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

["Huang\, Ying" <ying.huang@intel.com>]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote:
>
>> > Why do we need srcu here? Is it enough with rcu like below?
>> >
>> > It might have a bug/room to be optimized about performance/naming.
>> > I just wanted to show my intention.
>> 
>> Yes.  rcu should work too.  But if we use rcu, it may need to be called
>> several times to make sure the swap device under us doesn't go away, for
>> example, when checking si->max in __swp_swapcount() and
>> add_swap_count_continuation().  And I found we need rcu to protect swap
>> cache radix tree array too.  So I think it may be better to use one
>> calling to srcu_read_lock/unlock() instead of multiple callings to
>> rcu_read_lock/unlock().
>
> Or use stop_machine() ;)  It's very crude but it sure is simple.  Does
> anyone have a swapoff-intensive workload?

Sorry, I don't know how to solve the problem with stop_machine().

The problem we try to resolved is that, we have a swap entry, but that
swap entry can become invalid because of swappoff between we check it
and we use it.  So we need to prevent swapoff to be run between checking
and using.

I don't know how to use stop_machine() in swapoff to wait for all users
of swap entry to finish.  Anyone can help me on this?

Best Regards,
Huang, Ying