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SubjectRe: [PATCH 9/12] memcg allocate all page_cgroup at boot
On Fri, 26 Sep 2008 11:05:50 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Fri, 26 Sep 2008 10:43:36 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > > - /*
> > > > - * Check if our page_cgroup is valid
> > > > - */
> > > > - lock_page_cgroup(page);
> > > > - pc = page_get_page_cgroup(page);
> > > > - if (unlikely(!pc))
> > > > - goto unlock;
> > > > -
> > > > - VM_BUG_ON(pc->page != page);
> > > > + pc = lookup_page_cgroup(page);
> > > > + if (unlikely(!pc || !PageCgroupUsed(pc)))
> > > > + return;
> > > > + preempt_disable();
> > > > + lock_page_cgroup(pc);
> > > > + if (unlikely(page_mapped(page))) {
> > > > + unlock_page_cgroup(pc);
> > > > + preempt_enable();
> > > > + return;
> > > > + }
> > > Just for clarification, in what sequence will the page be mapped here?
> > > mem_cgroup_uncharge_page checks whether the page is mapped.
> > >
> > Please think about folloing situation.
> >
> > There is a SwapCache which is referred from 2 process, A, B.
> > A maps it.
> > B doesn't maps it.
> >
> > And now, process A exits.
> >
> > CPU0(process A) CPU1 (process B)
> >
> > zap_pte_range()
> > => page remove from rmap => charge() (do_swap_page)
> > => set page->mapcount->0
> > => uncharge() => set page->mapcount=1
> >
> > This race is what patch 12/12 is fixed.
> > This only happens on cursed SwapCache.
> >
> Sorry, my brain seems to be sleeping.. above page_mapped() check doesn't
> help this situation. Maybe this page_mapped() check is not necessary
> because it's of no use.
>
> I think this kind of problem will not be fixed until we handle SwapCache.
>
I've not fully understood yet what [12/12] does, but if we handle
swapcache properly, [12/12] would become unnecessary?

If so, how about handling swapcache instead of adding new interface?
I think it can be done independent of mem+swap.


Thanks,
Daisuke Nishimura.


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