Messages in this thread | | | Date | Wed, 28 May 2014 13:31:07 +0200 | From | Michal Hocko <> | Subject | Re: [patch 6/9] mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed |
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On Tue 27-05-14 15:45:00, Johannes Weiner wrote: > On Fri, May 23, 2014 at 03:20:43PM +0200, Michal Hocko wrote: > > On Wed 30-04-14 16:25:40, Johannes Weiner wrote: > > > There is a write barrier between setting pc->mem_cgroup and > > > PageCgroupUsed, which was added to allow LRU operations to lookup the > > > memcg LRU list of a page without acquiring the page_cgroup lock. But > > > ever since 38c5d72f3ebe ("memcg: simplify LRU handling by new rule"), > > > pages are ensured to be off-LRU while charging, so nobody else is > > > changing LRU state while pc->mem_cgroup is being written. > > > > This is quite confusing. Why do we have the lrucare path then? > > Some charge paths start with the page on the LRU, lrucare makes sure > it's off during the charge.
Yeah, I know I just wanted to point that the changelog might be confusing and so mentioning this aspect would be nice...
> > The code is quite tricky so this deserves a more detailed explanation > > IMO. > > > > There are only 3 paths which check both the flag and mem_cgroup ( > > without page_cgroup_lock) get_mctgt_type* and mem_cgroup_page_lruvec AFAICS. > > None of them have rmb so there was no guarantee about ordering anyway. > > Yeah, exactly. As per the changelog, this is a remnant of the way it > used to work but it's no longer needed because of guaranteed off-LRU > state. > > > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> > > > > Anyway, the change is welcome > > Acked-by: Michal Hocko <mhocko@suse.cz> > > Thanks!
-- Michal Hocko SUSE Labs
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