Re: [PATCH] Fix bug in try_to_free_pages and balance_pgdat when they fail to reclaim pages

["Martin J. Bligh" <mbligh@google.com>]

> > Well, it's zone->temp_priority, which was set to DEF_PRIORITY at the
> > top of the function, though I suppose something else might have
> > changed it since.
>
> Yes.

...

> > > If that happens, shouldn't prev_priority be set to 0?
> >
> > Yes, but it's not. We fall off the bottom of the loop, and set it
> > back to temp_priority. At best, the code is unclear.
>
> But temp_priority should be set to 0 at that point.

It that were true, it'd be great. But how?
This is everything that touches it:

0 mmzone.h     <global>             208 int temp_priority;
1 page_alloc.c free_area_init_core 2019 zone->temp_priority =
                                        zone->prev_priority = DEF_PRIORITY;
2 vmscan.c     shrink_zones         937 zone->temp_priority = priority;
3 vmscan.c     try_to_free_pages    987 zone->temp_priority = DEF_PRIORITY;
4 vmscan.c     try_to_free_pages   1031 zone->prev_priority =
                                        zone->temp_priority;
5 vmscan.c     balance_pgdat       1081 zone->temp_priority = DEF_PRIORITY;
6 vmscan.c     balance_pgdat       1143 zone->temp_priority = priority;
7 vmscan.c     balance_pgdat       1189 zone->prev_priority =
                                        zone->temp_priority;
8 vmstat.c     zoneinfo_show        593 zone->temp_priority,

Only thing that looks interesting here is shrink_zones.

> > I suppose shrink_zones() might in theory knock temp_priority down
> > as it goes, so it might come out right. But given that it's a global
> > (per zone), not per-reclaimer, I fail to see how that's really safe.
> > Supposing someone else has just started reclaim, and is still at
> > prio 12?
>
> But your loops are not exactly per reclaimer either. Granted there
> is a large race window in the current code, but this patch isn't the
> way to fix that particular problem.

Why not? Perhaps it's not a panacea, but it's a definite improvement.

> > Moreover, whilst try_to_free_pages calls shrink_zones, balance_pgdat
> > does not. Nothing else I can see sets temp_priority.
>
> balance_pgdat.

That's only called from kswapd. If we're in balance_pgdat, we ARE 
kswapd. We can't fix ourself. So effectively we're doing:

while (priority--) {
	if (we reclaimed OK)
		goto out;
}
out:
prev_priority = DEF_PRIORITY;

We've just walked the whole bloody list with priority set to 0.

We failed to reclaim a few pages.

We know the world is in deep pain.

Why the hell would we elevate prev_priority?

> Unnecesary and indicates something else is broken if you are seeing
> problems here.

You think we should set prev_priority up, when we've just walked the
whole list at prio 0 and can't reclaim anything? Unless so, I fail
to see how the patch is unnecessary.

And yes, I'm sure other things are broken, but again, this fixes a
clear bug.

> > I'm inclined to think the whole concept of temp_priority and
> > prev_priority are pretty broken. This may not fix the whole thing,
> > but it seems to me to make it better than it was before.
>
> I think it is broken too. I liked my split active lists, but at that point
> vmscan.c was in don't-touch mode.

I'm glad we agree it's broken. Whilst we await the 100th rewrite of the
VM, perhaps we can apply this simple fix?

> OK, so it sounds like temp_priority is being overwritten by the
> race. I'd consider throwing out temp_priority completely, and just
> going with adjusting prev_priority as we go.

I'm fine with that. Whole thing is racy as hell and pretty pointless
anyway. I'll make another patch up today.

> > Forward ported from an earlier version of 2.6 ... but I don't see
> > why we need extra heuristics here, it seems like a clear and fairly
> > simple bug. We're in deep crap with reclaim, and we go set the
> > global indicator back to "oh no, everything's fine". Not a good plan.
>
> All that reclaim_mapped code is pretty arbitrary anyway. What is needed
> is the zone_is_near_oom so we can decouple all those heuristics from
> the OOM decision. 

It seems what is needed is that we start to actually reclaim pages when
priority gets low. This is a very simple way of improving that.

> So do you still see the problem on upstream kernel
> without your patches applied?

I can't slap an upstream bleeding edge kernel across a few thousand
production machines, and wait to see if the world blows up, sorry.
If I can make a reproduce test case, I'll send it out, but thus far
we've been unsuccessful.

But I can see it happening in earlier versions, and I can read the
code in 2.6.18, and see obvious bugs.

M.
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