Re: [PATCH] memory cgroup: update root memory cgroup when node is onlined

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Sep 13, 2012 at 06:36:34PM -0700, Hugh Dickins wrote:
> On Thu, 13 Sep 2012, Johannes Weiner wrote:
> > On Thu, Sep 13, 2012 at 03:14:28PM +0800, Wen Congyang wrote:
> > > root_mem_cgroup->info.nodeinfo is initialized when the system boots.
> > > But NODE_DATA(nid) is null if the node is not onlined, so
> > > root_mem_cgroup->info.nodeinfo[nid]->zoneinfo[zone].lruvec.zone contains
> > > an invalid pointer. If we use numactl to bind a program to the node
> > > after onlining the node and its memory, it will cause the kernel
> > > panicked:
> > 
> > Is there any chance we could get rid of the zone backpointer in lruvec
> > again instead?
> 
> It could be done, but it would make me sad :(

We would not want that!

> > But can't
> > we just go back to passing the zone along with the lruvec down
> > vmscan.c paths?  I agree it's ugly to pass both, given their
> > relationship.  But I don't think the backpointer is any cleaner but in
> > addition less robust.
> 
> It's like how we use vma->mm: we could change everywhere to pass mm with
> vma, but it looks cleaner and cuts down on long arglists to have mm in vma.
> >From past experience, one of the things I worried about was adding extra
> args to the reclaim stack.

Ok, you certainly have a point.

> > That being said, the crashing code in particular makes me wonder:
> > 
> > static __always_inline void add_page_to_lru_list(struct page *page,
> > 				struct lruvec *lruvec, enum lru_list lru)
> > {
> > 	int nr_pages = hpage_nr_pages(page);
> > 	mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
> > 	list_add(&page->lru, &lruvec->lists[lru]);
> > 	__mod_zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru, nr_pages);
> > }
> > 
> > Why did we ever pass zone in here and then felt the need to replace it
> > with lruvec->zone in fa9add6 "mm/memcg: apply add/del_page to lruvec"?
> > A page does not roam between zones, its zone is a static property that
> > can be retrieved with page_zone().
> 
> Just as in vmscan.c, we have the lruvec to hand, and that's what we
> mainly want to operate upon, but there is also some need for zone.
> 
> (Both Konstantin and I were looking towards the day when we move the
> lru_lock into the lruvec, removing more dependence on "zone".  Pretty
> much the only reason that hasn't happened yet, is that we have not found
> time to make a performance case convincingly - but that's another topic.)
> 
> Yes, page_zone(page) is a static property of the page, but it's not
> necessarily cheap to evaluate: depends on how complex the memory model
> and the spare page flags space, doesn't it?  We both preferred to
> derive zone from lruvec where convenient.
> 
> How do you feel about this patch, and does it work for you guys?
> 
> You'd be right if you guessed that I started out without the
> mem_cgroup_zone_lruvec part of it, but oops in get_scan_count
> told me that's needed too.
> 
> Description to be filled in later: would it be needed for -stable,
> or is onlining already broken in other ways that you're now fixing up?
> 
> Reported-by: Tang Chen <tangchen@cn.fujitsu.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>

This looks good to me, thanks.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>