Re: [patch 6/9] mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed

[Michal Hocko]

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@xxxxxxxxxxx>
> > 
> > Anyway, the change is welcome
> > Acked-by: Michal Hocko <mhocko@xxxxxxx>
> 
> Thanks!

-- 
Michal Hocko
SUSE Labs
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