Subject: Re: Next April 28: boot failure on PowerPC with SLQB

From: Anton Vorontsov <[email-masked]>
Date: Thu, 30 Apr 2009 18:10:13 +0400

On Thu, Apr 30, 2009 at 03:05:42PM +0200, Nick Piggin wrote:
[...]
> ---
> SLQB: fix dumb early allocation cache
> 
> The dumb early allocation cache had a bug where it could allow allocation
> to go past the end of a page, which could cause crashes or random memory
> corruption. Fix this and simplify the logic.
> 
> Signed-off-by: Nick Piggin <[email-masked]>
> ---
>  mm/slqb.c |   19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> Index: linux-2.6/mm/slqb.c
> ===================================================================
> --- linux-2.6.orig/mm/slqb.c
> +++ linux-2.6/mm/slqb.c
> @@ -2185,8 +2185,11 @@ static void *kmem_cache_dyn_array_alloc(
>  {
>  	size_t size = sizeof(void *) * ids;
>  
> +	BUG_ON(!size);
> +
>  	if (unlikely(!slab_is_available())) {
>  		static void *nextmem;
> +		static size_t nextleft;
>  		void *ret;
>  
>  		/*
> @@ -2194,16 +2197,16 @@ static void *kmem_cache_dyn_array_alloc(
>  		 * never get freed by definition so we can do it rather
>  		 * simply.
>  		 */
> -		if (!nextmem) {
> -			nextmem = alloc_pages_exact(size, GFP_KERNEL);
> -			if (!nextmem)
> -				return NULL;
> +		if (size > nextleft) {
> +                        nextmem = alloc_pages_exact(size, GFP_KERNEL);
> +                        if (!nextmem)
> +                                return NULL;

Cosmetic issue: spaces instead of tabs are used on these
three lines.

> +			nextleft = roundup(size, PAGE_SIZE);
>  		}
> +
>  		ret = nextmem;
> -		nextmem = (void *)((unsigned long)ret + size);
> -		if ((unsigned long)ret >> PAGE_SHIFT !=
> -				(unsigned long)nextmem >> PAGE_SHIFT)
> -			nextmem = NULL;
> +		nextleft -= size;
> +		nextmem += size;
>  		memset(ret, 0, size);
>  		return ret;
>  	} else {

-- 
Anton Vorontsov
email: [email-masked]
irc://irc.freenode.net/bd2