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SubjectRe: [patch] mm: memcg: close race between charge and putback
On Thu, 8 Sep 2011 11:53:49 +0200
Johannes Weiner <jweiner@redhat.com> 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@redhat.com> 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@redhat.com> 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@redhat.com> 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



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