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SubjectRe: [RFC][PATCH 3/4] memcg: fix for mem_cgroup_hierarchical_reclaim
On Fri, 9 Jan 2009 11:03:23 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> * Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2009-01-08 19:15:01]:
>
> > If root_mem has no children, last_scaned_child is set to root_mem itself.
> > But after some children added to root_mem, mem_cgroup_get_next_node can
> > mem_cgroup_put the root_mem although root_mem has not been mem_cgroup_get.
> >
>
> Good catch!
>
Thanks :)

> > This patch fixes this behavior by:
> > - Set last_scanned_child to NULL if root_mem has no children or DFS search
> > has returned to root_mem itself(root_mem is not a "child" of root_mem).
> > Make mem_cgroup_get_first_node return root_mem in this case.
> > There are no mem_cgroup_get/put for root_mem.
> > - Rename mem_cgroup_get_next_node to __mem_cgroup_get_next_node, and
> > mem_cgroup_get_first_node to mem_cgroup_get_next_node.
> > Make mem_cgroup_hierarchical_reclaim call only new mem_cgroup_get_next_node.
> >
>
> How have you tested these changes? When I wrote up the patches, I did
> several tests to make sure that all nodes in the hierarchy are covered
> while reclaiming in order.
>
I do something like:

1. mount memcg (at/cgroup/memory)
2. enable hierarchy (if testing use_hierarchy==1 case)
3. mkdir /cgroup/memory/01
4. run some programs in /cgroup/memory/01
5. select the next directory to move to at random from 01/, 01/aa, 01/bb,
02/, 02/aa and 02/bb.
6. move all processes to next directory.
7. remove the old directory if possible.
8. wait for an random period.
9. goto 5.

Before this patch, I got sometimes general protection fault, which seemed
to be caused by unexpected free of mem_cgroup (and reuse the area for
another purpose).


BTW, I think "mem_cgroup_put(mem->last_scanned_child)" is also needed
at mem_cgroup_destroy to prevent memory leak.

I'll update my patch later.


Thanks,
Daisuke Nishimura.

> >
> > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > ---
> > mm/memcontrol.c | 37 +++++++++++++++++++++----------------
> > 1 files changed, 21 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 288e22c..dc38a0e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -622,7 +622,7 @@ unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
> > * called with hierarchy_mutex held
> > */
> > static struct mem_cgroup *
> > -mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> > +__mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> > {
> > struct cgroup *cgroup, *curr_cgroup, *root_cgroup;
> >
> > @@ -644,8 +644,8 @@ mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> > visit_parent:
> > if (curr_cgroup == root_cgroup) {
> > mem_cgroup_put(curr);
> > - curr = root_mem;
> > - mem_cgroup_get(curr);
> > + /* caller handles NULL case */
> > + curr = NULL;
> > goto done;
> > }
> >
> > @@ -668,7 +668,6 @@ visit_parent:
> > goto visit_parent;
> >
> > done:
> > - root_mem->last_scanned_child = curr;
> > return curr;
> > }
> >
> > @@ -678,20 +677,29 @@ done:
> > * that to reclaim free pages from.
> > */
> > static struct mem_cgroup *
> > -mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
> > +mem_cgroup_get_next_node(struct mem_cgroup *root_mem)
> > {
> > struct cgroup *cgroup;
> > struct mem_cgroup *ret;
> > bool obsolete;
> >
> > - obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
> > -
> > /*
> > * Scan all children under the mem_cgroup mem
> > */
> > mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
> > +
> > + obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
> > +
> > if (list_empty(&root_mem->css.cgroup->children)) {
> > - ret = root_mem;
> > + /*
> > + * root_mem might have children before and last_scanned_child
> > + * may point to one of them.
> > + */
> > + if (root_mem->last_scanned_child) {
> > + VM_BUG_ON(!obsolete);
> > + mem_cgroup_put(root_mem->last_scanned_child);
> > + }
> > + ret = NULL;
> > goto done;
> > }
> >
> > @@ -705,13 +713,13 @@ mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
> > ret = mem_cgroup_from_cont(cgroup);
> > mem_cgroup_get(ret);
> > } else
> > - ret = mem_cgroup_get_next_node(root_mem->last_scanned_child,
> > + ret = __mem_cgroup_get_next_node(root_mem->last_scanned_child,
> > root_mem);
> >
> > done:
> > root_mem->last_scanned_child = ret;
> > mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
> > - return ret;
> > + return (ret) ? ret : root_mem;
> > }
> >
> > static bool mem_cgroup_check_under_limit(struct mem_cgroup *mem)
> > @@ -769,21 +777,18 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> > if (!root_mem->use_hierarchy)
> > return ret;
> >
> > - next_mem = mem_cgroup_get_first_node(root_mem);
> > + next_mem = mem_cgroup_get_next_node(root_mem);
> >
> > while (next_mem != root_mem) {
> > if (mem_cgroup_is_obsolete(next_mem)) {
> > - mem_cgroup_put(next_mem);
> > - next_mem = mem_cgroup_get_first_node(root_mem);
> > + next_mem = mem_cgroup_get_next_node(root_mem);
> > continue;
> > }
> > ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
> > get_swappiness(next_mem));
> > if (mem_cgroup_check_under_limit(root_mem))
> > return 0;
> > - mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
> > - next_mem = mem_cgroup_get_next_node(next_mem, root_mem);
> > - mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
> > + next_mem = mem_cgroup_get_next_node(root_mem);
> > }
> > return ret;
> > }
> >
>
>
> Looks good to me, I need to test it though
>
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
>
> --
> Balbir


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