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

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

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.

M.
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