[RFC PATCH] swapfile: fix soft lockup in scan_swap_map_slots

[Chen Wandun <chenwandun@huawei.com>]

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).

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

Fixes: 048c27fd7281 ("[PATCH] swap: scan_swap_map latency breaks")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
---
 mm/swapfile.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index e99b291fb01a..5e7c98570ede 100644
--- 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);
-- 
2.25.1