Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 14 Feb 2019 15:33:18 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> > 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().
> 
> using stop_machine is generally discouraged. It is a gross
> synchronization.

This was discussed to death and I think the changelog explains the
conclusions adequately.  swapoff is super-rare so a stop_machine() in
that path is appropriate if its use permits more efficiency in the
regular swap code paths.  

> Besides that, since when do we have this problem?

What problem??