On Fri, 18 Nov 2022 21:38:50 +0800 Chen Wandun <chenwandun@xxxxxxxxxx> wrote:
> A soft lockup occur in scan free swap slot by constructing
> huge memory pressure.
> The test scenario is: 64 CPU cores, 64GB memory, and 28
> zram devices, the disksize of each zram device is 50MB.
>
> LATENCY_LIMIT is used to prevent soft lockup in function
> scan_swap_map_slots, but the real loop number would more
> than LATENCY_LIMIT because of "goto checks and goto scan"
> repeatly without decrease of latency limit.
>
> In order to fix it, move decrease latency_ration code in advance.
>
> There is also a suspicious place that will cause soft lockup in
> function get_swap_pages, in this function, the "goto start_over"
> may result in continuous scanning of swap partition, if there is
> no cond_sched in scan_swap_map_slots, it would cause soft lockup
> (I am not sure about this).
>
> ...
>
Looks sensible.
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -972,23 +972,23 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> scan:
> spin_unlock(&si->lock);
> while (++offset <= READ_ONCE(si->highest_bit)) {
> - if (swap_offset_available_and_locked(si, offset))
> - goto checks;
> if (unlikely(--latency_ration < 0)) {
> cond_resched();
> latency_ration = LATENCY_LIMIT;
> scanned_many = true;
> }
> + if (swap_offset_available_and_locked(si, offset))
> + goto checks;
> }
> offset = si->lowest_bit;
> while (offset < scan_base) {
> - if (swap_offset_available_and_locked(si, offset))
> - goto checks;
> if (unlikely(--latency_ration < 0)) {
> cond_resched();
> latency_ration = LATENCY_LIMIT;
> scanned_many = true;
> }
> + if (swap_offset_available_and_locked(si, offset))
> + goto checks;
> offset++;
> }
> spin_lock(&si->lock);
But this does somewhat alter the `scanned_many' logic. We'll now set
'scanned_many` earlier. What are the effects of this?
The ed43af10975eef7e changelog outlines tests which could be performed
to ensure we aren't regressing from this.
