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SubjectRe: [PATCH] memory cgroup: update root memory cgroup when node is onlined
On Thu, 13 Sep 2012, Johannes Weiner wrote:
> On Thu, Sep 13, 2012 at 03:14:28PM +0800, Wen Congyang wrote:
> > root_mem_cgroup->info.nodeinfo is initialized when the system boots.
> > But NODE_DATA(nid) is null if the node is not onlined, so
> > root_mem_cgroup->info.nodeinfo[nid]->zoneinfo[zone].lruvec.zone contains
> > an invalid pointer. If we use numactl to bind a program to the node
> > after onlining the node and its memory, it will cause the kernel
> > panicked:
>
> Is there any chance we could get rid of the zone backpointer in lruvec
> again instead?

It could be done, but it would make me sad :(

> Adding new nodes is a rare event and so updating every
> single memcg in the system might be just borderline crazy.

Not horribly crazy, but rather ugly, yes.

> But can't
> we just go back to passing the zone along with the lruvec down
> vmscan.c paths? I agree it's ugly to pass both, given their
> relationship. But I don't think the backpointer is any cleaner but in
> addition less robust.

It's like how we use vma->mm: we could change everywhere to pass mm with
vma, but it looks cleaner and cuts down on long arglists to have mm in vma.
From past experience, one of the things I worried about was adding extra
args to the reclaim stack.

>
> That being said, the crashing code in particular makes me wonder:
>
> static __always_inline void add_page_to_lru_list(struct page *page,
> struct lruvec *lruvec, enum lru_list lru)
> {
> int nr_pages = hpage_nr_pages(page);
> mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
> list_add(&page->lru, &lruvec->lists[lru]);
> __mod_zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru, nr_pages);
> }
>
> Why did we ever pass zone in here and then felt the need to replace it
> with lruvec->zone in fa9add6 "mm/memcg: apply add/del_page to lruvec"?
> A page does not roam between zones, its zone is a static property that
> can be retrieved with page_zone().

Just as in vmscan.c, we have the lruvec to hand, and that's what we
mainly want to operate upon, but there is also some need for zone.

(Both Konstantin and I were looking towards the day when we move the
lru_lock into the lruvec, removing more dependence on "zone". Pretty
much the only reason that hasn't happened yet, is that we have not found
time to make a performance case convincingly - but that's another topic.)

Yes, page_zone(page) is a static property of the page, but it's not
necessarily cheap to evaluate: depends on how complex the memory model
and the spare page flags space, doesn't it? We both preferred to
derive zone from lruvec where convenient.

How do you feel about this patch, and does it work for you guys?

You'd be right if you guessed that I started out without the
mem_cgroup_zone_lruvec part of it, but oops in get_scan_count
told me that's needed too.

Description to be filled in later: would it be needed for -stable,
or is onlining already broken in other ways that you're now fixing up?

Reported-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
include/linux/mmzone.h | 2 -
mm/memcontrol.c | 40 ++++++++++++++++++++++++++++++++-------
mm/mmzone.c | 6 -----
mm/page_alloc.c | 2 -
4 files changed, 36 insertions(+), 14 deletions(-)
--- 3.6-rc5/include/linux/mmzone.h 2012-08-03 08:31:26.892842267 -0700
+++ linux/include/linux/mmzone.h 2012-09-13 17:07:51.893772372 -0700
@@ -744,7 +744,7 @@ 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)
{
--- 3.6-rc5/mm/memcontrol.c 2012-08-03 08:31:27.060842270 -0700
+++ linux/mm/memcontrol.c 2012-09-13 17:46:36.870804625 -0700
@@ -1061,12 +1061,25 @@ 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.
+ */
+ if (unlikely(lruvec->zone != zone)) {
+ VM_BUG_ON(lruvec->zone);
+ lruvec->zone = zone;
+ }
+ return lruvec;
}

/*
@@ -1093,9 +1106,12 @@ 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;
@@ -1113,7 +1129,17 @@ 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.
+ */
+ if (unlikely(lruvec->zone != zone)) {
+ VM_BUG_ON(lruvec->zone);
+ lruvec->zone = zone;
+ }
+ return lruvec;
}

/**
@@ -4742,7 +4768,7 @@ 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;
--- 3.6-rc5/mm/mmzone.c 2012-08-03 08:31:27.064842271 -0700
+++ linux/mm/mmzone.c 2012-09-13 17:06:28.921766001 -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
}
--- 3.6-rc5/mm/page_alloc.c 2012-08-22 14:25:39.508279046 -0700
+++ linux/mm/page_alloc.c 2012-09-13 17:06:08.265763526 -0700
@@ -4456,7 +4456,7 @@ 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;


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