[PATCH] fix swap cluster offset

[PATCH] fix swap cluster offset

[Akinobu Mita]

When we've allocated SWAPFILE_CLUSTER pages, ->cluster_next should
be the first index of swap cluster. But current code probably sets it
wrong offset.

Signed-off-by: Akinobu Mita <mita@miraclelinux.com>

Index: work/mm/swapfile.c
===================================================================
--- work.orig/mm/swapfile.c
+++ work/mm/swapfile.c
@@ -116,7 +116,7 @@ static inline unsigned long scan_swap_ma
 				last_in_cluster = offset + SWAPFILE_CLUSTER;
 			else if (offset == last_in_cluster) {
 				spin_lock(&swap_lock);
-				si->cluster_next = offset-SWAPFILE_CLUSTER-1;
+				si->cluster_next = offset-SWAPFILE_CLUSTER+1;
 				goto cluster;
 			}
 			if (unlikely(--latency_ration < 0)) {

Re: [PATCH] fix swap cluster offset

[Hugh Dickins]

On Sun, 12 Mar 2006, Akinobu Mita wrote:

> When we've allocated SWAPFILE_CLUSTER pages, ->cluster_next should
> be the first index of swap cluster. But current code probably sets it
> wrong offset.
> 
> Signed-off-by: Akinobu Mita <mita@miraclelinux.com>

Acked-by: Hugh Dickins <hugh@veritas.com>

Very good eye!  My shame.
No need to say "probably" above, it was simply wrong.

By a stroke of luck, it's no worse than a slight inefficiency in an
algorithm only used when we're already going slow; but (if offset had
been signed, or not checked against highest_bit for unrelated reasons)
it could very easily have tried to access and modify swap_map[-1].

Anyway, thanks for catching that:
Andrew, please apply (but not desperate for 2.6.16).

Hugh

> Index: work/mm/swapfile.c
> ===================================================================
> --- work.orig/mm/swapfile.c
> +++ work/mm/swapfile.c
> @@ -116,7 +116,7 @@ static inline unsigned long scan_swap_ma
>  				last_in_cluster = offset + SWAPFILE_CLUSTER;
>  			else if (offset == last_in_cluster) {
>  				spin_lock(&swap_lock);
> -				si->cluster_next = offset-SWAPFILE_CLUSTER-1;
> +				si->cluster_next = offset-SWAPFILE_CLUSTER+1;
>  				goto cluster;
>  			}
>  			if (unlikely(--latency_ration < 0)) {