linux-kernel - Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150403185039.GB38424@linux.vnet.ibm.com>
Date:	Fri, 3 Apr 2015 11:50:39 -0700
From:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	anton@...bar.org, linuxppc-dev@...ts.ozlabs.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Dan Streetman <ddstreet@...e.org>
Subject: Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc
 reserves if node has no reclaimable pages

On 03.04.2015 [20:24:45 +0200], Michal Hocko wrote:
> On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote:
> > On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
> [...]
> > > I would expect kswapd would be looping endlessly because the zone
> > > wouldn't be balanced obviously. But I would be wrong... because
> > > pgdat_balanced is doing this:
> > > 		/*
> > > 		 * A special case here:
> > > 		 *
> > > 		 * balance_pgdat() skips over all_unreclaimable after
> > > 		 * DEF_PRIORITY. Effectively, it considers them balanced so
> > > 		 * they must be considered balanced here as well!
> > > 		 */
> > > 		if (!zone_reclaimable(zone)) {
> > > 			balanced_pages += zone->managed_pages;
> > > 			continue;
> > > 		}
> > > 
> > > and zone_reclaimable is false for you as you didn't have any
> > > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > > zones right?) and so the kswapd would be woken up easily. So it looks
> > > like a mess.
> > 
> > My understanding, and I could easily be wrong, is that kswapd2 (node 2
> > is the exhausted one) spins endlessly, because the reclaim logic sees
> > that we are reclaiming from somewhere but the allocation request for
> > node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
> > never complete, so we just continue to reclaim.
> 
> __GFP_THISNODE would be waking up kswapd2 again and again, that is true.

Right, one idea I had for this was ensuring that we perform reclaim with
somehow some knowledge of __GFP_THISNODE -- that is it needs to be
somewhat targetted in order to actually help satisfy the current
allocation. But it got pretty hairy fast and I didn't want to break the
world :)

> I am just wondering whether we will have any __GFP_THISNODE allocations
> for a node without CPUs (numa_node_id() shouldn't return such a node
> AFAICS). Maybe if somebody is bound to Node2 explicitly but I would
> consider this as a misconfiguration.

Right, I'd need to check what happens if in our setup you taskset to
node2 and tried to force memory to be local -- I think you'd either be
killed immediately, or the kernel will just disagree with your binding
since it's invalid (e.g., that will happen if you try to bind to a
memoryless node, I think).

Keep in mind that although in my config node2 had no CPUs, that's not a
hard & fast requirement. I do believe in a previous iteration of this
bug, the exhausted node had no free memory but did have cpus assigned to
it.

-Nish

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