Re: [rfc] [patch] mm: zone_reclaim fix for pseudo file systems

Ravikiran G Thirumalai
2007-07-31T01:36:49Z

On Mon, Jul 30, 2007 at 05:20:07PM -0700, Andrew Morton wrote:
>On Mon, 30 Jul 2007 17:01:38 -0700
>Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
>> >The (cheesy) way in which reclaim currently handles this sort of thing is
>> >to scan like mad, then to eventually set zone->all_unreclaimable. Once
>> >that has been set, the kernel will reduce the amount of scanning effort it
>> >puts into that zone by a very large amount. If the zone later comes back
>> >to life, all_unreclaimable gets cleared and things proceed as normal.
>>
>> I see. But this obviously does not work in this case. I have noticed the
>> process getting into 'system' and staying there for hours. I have never
>> noticed the app complete. Perhaps because I did not wait long enough.
>> So do you think a more aggressive auto setting/unsetting of 'all_unreclaimable'
>> is a better approach?
>
>The problem is that __zone_reclaim() doesn't use all_unreclaimable at all.
>You'll note that all the other callers of shrink_zone() do take avoiding
>action if the zone is in all_unreclaimable state, but __zone_reclaim() forgot
>to.

Ummm... zone_reclaim does look at all_unreclaimable:

int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
...
...
/*
* Avoid concurrent zone reclaims, do not reclaim in a zone that
* does
* not have reclaimable pages and if we should not delay the
* allocation
* then do not scan.
*/
if (!(gfp_mask & __GFP_WAIT) ||
zone->all_unreclaimable ||
atomic_read(&zone->reclaim_in_progress) > 0 ||
(current->flags & PF_MEMALLOC))
return 0;

I guess it is not being set correctly for unreclaimable (pseudo fs) pages.