linux-kernel - Re: [PATCH] mm: ratelimit end_swap_bio

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Date:   Sat, 6 Jan 2018 10:41:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: ratelimit end_swap_bio_write() error

On Sat 06-01-18 13:34:07, Sergey Senozhatsky wrote:
> Use the ratelimited printk() version for swap-device write error
> reporting. We can use ZRAM as a swap-device, and the tricky part
> here is that zsmalloc() stores compressed objects in memory, thus
> it has to allocates pages during swap-out. If the system is short
> on memory, then we begin to flood printk() log buffer with the
> same "Write-error on swap-device XXX" error messages and sometimes
> simply lockup the system.

Should we print an error in such a situation at all? Write-error
certainly sounds scare and it suggests something went really wrong.
My understading is that zram failed swap-out is not critical and
therefore the error message is not really useful. Or what should an
admin do when seeing it?

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  mm/page_io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_io.c b/mm/page_io.c
> index e93f1a4cacd7..422cd49bcba8 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -63,7 +63,7 @@ void end_swap_bio_write(struct bio *bio)
>  		 * Also clear PG_reclaim to avoid rotate_reclaimable_page()
>  		 */
>  		set_page_dirty(page);
> -		pr_alert("Write-error on swap-device (%u:%u:%llu)\n",
> +		pr_alert_ratelimited("Write-error on swap-device (%u:%u:%llu)\n",
>  			 MAJOR(bio_dev(bio)), MINOR(bio_dev(bio)),
>  			 (unsigned long long)bio->bi_iter.bi_sector);
>  		ClearPageReclaim(page);
> -- 
> 2.15.1

-- 
Michal Hocko
SUSE Labs