On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote: > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes: > > > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@xxxxxxxxx> 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? You can think of stop_machine() as being sort of like a reader-writer lock. The readers can be any section of code with preemption disabled, and the writer is the function passed to stop_machine(). Users running real-time applications on Linux don't tend to like stop_machine() much, but perhaps it is nevertheless the right tool for this particular job. Thanx, Paul -- 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>
