We found that the plist_for_each_entry_safe() loop in get_swap_pages()
could reach tens of thousands of times to find available space (extreme
case: cond_resched() is not called in the loop). The test scenario is like
commit de1ccfb64824 ("swapfile: fix soft lockup in scan_swap_map_slots")
does. On the other hand, scan_swap_map_slots() needs to reach LATENCY_LIMIT
before calling cond_resched(), so cond_resched() is needed when failed to
find available space to avoid softlockup.

Signed-off-by: Longlong Xia <xialonglong1@huawei.com>
---
 mm/swapfile.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 908a529bca12..4fa440e87cd6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1100,6 +1100,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 			goto check_out;
 		pr_debug("scan_swap_map of si %d failed to find offset\n",
 			si->type);
+		cond_resched();
 
 		spin_lock(&swap_avail_lock);
 nextsi:
-- 
2.25.1

Hi, Longlong,

Longlong Xia <xialonglong1@huawei.com> writes:

> We found that the plist_for_each_entry_safe() loop in get_swap_pages()
> could reach tens of thousands of times to find available space (extreme
> case: cond_resched() is not called in the loop). The test scenario is like
> commit de1ccfb64824 ("swapfile: fix soft lockup in scan_swap_map_slots")
> does. On the other hand, scan_swap_map_slots() needs to reach LATENCY_LIMIT
> before calling cond_resched(), so cond_resched() is needed when failed to
> find available space to avoid softlockup.

Thanks for the patch.  The patch itself looks good to me.

I think that it's better to describe the test case in the patch
description directly instead of referring the previous commit.

Best Regards,
Huang, Ying

> Signed-off-by: Longlong Xia <xialonglong1@huawei.com>
> ---
>  mm/swapfile.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 908a529bca12..4fa440e87cd6 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1100,6 +1100,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>  			goto check_out;
>  		pr_debug("scan_swap_map of si %d failed to find offset\n",
>  			si->type);
> +		cond_resched();
>  
>  		spin_lock(&swap_avail_lock);
>  nextsi: