Re: [PATCH 2/2] lib/btree: Fix possible NULL pointer dereference

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 May 2010 01:20:27 +0400
"Denis Kirjanov <kirjanov@gmail.com" <kirjanov@gmail.com> wrote:

> mempool_alloc can return null in atomic case.
> 
> Signed-off-by: Denis Kirjanov <kirjanov@gmail.com>
> ---
> diff --git a/lib/btree.c b/lib/btree.c
> index 41859a8..542c904 100644
> --- a/lib/btree.c
> +++ b/lib/btree.c
> @@ -95,7 +94,8 @@ static unsigned long *btree_node_alloc(struct btree_head *head, gfp_t gfp)
>  	unsigned long *node;
>  
>  	node = mempool_alloc(head->mempool, gfp);
> -	memset(node, 0, NODESIZE);
> +	if (likely(node))
> +		memset(node, 0, NODESIZE);
>  	return node;
>  }

hm, why is btree.c using mempools?  mempools are only appropriate when
it is known that objects will become available if the allocating task
simply waits for a while.  Typically, things like BIOs and
request-structs.  Simply waiting for the disk to complete some IO will
cause some objects to be returned to the mempool.

If waiting-and-doing-nothing fails to cause objects to be returned to
the pool then the mempool code can lock up.