-ref inode */
 			spin_unlock(&inode_lock);
 			return;
 		}
@@ -1077,7 +1077,6 @@ static void generic_forget_inode(struct 
 		write_inode_now(inode, 1);
 		spin_lock(&inode_lock);
 		inode->i_state &= ~I_WILL_FREE;
-		inodes_stat.nr_unused--;
 		hlist_del_init(&inode->i_hash);
 	}
 	list_del_init(&inode->i_list);
_


> Did I mention the part where Andrea and I are hunting a bug where the
> count of unused inodes goes negative and the everyone ends up spinning
> in shrink_icache_memory?

No.

>  Andrea's patch doesn't fix the spinning, but
> it might have fixed the unused inode count going negative.  We're
> waiting for another reproduce on the ppc64 race monster.

I assume you have BUG_ON(inode_stat.nr_unused < 0)s in there everywhere?

In fact WARN_ON(inode_stat.nr_unused < 100) might be better - something's
obviously doing a bogus decrement a lot of times.