(2012/11/27 3:47), Michal Hocko wrote:

mem_cgroup_iter curently relies on css->id when walking down a group
hierarchy tree. This is really awkward because the tree walk depends on
the groups creation ordering. The only guarantee is that a parent node
is visited before its children.
Example
  1) mkdir -p a a/d a/b/c
  2) mkdir -a a/b/c a/d
Will create the same trees but the tree walks will be different:
  1) a, d, b, c
  2) a, b, c, d

574bd9f7 (cgroup: implement generic child / descendant walk macros) has
introduced generic cgroup tree walkers which provide either pre-order
or post-order tree walk. This patch converts css->id based iteration
to pre-order tree walk to keep the semantic with the original iterator
where parent is always visited before its subtree.

cgroup_for_each_descendant_pre suggests using post_create and
pre_destroy for proper synchronization with groups addidition resp.
removal. This implementation doesn't use those because a new memory
cgroup is fully initialized in mem_cgroup_create and css reference
counting enforces that the group is alive for both the last seen cgroup
and the found one resp. it signals that the group is dead and it should
be skipped.

If the reclaim cookie is used we need to store the last visited group
into the iterator so we have to be careful that it doesn't disappear in
the mean time. Elevated reference count on the css keeps it alive even
though the group have been removed (parked waiting for the last dput so
that it can be freed).

V2
- use css_{get,put} for iter->last_visited rather than
   mem_cgroup_{get,put} because it is stronger wrt. cgroup life cycle
- cgroup_next_descendant_pre expects NULL pos for the first iterartion
   otherwise it might loop endlessly for intermediate node without any
   children.

Signed-off-by: Michal Hocko <redacted>

Acked-by: KAMEZAWA Hiroyuki <redacted>


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