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

[Ravikiran G Thirumalai <kiran@scalex86.org>]

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.

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