swapfile: fix soft lockup in scan_swap_map_slots

A softlockup occurs in scan free swap slot under 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 softlockups in scan_swap_map_slots(), but
the real loop number would more than LATENCY_LIMIT because of "goto checks
and goto scan" repeatly without decreasing latency limit.

In order to fix it, decrease latency_ration in advance.

There is also a suspicious place that will cause softlockups in
get_swap_pages().  In this function, the "goto start_over" may result in
continuous scanning of the swap partition.  If there is no cond_sched in
scan_swap_map_slots(), it would cause a softlockup (I am not sure about
this).

WARN: soft lockup - CPU#11 stuck for 11s! [kswapd0:466]
CPU: 11 PID: 466 Comm: kswapd@ Kdump: loaded Tainted: G
dump backtrace+0x0/0x1le4
show stack+0x20/@x2c
dump_stack+0xd8/0x140
watchdog print_info+0x48/0x54
watchdog_process_before_softlockup+0x98/0xa0
watchdog_timer_fn+0xlac/0x2d0
hrtimer_rum_queues+0xb0/0x130
hrtimer_interrupt+0x13c/0x3c0
arch_timer_handler_virt+0x3c/0x50
handLe_percpu_devid_irq+0x90/0x1f4
handle domain irq+0x84/0x100
gic_handle_irq+0x88/0x2b0
e11 ira+0xhB/Bx140
scan_swap_map_slots+0x678/0x890
get_swap_pages+0x29c/0x440
get_swap_page+0x120/0x2e0
add_to_swap+UX2U/0XyC
shrink_page_list+0x5d0/0x152c
shrink_inactive_list+0xl6c/Bx500
shrink_lruvec+0x270/0x304

WARN: soft lockup - CPU#32 stuck for 11s! [stress-ng:309915]
watchdog_timer_fn+0x1ac/0x2d0
__run_hrtimer+0x98/0x2a0
__hrtimer_run_queues+0xb0/0x130
hrtimer_interrupt+0x13c/0x3c0
arch_timer_handler_virt+0x3c/0x50
handle_percpu_devid_irq+0x90/0x1f4
__handle_domain_irq+0x84/0x100
gic_handle_irq+0x88/0x2b0
el1_irq+0xb8/0x140
get_swap_pages+0x1e8/0x440
get_swap_page+0x1c8/0x2e0
add_to_swap+0x20/0x9c
shrink_page_list+0x5d0/0x152c
reclaim_pages+0x160/0x310
madvise_cold_or_pageout_pte_range+0x7bc/0xe3c
walk_pmd_range.isra.0+0xac/0x22c
walk_pud_range+0xfc/0x1c0
walk_pgd_range+0x158/0x1b0
__walk_page_range+0x64/0x100
walk_page_range+0x104/0x150

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 048c27f ("[PATCH] swap: scan_swap_map latency breaks")
Signed-off-by: Chen Wandun <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Nanyong Sun <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Chen Wandun

mm/swapfile.c

@@ -973,23 +973,23 @@ 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);