From 59927fb984de1703c67bc640c3e522d8b5276c73 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 15 Mar 2012 15:17:07 -0700 Subject: [PATCH] memcg: free mem_cgroup by RCU to fix oops After fixing the GPF in mem_cgroup_lru_del_list(), three times one machine running a similar load (moving and removing memcgs while swapping) has oopsed in mem_cgroup_zone_nr_lru_pages(), when retrieving memcg zone numbers for get_scan_count() for shrink_mem_cgroup_zone(): this is where a struct mem_cgroup is first accessed after being chosen by mem_cgroup_iter(). Just what protects a struct mem_cgroup from being freed, in between mem_cgroup_iter()'s css_get_next() and its css_tryget()? css_tryget() fails once css->refcnt is zero with CSS_REMOVED set in flags, yes: but what if that memory is freed and reused for something else, which sets "refcnt" non-zero? Hmm, and scope for an indefinite freeze if refcnt is left at zero but flags are cleared. It's tempting to move the css_tryget() into css_get_next(), to make it really "get" the css, but I don't think that actually solves anything: the same difficulty in moving from css_id found to stable css remains. But we already have rcu_read_lock() around the two, so it's easily fixed if __mem_cgroup_free() just uses kfree_rcu() to free mem_cgroup. However, a big struct mem_cgroup is allocated with vzalloc() instead of kzalloc(), and we're not allowed to vfree() at interrupt time: there doesn't appear to be a general vfree_rcu() to help with this, so roll our own using schedule_work(). The compiler decently removes vfree_work() and vfree_rcu() when the config doesn't need them. Signed-off-by: Hugh Dickins Acked-by: KAMEZAWA Hiroyuki Acked-by: Johannes Weiner Cc: Konstantin Khlebnikov Cc: Tejun Heo Cc: Ying Han Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 53 +++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 47 insertions(+), 6 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index d0e57a3cda1..58a08fc7414 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -230,10 +230,30 @@ struct mem_cgroup { * the counter to account for memory usage */ struct res_counter res; - /* - * the counter to account for mem+swap usage. - */ - struct res_counter memsw; + + union { + /* + * the counter to account for mem+swap usage. + */ + struct res_counter memsw; + + /* + * rcu_freeing is used only when freeing struct mem_cgroup, + * so put it into a union to avoid wasting more memory. + * It must be disjoint from the css field. It could be + * in a union with the res field, but res plays a much + * larger part in mem_cgroup life than memsw, and might + * be of interest, even at time of free, when debugging. + * So share rcu_head with the less interesting memsw. + */ + struct rcu_head rcu_freeing; + /* + * But when using vfree(), that cannot be done at + * interrupt time, so we must then queue the work. + */ + struct work_struct work_freeing; + }; + /* * Per cgroup active and inactive list, similar to the * per zone LRU lists. @@ -4779,6 +4799,27 @@ out_free: return NULL; } +/* + * Helpers for freeing a vzalloc()ed mem_cgroup by RCU, + * but in process context. The work_freeing structure is overlaid + * on the rcu_freeing structure, which itself is overlaid on memsw. + */ +static void vfree_work(struct work_struct *work) +{ + struct mem_cgroup *memcg; + + memcg = container_of(work, struct mem_cgroup, work_freeing); + vfree(memcg); +} +static void vfree_rcu(struct rcu_head *rcu_head) +{ + struct mem_cgroup *memcg; + + memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing); + INIT_WORK(&memcg->work_freeing, vfree_work); + schedule_work(&memcg->work_freeing); +} + /* * At destroying mem_cgroup, references from swap_cgroup can remain. * (scanning all at force_empty is too costly...) @@ -4802,9 +4843,9 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg) free_percpu(memcg->stat); if (sizeof(struct mem_cgroup) < PAGE_SIZE) - kfree(memcg); + kfree_rcu(memcg, rcu_freeing); else - vfree(memcg); + call_rcu(&memcg->rcu_freeing, vfree_rcu); } static void mem_cgroup_get(struct mem_cgroup *memcg) -- 2.39.2