On Wed, Mar 07, 2012 at 10:01:50PM -0800, Hugh Dickins wrote: > 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. I don't think so, either, as long as the css_id can be found after the synchronize_rcu() between freezing the refcount and freeing the group. > 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 <hughd@xxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>