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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

> 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!

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