On Mon, 26 May 2014, Chen Yucong wrote:
> For cheap seek, when we scan the region between si->lowset_bit
> and scan_base, if san_base is greater than si->highest_bit, the
> scan operation between si->highest_bit and scan_base is not
> unnecessary.
>
> This patch can be used to avoid scanning invalidated region for
> cheap seek.
>
> Signed-off-by: Chen Yucong <slaoub@xxxxxxxxx>
I was going to suggest that you are adding a little code to a common
path, in order to optimize a very unlikely case: which does not seem
worthwhile to me.
But digging a little deeper, I think you have hit upon something more
interesting (though still in no need of your patch): it looks to me
like that is not even a common path, but dead code.
Shaohua, am I missing something, or does all SWP_SOLIDSTATE "seek is
cheap" now go your si->cluster_info scan_swap_map_try_ssd_cluster()
route? So that the "last_in_cluster < scan_base" loop in the body
of scan_swap_map() is just redundant, and should have been deleted?
Hugh
> ---
> mm/swapfile.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index beeeef8..7f0f27e 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -489,6 +489,7 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
> {
> unsigned long offset;
> unsigned long scan_base;
> + unsigned long upper_bound;
> unsigned long last_in_cluster = 0;
> int latency_ration = LATENCY_LIMIT;
>
> @@ -551,9 +552,11 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
>
> offset = si->lowest_bit;
> last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
> + upper_bound = (scan_base <= si->highest_bit) ?
> + scan_base : (si->highest_bit + 1);
>
> /* Locate the first empty (unaligned) cluster */
> - for (; last_in_cluster < scan_base; offset++) {
> + for (; last_in_cluster < upper_bound; offset++) {
> if (si->swap_map[offset])
> last_in_cluster = offset + SWAPFILE_CLUSTER;
> else if (offset == last_in_cluster) {
> --
> 1.7.10.4
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