Re: [PATCH] mm: Make vb_alloc() more foolproof

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

On Wed,  6 Jun 2012 00:40:56 +0200
Jan Kara <jack@suse.cz> wrote:

> If someone calls vb_alloc() (or vm_map_ram() for that matter) to allocate
> 0 bytes (0 pages), get_order() returns BITS_PER_LONG - PAGE_CACHE_SHIFT
> and interesting stuff happens. So make debugging such problems easier and
> warn about 0-size allocation.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/vmalloc.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 2aad499..bebee70 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -904,6 +904,15 @@ static void *vb_alloc(unsigned long size, gfp_t gfp_mask)
>  
>  	BUG_ON(size & ~PAGE_MASK);
>  	BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC);
> +	if (size == 0) {
> +		/*
> +		 * Allocating 0 bytes isn't what caller wants since
> +		 * get_order(0) returns funny result. Just warn and terminate
> +		 * early.
> +		 */
> +		WARN_ON(1);
> +		return NULL;
> +	}
>  	order = get_order(size);

Spose so.  You got bitten, I assume ;)

We can neaten the implementation:

--- a/mm/vmalloc.c~mm-make-vb_alloc-more-foolproof-fix
+++ a/mm/vmalloc.c
@@ -904,13 +904,12 @@ static void *vb_alloc(unsigned long size
 
 	BUG_ON(size & ~PAGE_MASK);
 	BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC);
-	if (size == 0) {
+	if (WARN_ON(size == 0)) {
 		/*
 		 * Allocating 0 bytes isn't what caller wants since
 		 * get_order(0) returns funny result. Just warn and terminate
 		 * early.
 		 */
-		WARN_ON(1);
 		return NULL;
 	}
 	order = get_order(size);

and that gives us the unlikely() hit too.

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