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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux