Re: Deadlock possibly caused by too_many_isolated.

[Minchan Kim]

On Tue, Oct 19, 2010 at 12:13 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>> On Tue, Oct 19, 2010 at 12:05 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> > On Tue, Oct 19, 2010 at 10:52:47AM +0800, Minchan Kim wrote:
>> >> Hi Wu,
>> >>
>> >> On Tue, Oct 19, 2010 at 11:35 AM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> >> >> @@ -2054,10 +2069,11 @@ rebalance:
>> >> >>                 goto got_pg;
>> >> >>
>> >> >>         /*
>> >> >> -        * If we failed to make any progress reclaiming, then we are
>> >> >> -        * running out of options and have to consider going OOM
>> >> >> +        * If we failed to make any progress reclaiming and there aren't
>> >> >> +        * many parallel reclaiming, then we are unning out of options and
>> >> >> +        * have to consider going OOM
>> >> >>          */
>> >> >> -       if (!did_some_progress) {
>> >> >> +       if (!did_some_progress && !too_many_isolated_zone(preferred_zone)) {
>> >> >>                 if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY)) {
>> >> >>                         if (oom_killer_disabled)
>> >> >>                                 goto nopage;
>> >> >
>> >> > This is simply wrong.
>> >> >
>> >> > It disabled this block for 99% system because there won't be enough
>> >> > tasks to make (!too_many_isolated_zone == true). As a result the LRU
>> >> > will be scanned like mad and no task get OOMed when it should be.
>> >>
>> >> If !too_many_isolated_zone is false, it means there are already many
>> >> direct reclaiming tasks.
>> >> So they could exit reclaim path and !too_many_isolated_zone will be true.
>> >> What am I missing now?
>> >
>> > Ah sorry, my brain get short circuited.. but I still feel uneasy with
>> > this change. It's not fixing the root cause and won't prevent too many
>> > LRU pages be isolated. It's too late to test too_many_isolated_zone()
>> > after direct reclaim returns (after sleeping for a long time).
>> >
>>
>> Intend to agree.
>> I think root cause is a infinite looping in too_many_isolated holding FS lock.
>> Would it be simple that too_many_isolated would be bail out after some try?
>
> How?
> A lot of caller don't have good recover logic when memory allocation fail occur.
>

I means following as.

1. shrink_inactive_list
2. if too_many_isolated is looping than 5 times, it marks some
variable to notice this fail is concurrent reclaim and bail out
3. __alloc_pages_slowpath see that did_some_progress is zero and the
mark which show bailout by concurrent reclaim.
4. Instead of OOM, congestion_wait and rebalance.

While I implement it, I knew it makes code rather ugly and I thought
lost is bigger than gain.

Okay. I will drop this idea.

Thanks for advising me, Wu, KOSAKI.
-- 
Kind regards,
Minchan Kim
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