linux-kernel - Re: mmotm 2015-01-22-15-04: qemu failure due to 'mm: memcontrol: remove unnecessary soft limit tree node test'

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Message-ID: <20150123173659.GB12036@phnom.home.cmpxchg.org>
Date:	Fri, 23 Jan 2015 12:36:59 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Christoph Lameter <cl@...ux.com>, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-next@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>, mhocko@...e.cz
Subject: Re: mmotm 2015-01-22-15-04: qemu failure due to 'mm: memcontrol:
 remove unnecessary soft limit tree node test'

On Fri, Jan 23, 2015 at 08:59:51AM -0800, Guenter Roeck wrote:
> On 01/23/2015 08:02 AM, Johannes Weiner wrote:
> >On Fri, Jan 23, 2015 at 09:17:44AM -0600, Christoph Lameter wrote:
> >>On Fri, 23 Jan 2015, Johannes Weiner wrote:
> >>
> >>>Is the assumption of this patch wrong?  Does the specified node have
> >>>to be online for the fallback to work?
> >>
> >>Nodes that are offline have no control structures allocated and thus
> >>allocations will likely segfault when the address of the controls
> >>structure for the node is accessed.
> >>
> >>If we wanted to prevent that then every allocation would have to add a
> >>check to see if the nodes are online which would impact performance.
> >
> >Okay, that makes sense, thank you.
> >
> >Andrew, can you please drop this patch?
> >
> Problem is that there are three patches.
> 
> 2537ffb mm: memcontrol: consolidate swap controller code
> 2f9b346 mm: memcontrol: consolidate memory controller initialization
> a40d0d2 mm: memcontrol: remove unnecessary soft limit tree node test
> 
> Reverting (or dropping) a40d0d2 alone is not possible since it modifies
> mem_cgroup_soft_limit_tree_init which is removed by 2f9b346.

("mm: memcontrol: consolidate swap controller code") gave me no issues
when rebasing, but ("mm: memcontrol: consolidate memory controller
initialization") needs updating.

So how about this one to replace ("mm: memcontrol: remove unnecessary
soft limit tree node test"):

---
From: Johannes Weiner <hannes@...xchg.org>
Subject: [patch] mm: memcontrol: simplify soft limit tree init code

- No need to test the node for N_MEMORY.  node_online() is enough for
  node fallback to work in slab, use NUMA_NO_NODE for everything else.

- Remove the BUG_ON() for allocation failure.  A NULL pointer crash is
  just as descriptive, and the absent return value check is obvious.

- Move local variables to the inner-most blocks.

- Point to the tree structure after its initialized, not before, it's
  just more logical that way.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/memcontrol.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fb9788af4a3e..88c67303d141 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4537,24 +4537,23 @@ EXPORT_SYMBOL(parent_mem_cgroup);
 
 static void __init mem_cgroup_soft_limit_tree_init(void)
 {
-	struct mem_cgroup_tree_per_node *rtpn;
-	struct mem_cgroup_tree_per_zone *rtpz;
-	int tmp, node, zone;
+	int node;
 
 	for_each_node(node) {
-		tmp = node;
-		if (!node_state(node, N_NORMAL_MEMORY))
-			tmp = -1;
-		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL, tmp);
-		BUG_ON(!rtpn);
+		struct mem_cgroup_tree_per_node *rtpn;
+		int zone;
 
-		soft_limit_tree.rb_tree_per_node[node] = rtpn;
+		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
+				    node_online(node) ? node : NUMA_NO_NODE);
 
 		for (zone = 0; zone < MAX_NR_ZONES; zone++) {
+			struct mem_cgroup_tree_per_zone *rtpz;
+
 			rtpz = &rtpn->rb_tree_per_zone[zone];
 			rtpz->rb_root = RB_ROOT;
 			spin_lock_init(&rtpz->lock);
 		}
+		soft_limit_tree.rb_tree_per_node[node] = rtpn;
 	}
 }
 
-- 
2.2.2
--
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