Messages in this thread | | | Date | Mon, 16 Oct 2006 23:36:02 -0700 | From | "Martin J. Bligh" <> | Subject | Re: [PATCH] Fix bug in try_to_free_pages and balance_pgdat when they fail to reclaim pages |
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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.
>> 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.
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?
Moreover, whilst try_to_free_pages calls shrink_zones, balance_pgdat does not. Nothing else I can see sets temp_priority.
> I don't agree the patch is correct.
You think it's doing something wrong? Or just unnecessary?
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.
> 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.
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.
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