Re: [BUG] kernel BUG at mm/memcontrol.c:1074!

[KAMEZAWA Hiroyuki]

On Wed, 18 Jan 2012 21:16:09 -0800 (PST)
Hugh Dickins <hughd@xxxxxxxxxx> wrote:

> On Thu, 19 Jan 2012, KAMEZAWA Hiroyuki wrote:
> > On Wed, 18 Jan 2012 19:41:44 -0800 (PST)
> > Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > > 
> > > I notice that, unlike Linus's git, this linux-next still has
> > > mm-isolate-pages-for-immediate-reclaim-on-their-own-lru.patch in.
> > > 
> > > I think that was well capable of oopsing in mem_cgroup_lru_del_list(),
> > > since it didn't always know which lru a page belongs to.
> > > 
> > > I'm going to be optimistic and assume that was the cause.
> > > 
> > Hmm, because the log hits !memcg at lru "del", the page should be added
> > to LRU somewhere and the lru must be determined by pc->mem_cgroup.
> > 
> > Once set, pc->mem_cgroup is not cleared, just overwritten. AFAIK, there is
> > only one chance to set pc->mem_cgroup as NULL... initalization.
> > I wonder why it hits lru_del() rather than lru_add()...
> > ................
> > 
> > Ahhhh, ok, it seems you are right. the patch has following kinds of codes
> > ==
> > +static void pagevec_putback_immediate_fn(struct page *page, void *arg)
> > +{
> > +       struct zone *zone = page_zone(page);
> > +
> > +       if (PageLRU(page)) {
> > +               enum lru_list lru = page_lru(page);
> > +               list_move(&page->lru, &zone->lru[lru].list);
> > +       }
> > +}
> > ==
> > ..this will bypass mem_cgroup_lru_add(), and we can see bug in lru_del()
> > rather than lru_add()..
> 
> I've not thought it through in detail (and your questioning reminds me
> that the worst I saw from that patch was updating of the wrong counts,
> leading to underflow, then livelock from the mismatch between empty list
> and enormous count: I never saw an oops from it, and may be mistaken).
> 
> > 
> > Another question is who pushes pages to LRU before setting pc->mem_cgroup..
> > Anyway, I think we need to fix memcg to be LRU_IMMEDIATE aware.
> 
> I don't think so: Mel agreed that the patch could not go forward as is,
> without an additional pageflag, and asked Andrew to drop it from mmotm
> in mail on 29th December (I didn't notice an mm-commits message to say
> akpm did drop it, and marc is blacked out in protest for today, so I
> cannot check: but certainly akpm left it out of his push to Linus).
> 
> Oh, and Mel noticed another bug in it on the 30th, that the PageLRU
> check in the function you quote above is wrong: see PATCH 11/11 thread.

Sure.

Hm, what I need to find is a path which adds page to LRU bypassing memcg's check...

Thanks,
-Kame


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