Re: Possible deadloop in direct reclaim?

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(7/25/13 9:22 PM), Bob Liu wrote:
> 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.

Right. This is a limitation of current slab shrinker's implementation.
We are welcome you contribution this area.


> 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.

sc->nr_reclaimed doesn't check how much pages freed from slab.




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