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

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Date:   Fri, 12 Jan 2018 13:27:52 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 Fri 12-01-18 13:41:33, Sergey Senozhatsky wrote:
> On (01/08/18 19:22), Sergey Senozhatsky wrote:
> [..]
> > > Your changelog is rather modest on the information.
> > 
> > fair point!
> > 
> > > Could you be more specific on how the problem actually happens how
> > > likely it is?
> > 
> > ok. so what we have is
> > 
> > 	slow_path / swap-out page
> > 	 __zram_bvec_write(page)
> > 	  compressed_page = zcomp_compress(page)
> > 	   zs_malloc(compressed_page)
> > 	    // no available zspage found, need to allocate new
> > 	     alloc_zspage()
> > 	     {
> > 		for (i = 0; i < class->pages_per_zspage; i++)
> > 		    page = alloc_page(gfp);
> > 		    if (!page)
> > 			    return NULL
> > 	     }
> > 
> > 	 return -ENOMEM
> > 	...
> > 	printk("Write-error on swap-device...");
> > 
> > 
> > zspage-s can consist of up to ->pages_per_zspage normal pages.
> > if alloc_page() fails then we can't allocate the entire zspage,
> > so we can't store the swapped out page, so it remains in ram
> > and we don't make any progress. so we try to swap another page
> > and may be do the whole zs_malloc()->alloc_zspage() again, may
> > be not. depending on how bad the OOM situation is there can be
> > few or many "Write-error on swap-device" errors.
> > 
> > > And again, I do not think the throttling is an appropriate counter
> > > measure. We do want to print those messages when a critical situation
> > > happens. If we have a fallback then simply do not print at all.
> > 
> > sure, but with the ratelimited printk we still print those messages.
> > we just don't print it for every single page we failed to write
> > to the device. the existing error messages can (*sometimes*) be noisy
> > and not very informative - "Write-error on swap-device (%u:%u:%llu)\n";
> > it's not like 1000 of those tell more than 1 or 10.
> 
> Michal, does that make sense? with the updated/reworked commit
> message will the patch be good enough?

I am sorry but I didn't get to look into this yet. I still _believe_
that the ratelimit is just papering over a real problem here. So I would
prefer if the real fix was done instead. Maybe that is not that easy
easy, I haven't checked. Maybe I just do not understand the issue here.
-- 
Michal Hocko
SUSE Labs