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SubjectRe: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone
On Tue, Jul 05, 2016 at 03:11:17PM +0900, Minchan Kim wrote:
> > - if (i < 0)
> > - goto out;
> > + /*
> > + * Only reclaim if there are no eligible zones. Check from
> > + * high to low zone to avoid prematurely clearing pgdat
> > + * congested state.
>
> I cannot understand "prematurely clearing pgdat congested state".
> Could you add more words to clear it out?
>

It's surprisingly difficult to concisely explain. Is this any better?

/*
* Only reclaim if there are no eligible zones. Check from
* high to low zone as allocations prefer higher zones.
* Scanning from low to high zone would allow congestion to be
* cleared during a very small window when a small low
* zone was balanced even under extreme pressure when the
* overall node may be congested.
*/
> > + */
> > + for (i = classzone_idx; i >= 0; i--) {
> > + zone = pgdat->node_zones + i;
> > + if (!populated_zone(zone))
> > + continue;
> > +
> > + if (zone_balanced(zone, sc.order, classzone_idx))
>
> If buffer_head is over limit, old logic force to reclaim highmem but
> this zone_balanced logic will prevent it.
>

The old logic was always busted on 64-bit because is_highmem would always
be 0. The original intent appears to be that buffer_heads_over_limit
would release the buffers when pages went inactive. There are a number
of things we treated inconsistently that get fixed up in the series and
buffer_heads_over_limit is one of them.

--
Mel Gorman
SUSE Labs

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