Re: [PATCH 5/6] mm: workingset: switch shadow entry tracking to radix tree exceptional counting

[Jan Kara]

On Mon 07-11-16 14:07:40, Johannes Weiner wrote:
> Currently, we track the shadow entries in the page cache in the upper
> bits of the radix_tree_node->count, behind the back of the radix tree
> implementation. Because the radix tree code has no awareness of them,
> we rely on random subtleties throughout the implementation (such as
> the node->count != 1 check in the shrinking code which is meant to
> exclude multi-entry nodes, but also happens to skip nodes with only
> one shadow entry since it's accounted in the upper bits). This is
> error prone and has, in fact, caused the bug fixed in d3798ae8c6f3
> ("mm: filemap: don't plant shadow entries without radix tree node").
> 
> To remove these subtleties, this patch moves shadow entry tracking
> from the upper bits of node->count to the existing counter for
> exceptional entries. node->count goes back to being a simple counter
> of valid entries in the tree node and can be shrunk to a single byte.

...

> diff --git a/mm/truncate.c b/mm/truncate.c
> index 6ae44571d4c7..d3ce5f261f47 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -53,7 +53,6 @@ static void clear_exceptional_entry(struct address_space *mapping,
>  	mapping->nrexceptional--;
>  	if (!node)
>  		goto unlock;
> -	workingset_node_shadows_dec(node);
>  	/*
>  	 * Don't track node without shadow entries.
>  	 *
> @@ -61,8 +60,7 @@ static void clear_exceptional_entry(struct address_space *mapping,
>  	 * The list_empty() test is safe as node->private_list is
>  	 * protected by mapping->tree_lock.
>  	 */
> -	if (!workingset_node_shadows(node) &&
> -	    !list_empty(&node->private_list))
> +	if (!node->exceptional && !list_empty(&node->private_list))
>  		list_lru_del(&workingset_shadow_nodes,
>  				&node->private_list);
>  	__radix_tree_delete_node(&mapping->page_tree, node);

Is this really correct now? The radix tree implementation can move a single
exceptional entry at index 0 from a node into a direct pointer and free
the node while it is still in the LRU list. Or am I missing something?
To fix this I'd prefer to just have a callback from radix tree code when it
is freeing a node, rather that trying to second-guess its implementation in
the page-cache code...

Otherwise the patch looks good to me and I really like the simplification!

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR