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SubjectRe: [PATCH] memcg: fix hotplugged memory zone oops
On Thu 01-11-12 18:28:02, Hugh Dickins wrote:
> When MEMCG is configured on (even when it's disabled by boot option),
> when adding or removing a page to/from its lru list, the zone pointer
> used for stats updates is nowadays taken from the struct lruvec.
> (On many configurations, calculating zone from page is slower.)
>
> But we have no code to update all the lruvecs (per zone, per memcg)
> when a memory node is hotadded. Here's an extract from the oops which
> results when running numactl to bind a program to a newly onlined node:
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000f60
> IP: [<ffffffff811870b9>] __mod_zone_page_state+0x9/0x60
> PGD 0
> Oops: 0000 [#1] SMP
> CPU 2
> Pid: 1219, comm: numactl Not tainted 3.6.0-rc5+ #180 Bochs Bochs
> Process numactl (pid: 1219, threadinfo ffff880039abc000, task ffff8800383c4ce0)
> Stack:
> ffff880039abdaf8 ffffffff8117390f ffff880039abdaf8 000000008167c601
> ffffffff81174162 ffff88003a480f00 0000000000000001 ffff8800395e0000
> ffff88003dbd0e80 0000000000000282 ffff880039abdb48 ffffffff81174181
> Call Trace:
> [<ffffffff8117390f>] __pagevec_lru_add_fn+0xdf/0x140
> [<ffffffff81174181>] pagevec_lru_move_fn+0xb1/0x100
> [<ffffffff811741ec>] __pagevec_lru_add+0x1c/0x30
> [<ffffffff81174383>] lru_add_drain_cpu+0xa3/0x130
> [<ffffffff8117443f>] lru_add_drain+0x2f/0x40
> ...
>
> The natural solution might be to use a memcg callback whenever memory
> is hotadded; but that solution has not been scoped out, and it happens
> that we do have an easy location at which to update lruvec->zone. The
> lruvec pointer is discovered either by mem_cgroup_zone_lruvec() or by
> mem_cgroup_page_lruvec(), and both of those do know the right zone.
>
> So check and set lruvec->zone in those; and remove the inadequate
> attempt to set lruvec->zone from lruvec_init(), which is called
> before NODE_DATA(node) has been allocated in such cases.
>
> Ah, there was one exceptionr. For no particularly good reason,
> mem_cgroup_force_empty_list() has its own code for deciding lruvec.
> Change it to use the standard mem_cgroup_zone_lruvec() and
> mem_cgroup_get_lru_size() too. In fact it was already safe against
> such an oops (the lru lists in danger could only be empty),
> but we're better proofed against future changes this way.
>
> Reported-by: Tang Chen <tangchen@cn.fujitsu.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
> Cc: Wen Congyang <wency@cn.fujitsu.com>
> Cc: stable@vger.kernel.org

OK, it adds "an overhead" also when there is no hotadd going on but this
is just one additional mem access&cmp&je so it shouldn't be noticable
(lruvec->zone is used most of the time later so it not a pointless
load).
It is also easier to backport for stable. But is there any reason to fix
it later properly in the hotadd hook?

Anyway
Acked-by: Michal Hocko <mhocko@suse.cz>

Thanks!
> ---
> I've marked this for stable (3.6) since we introduced the problem
> in 3.5 (now closed to stable); but I have no idea if this is the
> only fix needed to get memory hotadd working with memcg in 3.6,
> and received no answer when I enquired twice before.
>
> include/linux/mmzone.h | 2 -
> mm/memcontrol.c | 46 +++++++++++++++++++++++++++++----------
> mm/mmzone.c | 6 -----
> mm/page_alloc.c | 2 -
> 4 files changed, 38 insertions(+), 18 deletions(-)
>
> --- 3.7-rc3/include/linux/mmzone.h 2012-10-14 16:16:57.665308933 -0700
> +++ linux/include/linux/mmzone.h 2012-11-01 14:31:04.284185741 -0700
> @@ -752,7 +752,7 @@ extern int init_currently_empty_zone(str
> unsigned long size,
> enum memmap_context context);
>
> -extern void lruvec_init(struct lruvec *lruvec, struct zone *zone);
> +extern void lruvec_init(struct lruvec *lruvec);
>
> static inline struct zone *lruvec_zone(struct lruvec *lruvec)
> {
> --- 3.7-rc3/mm/memcontrol.c 2012-10-14 16:16:58.341309118 -0700
> +++ linux/mm/memcontrol.c 2012-11-01 14:31:04.284185741 -0700
> @@ -1055,12 +1055,24 @@ struct lruvec *mem_cgroup_zone_lruvec(st
> struct mem_cgroup *memcg)
> {
> struct mem_cgroup_per_zone *mz;
> + struct lruvec *lruvec;
>
> - if (mem_cgroup_disabled())
> - return &zone->lruvec;
> + if (mem_cgroup_disabled()) {
> + lruvec = &zone->lruvec;
> + goto out;
> + }
>
> mz = mem_cgroup_zoneinfo(memcg, zone_to_nid(zone), zone_idx(zone));
> - return &mz->lruvec;
> + lruvec = &mz->lruvec;
> +out:
> + /*
> + * Since a node can be onlined after the mem_cgroup was created,
> + * we have to be prepared to initialize lruvec->zone here;
> + * and if offlined then reonlined, we need to reinitialize it.
> + */
> + if (unlikely(lruvec->zone != zone))
> + lruvec->zone = zone;
> + return lruvec;
> }
>
> /*
> @@ -1087,9 +1099,12 @@ struct lruvec *mem_cgroup_page_lruvec(st
> struct mem_cgroup_per_zone *mz;
> struct mem_cgroup *memcg;
> struct page_cgroup *pc;
> + struct lruvec *lruvec;
>
> - if (mem_cgroup_disabled())
> - return &zone->lruvec;
> + if (mem_cgroup_disabled()) {
> + lruvec = &zone->lruvec;
> + goto out;
> + }
>
> pc = lookup_page_cgroup(page);
> memcg = pc->mem_cgroup;
> @@ -1107,7 +1122,16 @@ struct lruvec *mem_cgroup_page_lruvec(st
> pc->mem_cgroup = memcg = root_mem_cgroup;
>
> mz = page_cgroup_zoneinfo(memcg, page);
> - return &mz->lruvec;
> + lruvec = &mz->lruvec;
> +out:
> + /*
> + * Since a node can be onlined after the mem_cgroup was created,
> + * we have to be prepared to initialize lruvec->zone here;
> + * and if offlined then reonlined, we need to reinitialize it.
> + */
> + if (unlikely(lruvec->zone != zone))
> + lruvec->zone = zone;
> + return lruvec;
> }
>
> /**
> @@ -3688,17 +3712,17 @@ unsigned long mem_cgroup_soft_limit_recl
> static bool mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
> int node, int zid, enum lru_list lru)
> {
> - struct mem_cgroup_per_zone *mz;
> + struct lruvec *lruvec;
> unsigned long flags, loop;
> struct list_head *list;
> struct page *busy;
> struct zone *zone;
>
> zone = &NODE_DATA(node)->node_zones[zid];
> - mz = mem_cgroup_zoneinfo(memcg, node, zid);
> - list = &mz->lruvec.lists[lru];
> + lruvec = mem_cgroup_zone_lruvec(zone, memcg);
> + list = &lruvec->lists[lru];
>
> - loop = mz->lru_size[lru];
> + loop = mem_cgroup_get_lru_size(lruvec, lru);
> /* give some margin against EBUSY etc...*/
> loop += 256;
> busy = NULL;
> @@ -4736,7 +4760,7 @@ static int alloc_mem_cgroup_per_zone_inf
>
> for (zone = 0; zone < MAX_NR_ZONES; zone++) {
> mz = &pn->zoneinfo[zone];
> - lruvec_init(&mz->lruvec, &NODE_DATA(node)->node_zones[zone]);
> + lruvec_init(&mz->lruvec);
> mz->usage_in_excess = 0;
> mz->on_tree = false;
> mz->memcg = memcg;
> --- 3.7-rc3/mm/mmzone.c 2012-09-30 16:47:46.000000000 -0700
> +++ linux/mm/mmzone.c 2012-11-01 14:31:04.284185741 -0700
> @@ -87,7 +87,7 @@ int memmap_valid_within(unsigned long pf
> }
> #endif /* CONFIG_ARCH_HAS_HOLES_MEMORYMODEL */
>
> -void lruvec_init(struct lruvec *lruvec, struct zone *zone)
> +void lruvec_init(struct lruvec *lruvec)
> {
> enum lru_list lru;
>
> @@ -95,8 +95,4 @@ void lruvec_init(struct lruvec *lruvec,
>
> for_each_lru(lru)
> INIT_LIST_HEAD(&lruvec->lists[lru]);
> -
> -#ifdef CONFIG_MEMCG
> - lruvec->zone = zone;
> -#endif
> }
> --- 3.7-rc3/mm/page_alloc.c 2012-10-28 13:48:00.021774166 -0700
> +++ linux/mm/page_alloc.c 2012-11-01 14:31:04.284185741 -0700
> @@ -4505,7 +4505,7 @@ static void __paginginit free_area_init_
> zone->zone_pgdat = pgdat;
>
> zone_pcp_init(zone);
> - lruvec_init(&zone->lruvec, zone);
> + lruvec_init(&zone->lruvec);
> if (!size)
> continue;
>

--
Michal Hocko
SUSE Labs


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