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

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Feb 14, 2019 at 04:07:37PM +0800, Huang, Ying wrote:
> Before, we choose to use stop_machine() to reduce the overhead of hot
> path (page fault handler) as much as possible.  But now, I found
> rcu_read_lock_sched() is just a wrapper of preempt_disable().  So maybe
> we can switch to RCU version now.

rcu_read_lock looks more efficient than rcu_read_lock_sched. So for
this purpose in the fast path rcu_read_lock()/unlock() should be the
preferred methods, no need to force preempt_disable() (except for
debug purposes if sleep debug is enabled). Server builds are done with
voluntary preempt (no preempt shouldn't even exist as config option)
and there rcu_read_lock might be just a noop.

Against a fast path rcu_read_lock/unlock before the consolidation
synchronize_rcu would have been enough, now after the consolidation
even more certain that it's enough because it's equivalent to _mult.