Re: [PATCHv5 4/8] zswap: add to mm/

[Seth Jennings]

On 02/18/2013 01:49 PM, Cody P Schafer wrote:
> On 02/18/2013 11:24 AM, Seth Jennings wrote:
>> On 02/15/2013 10:04 PM, Ric Mason wrote:
>>> On 02/14/2013 02:38 AM, Seth Jennings wrote:
>> <snip>
>>>> +/* invalidates all pages for the given swap type */
>>>> +static void zswap_frontswap_invalidate_area(unsigned type)
>>>> +{
>>>> +    struct zswap_tree *tree = zswap_trees[type];
>>>> +    struct rb_node *node, *next;
>>>> +    struct zswap_entry *entry;
>>>> +
>>>> +    if (!tree)
>>>> +        return;
>>>> +
>>>> +    /* walk the tree and free everything */
>>>> +    spin_lock(&tree->lock);
>>>> +    node = rb_first(&tree->rbroot);
>>>> +    while (node) {
>>>> +        entry = rb_entry(node, struct zswap_entry, rbnode);
>>>> +        zs_free(tree->pool, entry->handle);
>>>> +        next = rb_next(node);
>>>> +        zswap_entry_cache_free(entry);
>>>> +        node = next;
>>>> +    }
>>>> +    tree->rbroot = RB_ROOT;
>>>
>>> Why don't need rb_erase for every nodes?
>>
>> We are freeing the entire tree here.  try_to_unuse() in the swapoff
>> syscall should have already emptied the tree, but this is here for
>> completeness.
>>
>> rb_erase() will do things like rebalancing the tree; something that
>> just wastes time since we are in the process of freeing the whole
>> tree.  We are holding the tree lock here so we are sure that no one
>> else is accessing the tree while it is in this transient broken state.
> 
> If we have a sub-tree like:
>     ...
>    /
>   A
>  / \
> B   C
> 
> B == rb_next(tree)
> A == rb_next(B)
> C == rb_next(A)
> 
> The current code free's A (via zswap_entry_cache_free()) prior to
> examining C, and thus rb_next(C) results in a use after free of A.
> 
> You can solve this by doing a post-order traversal of the tree, either
> 
> a) in the destructive manner used in a number of filesystems, see
> fs/ubifs/orphan.c ubifs_add_orphan(), for example.
> 
> b) or by doing something similar to this commit:
> https://github.com/jmesmon/linux/commit/d9e43aaf9e8a447d6802531d95a1767532339fad
> , which I've been using for some yet-to-be-merged code.

Great catch! I'll fix this up.

Thanks,
Seth

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