Re: Oops in a driver while using SLUB as a SLAB allocator

[Hugh Dickins]

On Fri, 22 Jun 2007, Christoph Lameter wrote:
> 
> Add VM_BUG_ON in case someone uses page_mapping on a slab page
> 
> Detect slab objects being passed to the page oriented functions of the VM.
> 
> It is not sufficient to simply return NULL because the functions calling
> page_mapping may depend on other items of the page_struct also to be setup
> properly. Moreover the slab object may not be properly aligned. The page 
> orientedfunctions of the VM expect to operate on page aligned, page sized 
> objects. operations on objects straddling page boundaries may only affect 
> the objects partially which may lead to surprising results.
> 
> It is better to detect eventually remaining uses and eliminate them.
> 
> Signed-off-by: Christoph Lameter <clameter@xxxxxxx>

I'm quite happy with this approach for 2.6.23-rc, along with your ARM
dma_map patch which (if I understood aright) rmk approved.  Except,
haven't you misplaced that VM_BUG_ON between an if and its else?
And I'd much prefer you to make it an outright BUG_ON, because
many testers have those VM_BUG_ONs configured out.

Hugh

> 
> ---
>  include/linux/mm.h |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h	2007-06-22 10:33:27.000000000 -0700
> +++ linux-2.6/include/linux/mm.h	2007-06-22 10:34:09.000000000 -0700
> @@ -603,10 +603,7 @@ static inline struct address_space *page
>  
>  	if (unlikely(PageSwapCache(page)))
>  		mapping = &swapper_space;
> -#ifdef CONFIG_SLUB
> -	else if (unlikely(PageSlab(page)))
> -		mapping = NULL;
> -#endif
> +	VM_BUG_ON(PageSlab(page));
>  	else if (unlikely((unsigned long)mapping & PAGE_MAPPING_ANON))
>  		mapping = NULL;
>  	return mapping;
> 
-
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