On Thu, 8 Sep 2011 11:53:49 +0200 Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > On Thu, Sep 08, 2011 at 06:42:21PM +0900, KAMEZAWA Hiroyuki wrote: > > On Thu, 8 Sep 2011 11:33:16 +0200 > > Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > > > > > On Thu, Sep 08, 2011 at 06:19:01PM +0900, KAMEZAWA Hiroyuki wrote: > > > > On Thu, 8 Sep 2011 10:54:04 +0200 > > > > Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > > > > > > > > > On Thu, Sep 08, 2011 at 05:30:42PM +0900, KAMEZAWA Hiroyuki wrote: > > > > > > On Thu, 8 Sep 2011 09:40:22 +0200 > > > > > > Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > > > > > > > > > > > > > There is a potential race between a thread charging a page and another > > > > > > > thread putting it back to the LRU list: > > > > > > > > > > > > > > charge: putback: > > > > > > > SetPageCgroupUsed SetPageLRU > > > > > > > PageLRU && add to memcg LRU PageCgroupUsed && add to memcg LRU > > > > > > > > > > > > > > > > > > > I assumed that all pages are charged before added to LRU. > > > > > > (i.e. event happens in charge->lru_lock->putback order.) > > > > > > > > > > > > But hmm, this assumption may be bad for maintainance. > > > > > > Do you find a code which adds pages to LRU before charge ? > > > > > > > > > > > > Hmm, if there are codes which recharge the page to other memcg, > > > > > > it will cause bug and my assumption may be harmful. > > > > > > > > > > Swap slots are read optimistically into swapcache and put to the LRU, > > > > > then charged upon fault. > > > > > > > > Yes, then swap charge removes page from LRU before charge. > > > > IIUC, it needed to do so because page->mem_cgroup may be replaced. > > > > > > But only from the memcg LRU. It's still on the global per-zone LRU, > > > so reclaim could isolate/putback it during the charge. And then > > > > > > > > > > charge: putback: > > > > > > > SetPageCgroupUsed SetPageLRU > > > > > > > PageLRU && add to memcg LRU PageCgroupUsed && add to memcg LRU > > > > > > applies. > > > > Hmm, in this case, I thought memcg puts back the page to its LRU by itself > > under lru_loc after charge and the race was hidden. > > But it locklessly checks PageLRU and bails if it's cleared and that is I think PageLRU check is done under zone->lru_lock. > the problem: it's not guaranteed that PageLRU is observed on the > charging CPU when the putback side bailed because of PageCgroupUsed. > zone->lru_lock is no help ? > My barrier puts this in order and makes sure one of the two succeeds. > 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>