On Mon 02-04-18 19:50:50, Wang Long wrote: > > Hi, Johannes Weiner and Tejun Heo > > I use linux-4.4.y to test the new cgroup controller io and the current > stable kernel linux-4.4.y has the follow logic > > > int clear_page_dirty_for_io(struct page *page){ > ... > ... > memcg = mem_cgroup_begin_page_stat(page); ----------(a) > wb = unlocked_inode_to_wb_begin(inode, &locked); ---------(b) > if (TestClearPageDirty(page)) { > mem_cgroup_dec_page_stat(memcg, > MEM_CGROUP_STAT_DIRTY); > dec_zone_page_state(page, NR_FILE_DIRTY); > dec_wb_stat(wb, WB_RECLAIMABLE); > ret =1; > } > unlocked_inode_to_wb_end(inode, locked); -----------(c) > mem_cgroup_end_page_stat(memcg); -------------(d) > return ret; > ... > ... > } > > > when memcg is moving, and I_WB_SWITCH flags for inode is set. the logic > is the following: > > > spin_lock_irqsave(&memcg->move_lock, flags); -------------(a) > spin_lock_irq(&inode->i_mapping->tree_lock); ------------(b) > spin_unlock_irq(&inode->i_mapping->tree_lock); -----------(c) > spin_unlock_irqrestore(&memcg->move_lock, flags); -----------(d) > > > after (c) , the local irq is enabled. I think it is not correct. > > We get a deadlock backtrace after (c), the cpu get an softirq and in the > irq it also call mem_cgroup_begin_page_stat to lock the same > memcg->move_lock. > > Since the conditions are too harsh, this scenario is difficult to > reproduce. But it really exists. > > So how about change (b) (c) to spin_lock_irqsave/spin_lock_irqrestore?
Yes, it seems we really need this even for the current tree. Please note that At least clear_page_dirty_for_io doesn't lock memcg anymore. __cancel_dirty_page still uses lock_page_memcg though (former mem_cgroup_begin_page_stat). -- Michal Hocko SUSE Labs