Messages in this thread Patch in this message | | | Date | Fri, 26 Jun 2009 09:57:45 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | [PATCH] memcg: add commens for expaing memory barrier (Was Re: Low overhead patches for the memory cgroup controller (v5) |
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On Tue, 23 Jun 2009 09:01:16 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote: > > Do we still need the smp_wmb()? > > > > It's hard to say, because we forgot to document it :( > > > Sorry for lack of documentation. > > pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise, > A routine believes USED bit will see bad pc->mem_cgroup. > > I'd like to add a comment later (againt new mmotm.) >
Ok, it's now. == From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge().
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> Cc: Balbir Singh <balbir@linux.vnet.ibm.com> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> --- mm/memcontrol.c | 7 +++++++ 1 file changed, 7 insertions(+)
Index: mmotm-2.6.31-Jun25/mm/memcontrol.c =================================================================== --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c +++ mmotm-2.6.31-Jun25/mm/memcontrol.c @@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(s } pc->mem_cgroup = mem; + /* + * We access a page_cgroup asynchronously without lock_page_cgroup(). + * Especially when a page_cgroup is taken from a page, pc->mem_cgroup + * is accessed after testing USED bit. To make pc->mem_cgroup visible + * before USED bit, we need memory barrier here. + * See mem_cgroup_add_lru_list(), etc. + */ smp_wmb(); switch (ctype) { case MEM_CGROUP_CHARGE_TYPE_CACHE:
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