Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()

Huang, Ying
2020-04-23T05:57:42Z

Wei Yang <richard.weiyang@gmail.com> writes:

> After commit c60aa176c6de8 ("swapfile: swap allocation cycle if
> nonrot"), swap allocation is cyclic. Current approach is done with two
> separate loop on the upper and lower half. This looks a little
> redundant.

I can understand that the redundant code doesn't smell good. But I
don't think the new code is easier to be understood than the original
one.

> From another point of view, the loop iterates [lowest_bit, highest_bit]
> range starting with (offset + 1) but except scan_base. So we can
> simplify the loop with condition (next_offset() != scan_base) by
> introducing next_offset() which makes sure offset fit in that range
> with correct order.
>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Hugh Dickins <hughd@google.com>
> CC: "Huang, Ying" <ying.huang@intel.com>
>
> ---
> v2:
> * return scan_base if the lower part is eaten
> * only start over when iterating on the upper part
> ---
> mm/swapfile.c | 31 ++++++++++++++-----------------
> 1 file changed, 14 insertions(+), 17 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f903e5a165d5..0005a4a1c1b4 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -729,6 +729,19 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> }
> }
>
> +static unsigned long next_offset(struct swap_info_struct *si,
> + unsigned long *offset, unsigned long scan_base)
> +{
> + /* only start over when iterating on the upper part */
> + if (++(*offset) > si->highest_bit && *offset > scan_base) {
> + *offset = si->lowest_bit;
> + /* someone has eaten the lower part */
> + if (si->lowest_bit >= scan_base)
> + return scan_base;
> + }

if "offset > si->highest_bit" is true and "offset < scan_base" is true,
scan_base need to be returned.

Again, the new code doesn't make it easier to find this kind of issues.

Best Regards,
Huang, Ying