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    SubjectRe: [RFC PATCH 2/2] Don't continue reclaim if the system have plenty free memory
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    On Thu, Jul 9, 2009 at 2:08 PM, KOSAKI
    Motohiro<kosaki.motohiro@jp.fujitsu.com> wrote:
    >> Hi, Kosaki.
    >>
    >> On Tue, Jul 7, 2009 at 6:48 PM, KOSAKI
    >> Motohiro<kosaki.motohiro@jp.fujitsu.com> wrote:
    >> > Subject: [PATCH] Don't continue reclaim if the system have plenty free memory
    >> >
    >> > On concurrent reclaim situation, if one reclaimer makes OOM, maybe other
    >> > reclaimer can stop reclaim because OOM killer makes enough free memory.
    >> >
    >> > But current kernel doesn't have its logic. Then, we can face following accidental
    >> > 2nd OOM scenario.
    >> >
    >> > 1. System memory is used by only one big process.
    >> > 2. memory shortage occur and concurrent reclaim start.
    >> > 3. One reclaimer makes OOM and OOM killer kill above big process.
    >> > 4. Almost reclaimable page will be freed.
    >> > 5. Another reclaimer can't find any reclaimable page because those pages are
    >> > ? already freed.
    >> > 6. Then, system makes accidental and unnecessary 2nd OOM killer.
    >> >
    >>
    >> Did you see the this situation ?
    >> Why I ask is that we have already a routine for preventing parallel
    >> OOM killing in __alloc_pages_may_oom.
    >>
    >> Couldn't it protect your scenario ?
    >
    > Can you please see actual code of this patch?

    I mean follow as,

    static inline struct page *
    __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
    struct zonelist *zonelist, enum zone_type high_zoneidx,
    ...
    <snip>

    /*
    * Go through the zonelist yet one more time, keep very high watermark
    * here, this is only to catch a parallel oom killing, we must fail if
    * we're still under heavy pressure.
    */
    page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask,
    order, zonelist, high_zoneidx,
    ALLOC_WMARK_HIGH|ALLOC_CPUSET,
    preferred_zone, migratetype);


    > Those two patches fix different problem.
    >
    > 1/2 fixes the issue of that concurrent direct reclaimer makes
    > too many isolated pages.
    > 2/2 fixes the issue of that reclaim and exit race makes accidental oom.
    >
    >
    >> If it can't, Could you explain the scenario in more detail ?
    >
    > __alloc_pages_may_oom() check don't effect the threads of already
    > entered reclaim. it's obvious.

    Threads which are entered into direct reclaim mode will call
    __alloc_pages_may_oom before out_of_memory.
    At that time, if one big process is killed a while ago,
    get_page_from_freelist in __alloc_pages_may_oom will be succeeded at
    last. So I think it doesn't occur OOM.

    But in that case, we suffered from unnecessary page scanning per each
    priority(12~0). So in this case, your patch is good to me. then you
    would be better to change log. :)

    --
    Kind regards,
    Minchan Kim


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