On Tue, 21 Jan 2014, Jamie Liu wrote:
> In the second half of scan_swap_map()'s scan loop, offset is set to
> si->lowest_bit and then incremented before entering the loop for the
> first time, causing si->swap_map[si->lowest_bit] to be skipped.
>
> Signed-off-by: Jamie Liu <jamieliu@xxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Good catch. At first I was puzzled that this off-by-one could have
gone unnoticed for so long (ever since 2.6.29); but now I think that
almost always we have a good amount of slack, in those pages duplicated
between swap and swapcache, which can be reclaimed at the vm_swap_full()
check, and so conceal this loss of a single slot.
> ---
> mm/swapfile.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 612a7c9..6635081 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -616,7 +616,7 @@ scan:
> }
> }
> offset = si->lowest_bit;
> - while (++offset < scan_base) {
> + while (offset < scan_base) {
> if (!si->swap_map[offset]) {
> spin_lock(&si->lock);
> goto checks;
> @@ -629,6 +629,7 @@ scan:
> cond_resched();
> latency_ration = LATENCY_LIMIT;
> }
> + offset++;
> }
> spin_lock(&si->lock);
>
> --
> 1.8.5.3
>
>
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