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SubjectRe: VM in v2.4.0test9
On Wed, 4 Oct 2000, Roger Larsson wrote:
> Rik van Riel wrote:
> > On Wed, 4 Oct 2000, Rik van Riel wrote:
> >
> > > > > First, you have MORE free memory than freepages.high. In this
> > > > > case I really don't see why __alloc_pages() wouldn't give the
> > > > > memory to your processes ....
> > > >
> > > > Hmm...
> > > > Can't it be a zone problem?
> > > > Free pages is the total free - all zones.
> > > > But suppose you want a page from a specific zone - DMA, the more
> > > > memory you have the less likely that you have a DMA page free...
> > > > Does all test take this into consideration?
> > >
> > > Free_shortage() /should/ take this into consideration, and unless
> > > I'm wrong, it does ;)
> >
> > Also, a zone problem CANNOT cause the problem in
> > David's 16MB test ...
> >
> > (this is getting stranger and stranger)
>
> Now I know from where the 125 pages limit comes from.
> static int zone_balance_ratio[MAX_NR_ZONES] = { 32, 128, 128, };
> 16M/4k/32 = 125
>
> Probably there is a mismatch between zone->free_pages and
> free_pages.{min,low,high}

Arghhhhhhhhhhhhhhhhhhhhhhhhh....

The potential for this bug has been around since 2.3.51, when
different balance_ratios for different zones became possible.

The bug hasn't bitten us yet since then because 1) the balance
ratio for ZONE_DMA wasn't changed until some time later and
2) we didn't use freepages.{min,low,high}.

Now that we /are/ using the values in the freepages array again,
we're running into the very big problem that freepages.high has
a value LOWER than zone->pages_min for the DMA zone, on 16MB
machines. This caused David's system to behave the way it did.

Does the attached patch fix the problem?

regards,

Rik
--
"What you're running that piece of shit Gnome?!?!"
-- Miguel de Icaza, UKUUG 2000

http://www.conectiva.com/ http://www.surriel.com/


--- mm/page_alloc.c.orig Tue Oct 3 10:20:41 2000
+++ mm/page_alloc.c Wed Oct 4 17:01:52 2000
@@ -795,21 +795,6 @@

printk("On node %d totalpages: %lu\n", nid, realtotalpages);

- /*
- * Select nr of pages we try to keep free for important stuff
- * with a minimum of 10 pages and a maximum of 256 pages, so
- * that we don't waste too much memory on large systems.
- * This is fairly arbitrary, but based on some behaviour
- * analysis.
- */
- i = realtotalpages >> 7;
- if (i < 10)
- i = 10;
- if (i > 256)
- i = 256;
- freepages.min += i;
- freepages.low += i * 2;
- freepages.high += i * 3;
memlist_init(&active_list);
memlist_init(&inactive_dirty_list);

@@ -875,6 +860,20 @@
zone->pages_min = mask;
zone->pages_low = mask*2;
zone->pages_high = mask*3;
+ /*
+ * Add these free targets to the global free target;
+ * we have to be SURE that freepages.high is higher
+ * than SUM [zone->pages_min] for all zones, otherwise
+ * we may have bad bad problems.
+ *
+ * This means we cannot make the freepages array writable
+ * in /proc, but have to add a separate extra_free_target
+ * for people who require it to catch load spikes in eg.
+ * gigabit ethernet routing...
+ */
+ freepages.min += mask;
+ freepages.low += mask*2;
+ freepages.high += mask*3;
zone->zone_mem_map = mem_map + offset;
zone->zone_start_mapnr = offset;
zone->zone_start_paddr = zone_start_paddr;
-
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