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Date: Tue, 27 May 2014 15:45:00 -0400 From: Johannes Weiner <hannes@...xchg.org> To: Michal Hocko <mhocko@...e.cz> Cc: linux-mm@...ck.org, Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [patch 6/9] mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed 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@...xchg.org> > > Anyway, the change is welcome > Acked-by: Michal Hocko <mhocko@...e.cz> Thanks! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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