On Tue, 2010-03-02 at 23:14 +0100, Andrea Righi wrote: > > I agree mem_cgroup_has_dirty_limit() is nicer. But we must do that under > RCU, so something like: > > rcu_read_lock(); > if (mem_cgroup_has_dirty_limit()) > mem_cgroup_get_page_stat() > else > global_page_state() > rcu_read_unlock(); > > That is bad when mem_cgroup_has_dirty_limit() always returns false > (e.g., when memory cgroups are disabled). So I fallback to the old > interface. Why is it that mem_cgroup_has_dirty_limit() needs RCU when mem_cgroup_get_page_stat() doesn't? That is, simply make mem_cgroup_has_dirty_limit() not require RCU in the same way *_get_page_stat() doesn't either. > What do you think about: > > mem_cgroup_lock(); > if (mem_cgroup_has_dirty_limit()) > mem_cgroup_get_page_stat() > else > global_page_state() > mem_cgroup_unlock(); > > Where mem_cgroup_read_lock/unlock() simply expand to nothing when > memory cgroups are disabled. I think you're engineering the wrong way around. > > > > That allows for a 0 dirty limit (which should work and basically makes > > all io synchronous). > > IMHO it is better to reserve 0 for the special value "disabled" like the > global settings. A synchronous IO can be also achieved using a dirty > limit of 1. Why?! 0 clearly states no writeback cache, IOW sync writes, a 1 byte/page writeback cache effectively reduces to the same thing, but its not the same thing conceptually. If you want to put the size and enable into a single variable pick -1 for disable or so. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>