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

[Nick Piggin]

Martin J. Bligh wrote:

> Nick Piggin wrote:
>
> > Martin Bligh wrote:
> >
> > > The same bug is contained in both try_to_free_pages and balance_pgdat.
> > > On reclaiming the requisite number of pages we correctly set
> > > prev_priority back to DEF_PRIORITY.
> >
> >
> >
> > AFAIKS, we set prev_priority to the priority at which the zone was
> > deemed to require no more reclaiming, not DEF_PRIORITY.
>
>
> 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.

> > > However, we ALSO do this even
> > > if we loop over all priorities and fail to reclaim.
> >
> >
> >
> > 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.

> 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.

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


balance_pgdat.

> > I don't agree the patch is correct.
>
> You think it's doing something wrong? Or just unnecessary?


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

> 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.

> > We saw problems with this before releasing SLES10 too. See
> > zone_is_near_oom and other changesets from around that era. I would
> > like to know what workload was prevented from going OOM with these
> > changes, but zone_is_near_oom didn't help -- it must have been very
> > marginal (or there may indeed be a bug somewhere).
>
>
> Google production workload. Multiple reclaimers operating - one is
> down to priority 0 on the reclaim, but distress is still set to 0,
> thanks to prev_priority being borked. Hence we don't reclaim mapped
> pages, the reclaim fails, OOM killer kicks in.


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.

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

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