Re: [PATCH 04/19] fs: Implement lazy LRU updates for inodes.

Nick Piggin
2010-10-16T17:29:29Z

On Sat, Oct 16, 2010 at 12:59:30PM -0400, Christoph Hellwig wrote:
> On Sat, Oct 16, 2010 at 08:29:16PM +1100, Nick Piggin wrote:
> > On Sat, Oct 16, 2010 at 07:13:58PM +1100, Dave Chinner wrote:
> > > @@ -502,11 +527,15 @@ static void prune_icache(int nr_to_scan)
> > > iput(inode);
> > > spin_lock(&inode_lock);
> > >
> > > - if (inode != list_entry(inode_unused.next,
> > > - struct inode, i_list))
> > > - continue; /* wrong inode or list_empty */
> > > - if (!can_unuse(inode))
> > > + /*
> > > + * if we can't reclaim this inode immediately, give it
> > > + * another pass through the free list so we don't spin
> > > + * on it.
> > > + */
> > > + if (!can_unuse(inode)) {
> > > + list_move(&inode->i_list, &inode_unused);
> > > continue;
> > > + }
> > > }
> > > list_move(&inode->i_list, &freeable);
> > > WARN_ON(inode->i_state & I_NEW);
> >
> > This is a bug, actually 2 bugs, which is why I omitted it in the version
> > you picked up. I agree we want the optimisation though, so I've added it
> > back in my tree.
> >
> > After you iput() and then re take the inode lock, you can't reference
> > the inode because you don't know what happened to it. You need to keep
> > that pointer check to verify it is still there.
>
> I don't think the pointer check will work either. By the time we retake
> the lru lock the inode might already have been reaped through a call
> to invalidate_inodes. There's no way we can do anything with it after

I don't think you're right. If we re take inode_lock, ensure it is on
the LRU, and call the can_unuse checks, there is no more problem than
the regular loop taking items from the LRU, AFAIKS.

> iput. What we could do is using variant of can_unuse to decide to move
> the inode to the front of the lru before doing the iput. That way
> we'll get to it next after retaking the lru lock if it's still there.

This might actually be the better approach anyway (even for upstream)
-- it means we don't have to worry about the "check head element"
heuristic of the LRU check which could get false negatives if there is
a lot of concurrency on the LRU.