"Huang, Ying" <ying.huang@xxxxxxxxx> writes: > From: Huang Ying <ying.huang@xxxxxxxxx> > > When the swapin is performed, after getting the swap entry information > from the page table, system will swap in the swap entry, without any > lock held to prevent the swap device from being swapoff. This may > cause the race like below, > > CPU 1 CPU 2 > ----- ----- > do_swap_page > swapin_readahead > __read_swap_cache_async > swapoff swapcache_prepare > p->swap_map = NULL __swap_duplicate > p->swap_map[?] /* !!! NULL pointer access */ > > Because swapoff is usually done when system shutdown only, the race > may not hit many people in practice. But it is still a race need to > be fixed. > > To fix the race, get_swap_device() is added to check whether the > specified swap entry is valid in its swap device. If so, it will keep > the swap entry valid via preventing the swap device from being > swapoff, until put_swap_device() is called. > > Because swapoff() is very rare code path, to make the normal path runs > as fast as possible, disabling preemption + stop_machine() instead of > reference count is used to implement get/put_swap_device(). From > get_swap_device() to put_swap_device(), the preemption is disabled, so > stop_machine() in swapoff() will wait until put_swap_device() is > called. > > In addition to swap_map, cluster_info, etc. data structure in the > struct swap_info_struct, the swap cache radix tree will be freed after > swapoff, so this patch fixes the race between swap cache looking up > and swapoff too. > > Races between some other swap cache usages protected via disabling > preemption and swapoff are fixed too via calling stop_machine() > between clearing PageSwapCache() and freeing swap cache data > structure. > > Alternative implementation could be replacing disable preemption with > rcu_read_lock_sched and stop_machine() with synchronize_sched(). > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Cc: Minchan Kim <minchan@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > Cc: Shaohua Li <shli@xxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Cc: David Rientjes <rientjes@xxxxxxxxxx> > Cc: Rik van Riel <riel@xxxxxxxxxx> > Cc: Jan Kara <jack@xxxxxxx> > Cc: Dave Jiang <dave.jiang@xxxxxxxxx> > Cc: Aaron Lu <aaron.lu@xxxxxxxxx> > Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > > Changelog: > > v5: > > - Replace RCU with stop_machine() 2 versions (-V5a and -V5b) have been sent, one is implemented with stop_machine(), the other is implemented with RCU-sched. RCU-sched based version is better for real time users. Both are OK for me. Best Regards, Huang, Ying -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
