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Re: [PATCH 5/9] lib: radix-tree: check accounting of existing slot replacement users

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On Thu 17-11-16 14:30:21, Johannes Weiner wrote:
> The bug in khugepaged fixed earlier in this series shows that radix
> tree slot replacement is fragile; and it will become more so when not
> only NULL<->!NULL transitions need to be caught but transitions from
> and to exceptional entries as well. We need checks.
> 
> Re-implement radix_tree_replace_slot() on top of the sanity-checked
> __radix_tree_replace(). This requires existing callers to also pass
> the radix tree root, but it'll warn us when somebody replaces slots
> with contents that need proper accounting (transitions between NULL
> entries, real entries, exceptional entries) and where a replacement
> through the slot pointer would corrupt the radix tree node counts.
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

One nit below:

> @@ -785,6 +776,50 @@ void __radix_tree_replace(struct radix_tree_root *root,
>  }
>  
>  /**
> + * __radix_tree_replace		- replace item in a slot
> + * @root:	radix tree root
> + * @node:	pointer to tree node
> + * @slot:	pointer to slot in @node
> + * @item:	new item to store in the slot.
> + *
> + * For use with __radix_tree_lookup().  Caller must hold tree write locked
> + * across slot lookup and replacement.
> + */

I'd comment here that even this function cannot be used for NULL <->
non-NULL replacements. For that are radix_tree_delete() and
radix_tree_insert().

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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[PATCH 0/9] mm: workingset: radix tree subtleties & single-page file refaults v3 Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:20 +0100
  [PATCH 4/9] lib: radix-tree: native accounting of exceptional entries Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 4/9] lib: radix-tree: native accounting of exceptional  entries Jan Kara <jack@suse.cz> - 2016-11-18 08:40 +0100
  [PATCH 6/9] lib: radix-tree: add entry deletion support to  __radix_tree_replace() Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 6/9] lib: radix-tree: add entry deletion support to  __radix_tree_replace() Jan Kara <jack@suse.cz> - 2016-11-18 09:20 +0100
  [PATCH 8/9] mm: workingset: move shadow entry tracking to radix tree  exceptional tracking Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 8/9] mm: workingset: move shadow entry tracking to radix  tree exceptional tracking Jan Kara <jack@suse.cz> - 2016-11-18 09:30 +0100
  [PATCH 7/9] lib: radix-tree: update callback for changing leaf nodes Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 7/9] lib: radix-tree: update callback for changing leaf  nodes Jan Kara <jack@suse.cz> - 2016-11-18 09:30 +0100
  [PATCH 5/9] lib: radix-tree: check accounting of existing slot  replacement users Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 5/9] lib: radix-tree: check accounting of existing slot  replacement users Jan Kara <jack@suse.cz> - 2016-11-18 08:50 +0100
  [PATCH 9/9] mm: workingset: restore refault tracking for single-page  files Johannes Weiner <hannes@cmpxchg.org> - 2016-11-17 20:40 +0100
    Re: [PATCH 9/9] mm: workingset: restore refault tracking for  single-page files Jan Kara <jack@suse.cz> - 2016-11-18 09:40 +0100

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