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mm: memcontrol: do not iterate uninitialized memcgs
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commit 2f7dd7a upstream.

The cgroup iterators yield css objects that have not yet gone through
css_online(), but they are not complete memcgs at this point and so the
memcg iterators should not return them.  Commit d8ad305 ("mm/memcg:
iteration skip memcgs not yet fully initialized") set out to implement
exactly this, but it uses CSS_ONLINE, a cgroup-internal flag that does
not meet the ordering requirements for memcg, and so the iterator may
skip over initialized groups, or return partially initialized memcgs.

The cgroup core can not reasonably provide a clear answer on whether the
object around the css has been fully initialized, as that depends on
controller-specific locking and lifetime rules.  Thus, introduce a
memcg-specific flag that is set after the memcg has been initialized in
css_online(), and read before mem_cgroup_iter() callers access the memcg
members.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
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Johannes Weiner authored and Jiri Slaby committed Oct 31, 2014
1 parent d9fc4e6 commit 19911e1
Showing 1 changed file with 30 additions and 3 deletions.
33 changes: 30 additions & 3 deletions mm/memcontrol.c
Original file line number Diff line number Diff line change
@@ -250,6 +250,9 @@ struct mem_cgroup {
/* vmpressure notifications */
struct vmpressure vmpressure;

/* css_online() has been completed */
int initialized;

/*
* the counter to account for mem+swap usage.
*/
@@ -1089,9 +1092,23 @@ static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
* skipping css reference should be safe.
*/
if (next_css) {
if ((next_css == &root->css) ||
((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
return mem_cgroup_from_css(next_css);
struct mem_cgroup *memcg = mem_cgroup_from_css(next_css);

if (next_css == &root->css)
return memcg;

if (css_tryget(next_css)) {
if (memcg->initialized) {
/*
* Make sure the memcg is initialized:
* mem_cgroup_css_online() orders the the
* initialization against setting the flag.
*/
smp_rmb();
return memcg;
}
css_put(next_css);
}

prev_css = next_css;
goto skip_node;
@@ -6331,6 +6348,16 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)

error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
mutex_unlock(&memcg_create_mutex);

if (!error) {
/*
* Make sure the memcg is initialized: mem_cgroup_iter()
* orders reading memcg->initialized against its callers
* reading the memcg members.
*/
smp_wmb();
memcg->initialized = 1;
}
return error;
}

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