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    SubjectRe: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely
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    > On Thu, Mar 24, 2011 at 3:16 PM, KOSAKI Motohiro
    > <kosaki.motohiro@jp.fujitsu.com> 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,  hibernation 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.

    Ok, _If_ I want.
    However, I haven't seen desktop people customize it.


    > >> 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.
    > >
    > >        static bool zone_reclaimable(struct zone *zone)
    > >        {
    > >                if (zone->all_unreclaimable)
    > >                        return false;
    > >
    > >                return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
    > >        }
    > >
    > >
    > > The function require a reviewer know
    > >
    > >  o pages_scanned and all_unreclaimable are racy
    >
    > Yes. That part should be written down of comment.
    >
    > >  o at hibernation, zone->all_unreclaimable can be false negative,
    > >   but can't be false positive.
    >
    > The comment of all_unreclaimable already does explain it well, I think.

    Where is?


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

    I meant the comments says

             * So let's check zone's unreclaimable in direct reclaim as well as kswapd.

    but now it isn't aswell as kswapd.

    I think it's critical important. If people can't understand why the
    algorithm was choosed, anyone will break the code again sooner or later.


    > > 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.

    Ok. Let's try.

    Please concern why priority=0 is not enough. zone_reclaimable_pages(zone) * 6
    is a conservative value of worry about multi thread race. While one task
    is reclaiming, others can allocate/free memory concurrently. therefore,
    even after priority=0, we have a chance getting reclaimable pages on lru.
    But, in hibernation case, almost all tasks was freezed before hibernation
    call shrink_all_memory(). therefore, there is no race. priority=0 reclaim
    can cover all lru pages.

    Is this enough explanation for you?


    >
    > Thanks for the comment, Kosaki.




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