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

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Date:   Fri, 18 Nov 2016 08:46:38 +0100
From:   Jan Kara <jack@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 5/9] lib: radix-tree: check accounting of existing slot
 replacement users

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@...e.cz>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@...e.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@...e.com>
SUSE Labs, CR