Re: Possible deadloop in direct reclaim?

[Bob Liu <lliubbo@gmail.com>]

Hi Kosaki,

On Fri, Jul 26, 2013 at 2:14 AM, KOSAKI Motohiro
<kosaki.motohiro@gmail.com> wrote:
>> How about replace the checking in kswapd_shrink_zone()?
>>
>> @@ -2824,7 +2824,7 @@ static bool kswapd_shrink_zone(struct zone *zone,
>>         /* Account for the number of pages attempted to reclaim */
>>         *nr_attempted += sc->nr_to_reclaim;
>>
>> -       if (nr_slab == 0 && !zone_reclaimable(zone))
>> +       if (sc->nr_reclaimed == 0 && !zone_reclaimable(zone))
>>                 zone->all_unreclaimable = 1;
>>
>>         zone_clear_flag(zone, ZONE_WRITEBACK);
>>
>>
>> I think the current check is wrong, reclaimed a slab doesn't mean
>> reclaimed a page.
>
> The code is correct, at least, it works as intentional. page reclaim
> status is checked by zone_reclaimable() and slab shrinking status is
> checked by nr_slab.

I'm afraid in some special cases, nr_slab = 1 or any small number
which means we reclaimed some slab objects.
Then we don't set zone->all_unreclaimeabled =1.

But even though we reclaimed some slab objects, there may be no pages freed.
Because one page may contain several objects.

If we reclaimed some slab objects but without actual pages, we need to
set zone->all_unreclaimeabled=1!
So I think we should check sc->nr_reclaimed == 0 instead of nr_slab == 0.

-- 
Regards,
--Bob

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