On Tue, 20 Dec 2011 09:24:47 +0900 Hiroyuki Kamezawa <kamezawa.hiroyuki@xxxxxxxxx> wrote: > 2011/12/20 Hugh Dickins <hughd@xxxxxxxxxx>: > > On Mon, 19 Dec 2011, KAMEZAWA Hiroyuki wrote: > >> From d620ff605a3a592c2b1de3a046498ce5cd3d3c50 Mon Sep 17 00:00:00 2001 > >> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > >> Date: Mon, 19 Dec 2011 16:55:10 +0900 > >> Subject: [PATCH 2/2] memcg: reset lru to root_mem_cgroup in special cases. > >> > >> This patch is a fix for memcg-simplify-lru-handling-by-new-rule.patch > >> > >> After the patch, all pages which will be onto LRU must have sane > >> pc->mem_cgroup. But, in special case, it's not set. > >> > >> If task->mm is NULL or task is TIF_MEMDIE or fatal_signal_pending(), > >> try_charge() is bypassed and the new charge will not be charged. And > >> pc->mem_cgroup is unset even if the page will be used/mapped and added > >> to LRU. To avoid this, this patch charges such pages to root_mem_cgroup, > >> then, pc->mem_cgroup will be handled correctly. > >> > >> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > >> --- > >> mm/memcontrol.c | 2 +- > >> 1 files changed, 1 insertions(+), 1 deletions(-) > >> > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c > >> index 0d6d21c..9268e8e 100644 > >> --- a/mm/memcontrol.c > >> +++ b/mm/memcontrol.c > >> @@ -2324,7 +2324,7 @@ nomem: > >> *ptr = NULL; > >> return -ENOMEM; > >> bypass: > >> - *ptr = NULL; > >> + *ptr = root_mem_cgroup; > >> return 0; > >> } > >> > >> -- > > > Thank you for review. > > > I'm dubious about this patch: certainly you have not fully justified it. > > > I sometimes see panics (in !pc->mem_cgroup check in lru code) > when I stops test programs by Ctrl-C or some. That was because > of this path. I checked this by adding a debug code to make > pc->mem_cgroup = NULL in prep_new_page. > > > I speak from experience: I did *exactly* the same at "bypass" when > > I introduced our mem_cgroup_reset_page(), which corresponds to your > > mem_cgroup_reset_owner(); it seemed right to me that a successful > > (return 0) call to try_charge() should provide a good *ptr. > > > ok. > > > But others (Ying and Greg) pointed out that it changes the semantics > > of __mem_cgroup_try_charge() in this case, so you need to justify the > > change to all those places which do something like "if (ret || !memcg)" > > after calling it. Perhaps it is a good change everywhere, but that's > > not obvious, so we chose caution. > > > > > Doesn't it lead to bypass pages being marked as charged to root, so > > they don't get charged to the right owner next time they're touched? > > > Yes. You're right. > Hm. So, it seems I should add reset_owner() to the !memcg path > rather than here. > Considering this again.. Now, we catch 'charge' event only once in lifetime of anon/file page. So, it doesn't depend on that it's marked as PCG_USED or not. > > In our internal kernel, I restored "bypass" to set *ptr = NULL as > > before, but routed those callers that need it to continue on to > > __mem_cgroup_commit_charge() when it's NULL, and let that do a > > quick little mem_cgroup_reset_page() to root_mem_cgroup for this. > > > Yes, I'll prepare v2. > But ok, I'll go this way with some more description. Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>