Re: [Xen-devel] [PATCH] Trivial fix for latent bug in page_alloc.c

[David Hopwood <david.nospam.hopwood@xxxxxxxxxxxxxxxx>]

Rusty Russell wrote:
> Was reading through page_alloc.c, and the "find smallest order" loop
> assumes MIN_ORDER is 0.  Easiest fix is to get rid of "MIN_ORDER" and
> hence NR_ORDERS, and simplify code.
>
> Rusty.
>
> --- xen/common/page_alloc.c.~1~ 2005-01-11 15:35:54.000000000 +1100
> +++ xen/common/page_alloc.c     2005-01-19 14:51:47.000000000 +1100
> @@ -203,10 +203,8 @@
>  #define NR_ZONES    2
>  
>  /* Up to 2^10 pages can be allocated at once. */
> -#define MIN_ORDER  0
>  #define MAX_ORDER 10
> -#define NR_ORDERS (MAX_ORDER - MIN_ORDER + 1)
> -static struct list_head heap[NR_ZONES][NR_ORDERS];
> +static struct list_head heap[NR_ZONES][MAX_ORDER];
This patch is broken because it replaces NR_ORDERS with MAX_ORDER,
not with MAX_ORDER+1.

   #define MAX_ORDER 10
   static struct list_head heap[NR_ZONES][MAX_ORDER+1];

>  static unsigned long avail[NR_ZONES];
>  
> @@ -220,7 +218,7 @@
>      memset(avail, 0, sizeof(avail));
>  
>      for ( i = 0; i < NR_ZONES; i++ )
> -        for ( j = 0; j < NR_ORDERS; j++ )
> +        for ( j = 0; j < MAX_ORDER; j++ )
           for ( j = 0; j <= MAX_ORDER; j++ )

>              INIT_LIST_HEAD(&heap[i][j]);
>  
>      /* Pages that are free now go to the domain sub-allocator. */
> @@ -251,17 +249,18 @@
>      int i;
>      struct pfn_info *pg;
>  
> -    if ( unlikely(order < MIN_ORDER) || unlikely(order > MAX_ORDER) )
> +    ASSERT(order >= 0);
> +    if ( unlikely(order >= MAX_ORDER) )
>          return NULL;
Why is it valid to change 'return NULL' to a failed ASSERT?
Also changing > to >= is wrong.

       if ( unlikely(order < 0) || unlikely(order > MAX_ORDER) )
           return NULL;

>      spin_lock(&heap_lock);
>  
>      /* Find smallest order which can satisfy the request. */
> -    for ( i = order; i < NR_ORDERS; i++ )
> +    for ( i = order; i < MAX_ORDER; i++ )
       for ( i = order; i <= MAX_ORDER; i++ )

>         if ( !list_empty(&heap[zone][i]) )
>             break;
>  
> -    if ( i == NR_ORDERS ) 
> +    if ( i == MAX_ORDER ) 
       if ( i > MAX_ORDER)

>          goto no_memory;
>   
>      pg = list_entry(heap[zone][i].next, struct pfn_info, list);
--
David Hopwood <david.nospam.hopwood@xxxxxxxxxxxxxxxx>



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