"Huang, Ying" <[email protected]> writes: > From: Huang Ying <[email protected]> > > 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 <[email protected]> > Cc: Paul E. McKenney <[email protected]> > Cc: Minchan Kim <[email protected]> > Cc: Johannes Weiner <[email protected]> > Cc: Tim Chen <[email protected]> > Cc: Shaohua Li <[email protected]> > Cc: Mel Gorman <[email protected]> > Cc: "Jérôme Glisse" <[email protected]> > Cc: Michal Hocko <[email protected]> > Cc: Andrea Arcangeli <[email protected]> > Cc: David Rientjes <[email protected]> > Cc: Rik van Riel <[email protected]> > Cc: Jan Kara <[email protected]> > Cc: Dave Jiang <[email protected]> > Cc: Aaron Lu <[email protected]> > Signed-off-by: "Huang, Ying" <[email protected]> > > 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
