Now that everybody loves memcg, configures it on, and would not dream
of booting with cgroup_disable=memory, it can pass unnoticed for weeks
that memcg-less page reclaim is completely broken.
mmotm's "memcg: enhance memcg iterator to support predicates" replaces
__shrink_zone()'s "do { } while (memcg);" loop by a "while (memcg) {}"
loop: which is nicer for memcg, but does nothing for !CONFIG_MEMCG or
cgroup_disable=memory. Page reclaim hangs, making no progress.
Adding mem_cgroup_disabled() and once++ test there is ugly. Ideally,
even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
which would get around this: but that's not so at present.
However, it appears that nothing actually dereferences the memcg pointer
in the mem_cgroup_disabled() case, here or anywhere else that case can
reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
global reclaim).
So, simply pass back an ordinarily-oopsing non-NULL address the first
time, and we shall hear about it if I'm wrong.
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
By all means fold in to
memcg-enhance-memcg-iterator-to-support-predicates.patch
include/linux/memcontrol.h | 3 ++-
mm/memcontrol.c | 6 ++++--
2 files changed, 6 insertions(+), 3 deletions(-)
--- 3.11-rc5-mm1/include/linux/memcontrol.h 2013-08-15 18:10:50.504539510 -0700
+++ linux/include/linux/memcontrol.h 2013-08-18 12:30:58.116460318 -0700
@@ -370,7 +370,8 @@ mem_cgroup_iter_cond(struct mem_cgroup *
struct mem_cgroup_reclaim_cookie *reclaim,
mem_cgroup_iter_filter cond)
{
- return NULL;
+ /* first call must return non-NULL, second return NULL */
+ return (struct mem_cgroup *)(unsigned long)!prev;
}
static inline struct mem_cgroup *
--- 3.11-rc5-mm1/mm/memcontrol.c 2013-08-15 18:10:50.720539516 -0700
+++ linux/mm/memcontrol.c 2013-08-18 12:29:15.352460818 -0700
@@ -1086,8 +1086,10 @@ struct mem_cgroup *mem_cgroup_iter_cond(
struct mem_cgroup *memcg = NULL;
struct mem_cgroup *last_visited = NULL;
- if (mem_cgroup_disabled())
- return NULL;
+ if (mem_cgroup_disabled()) {
+ /* first call must return non-NULL, second return NULL */
+ return (struct mem_cgroup *)(unsigned long)!prev;
+ }
if (!root)
root = root_mem_cgroup;
--
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