Re: OOM killer changes

[Ralf-Peter Rohbeck <Ralf-Peter.Rohbeck@quantum.com>]

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This time the OOM killer hit much quicker. No btrfs balance, just 
compiling the kernel with the new change did it.
Much smaller logs so I'm attaching them.

Ralf-Peter
On 15.08.2016 08:01, Michal Hocko wrote:
> On Mon 15-08-16 11:16:36, Vlastimil Babka wrote:
>> On 08/15/2016 06:48 AM, Ralf-Peter Rohbeck wrote:
>>> On 02.08.2016 12:25, Ralf-Peter Rohbeck wrote:
>>> Took me a little longer than expected due to work. The failure wouldn't
>>> happen for a while and so I started a couple of scripts and let them
>>> run. When I checked today the server didn't respond on the network and
>>> sure enough it had killed everything. This is with 4.7.0 with the config
>>> based on Debian 4.7-rc7.
>>>
>>> trace_pipe got a little big (5GB) so I uploaded the logs to
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__filebin.net_box0wycfouvhl6sr_OOM-5F4.7.0.tar.bz2&d=DQIBAg&c=8S5idjlO_n28Ko3lg6lskTMwneSC-WqZ5EBTEEvDlkg&r=yGQdEpZknbtYvR0TyhkCGu-ifLklIvXIf740poRFltQ&m=TBVC4CIIUzJlmpDNapp31jIbz3Gy1M-aQ9jhrv0U56I&s=ozhbhqcuwlWiU1Cd8PZGl5-CC69-m-sNUitSYI2ry1Y&e= . before_btrfs is
>>> before the btrfs filesystems were mounted.
>>> I did run a btrfs balance because it creates IO load and I needed to
>>> balance anyway. Maybe that's what caused it?
>> pgmigrate_success        46738962
>> pgmigrate_fail          135649772
>> compact_migrate_scanned 309726659
>> compact_free_scanned   9715615169
>> compact_isolated        229689596
>> compact_stall 4777
>> compact_fail 3068
>> compact_success 1709
>> compact_daemon_wake 207834
>>
>> The migration failures are quite enormous. Very quick analysis of the
>> trace seems to confirm that these are mostly "real", as opposed to result
>> of failure to isolate free pages for migration targets, although the free
>> scanner spent a lot of time:
>>
>>> grep "nr_failed=32" -B1 trace_pipe.log | grep isolate_freepages.*nr_taken=0 | wc -l
>> 3246
>>
>> So is it one of the cases where fs is unable to migrate dirty/writeback pages?
> It smells that way. Now we should find out why and what can we do about
> that. I suspect that try_to_release_page is not able to release the page
> for migration. Btrfs doesn't seem to have migratepage for page cache
> pages so it should go via fallback_migrate_page.
>
> The following diff should tell us whether this is really the case. Just
> open trace_pipe and see whether this path really triggered.
> ---
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 72c09dea6526..120e2e5fcbea 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -729,8 +729,10 @@ static int fallback_migrate_page(struct address_space *mapping,
>   	 * We must have no buffers or drop them.
>   	 */
>   	if (page_has_private(page) &&
> -	    !try_to_release_page(page, GFP_KERNEL))
> +	    !try_to_release_page(page, GFP_KERNEL)) {
> +		trace_printk("try_to_release_page failed for a_ops:%pS\n", page->a_ops);
>   		return -EAGAIN;
> +	}
>   
>   	return migrate_page(mapping, newpage, page, mode);
>   }


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