Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from directreclaim path completely

[Minchan Kim]

On Thu, Mar 24, 2011 at 3:16 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> Hi
>
>> Thanks for your effort, Kosaki.
>> But I still doubt this patch is good.
>>
>> This patch makes early oom killing in hibernation as it skip
>> all_unreclaimable check.
>> Normally, Âhibernation needs many memory so page_reclaim pressure
>> would be big in small memory system. So I don't like early give up.
>
> Wait. When occur big pressure? hibernation reclaim pressure
> (sc->nr_to_recliam) depend on physical memory size. therefore
> a pressure seems to don't depend on the size.

It depends on physical memory size and /sys/power/image_size.
If you want to tune image size bigger, reclaim pressure would be big.

>
>
>> Do you think my patch has a problem? Personally, I think it's very
>> simple and clear. :)
>
> To be honest, I dislike following parts. It's madness on madness.
>
> Â Â Â Âstatic bool zone_reclaimable(struct zone *zone)
> Â Â Â Â{
> Â Â Â Â Â Â Â Âif (zone->all_unreclaimable)
> Â Â Â Â Â Â Â Â Â Â Â Âreturn false;
>
> Â Â Â Â Â Â Â Âreturn zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
> Â Â Â Â}
>
>
> The function require a reviewer know
>
> Âo pages_scanned and all_unreclaimable are racy

Yes. That part should be written down of comment.

> Âo at hibernation, zone->all_unreclaimable can be false negative,
> Â but can't be false positive.

The comment of all_unreclaimable already does explain it well, I think.

>
> And, a function comment of all_unreclaimable() says
>
> Â Â Â Â /*
> Â Â Â Â Â* As hibernation is going on, kswapd is freezed so that it can't mark
> Â Â Â Â Â* the zone into all_unreclaimable. It can't handle OOM during hibernation.
> Â Â Â Â Â* So let's check zone's unreclaimable in direct reclaim as well as kswapd.
> Â Â Â Â Â*/
>
> But, now it is no longer copy of kswapd algorithm.

The comment don't say it should be a copy of kswapd.

>
> If you strongly prefer this idea even if you hear above explanation,
> please consider to add much and much comments. I can't say
> current your patch is enough readable/reviewable.

My patch isn't a formal patch for merge but just a concept to show.
If you agree the idea, of course, I will add more concrete comment
when I send formal patch.

Before, I would like to get a your agreement. :)
If you solve my concern(early give up in hibernation) in your patch, I
don't insist on my patch, either.

Thanks for the comment, Kosaki.

>
> Thanks.
>
>
>



-- 
Kind regards,
Minchan Kim
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