Re: [RFC PATCH 2/2] mm/vmscan.c: Prevent allocating shrinker_info on offlined nodes

[Yang Shi <shy828301@gmail.com>]

On Mon, Dec 6, 2021 at 11:01 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.12.21 19:42, Yang Shi wrote:
> > On Mon, Dec 6, 2021 at 5:19 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> >>
> >> On 06.12.2021 13:45, David Hildenbrand wrote:
> >>>> This doesn't seen complete. Slab shrinkers are used in the reclaim
> >>>> context. Previously offline nodes could be onlined later and this would
> >>>> lead to NULL ptr because there is no hook to allocate new shrinker
> >>>> infos. This would be also really impractical because this would have to
> >>>> update all existing memcgs...
> >>>
> >>> Instead of going through the trouble of updating...
> >>>
> >>> ...  maybe just keep for_each_node() and check if the target node is
> >>> offline. If it's offline, just allocate from the first online node.
> >>> After all, we're not using __GFP_THISNODE, so there are no guarantees
> >>> either way ...
> >>
> >> Hm, can't we add shrinker maps allocation to __try_online_node() in addition
> >> to this patch?
> >
> > I think the below fix (an example, doesn't cover all affected
> > callsites) should be good enough for now? It doesn't touch the hot
> > path of the page allocator.
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index fb9584641ac7..1252a33f7c28 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -222,13 +222,15 @@ static int expand_one_shrinker_info(struct
> > mem_cgroup *memcg,
> >         int size = map_size + defer_size;
> >
> >         for_each_node(nid) {
> > +               int tmp = nid;
> >                 pn = memcg->nodeinfo[nid];
> >                 old = shrinker_info_protected(memcg, nid);
> >                 /* Not yet online memcg */
> >                 if (!old)
> >                         return 0;
> > -
> > -               new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
> > +               if (!node_online(nid))
> > +                       tmp = -1;
> > +               new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, tmp);
> >                 if (!new)
> >                         return -ENOMEM;
> >
> > It used to use kvmalloc instead of kvmalloc_node(). The commit
> > 86daf94efb11d7319fbef5e480018c4807add6ef ("mm/memcontrol.c: allocate
> > shrinker_map on appropriate NUMA node") changed to use *_node()
> > version. The justification was that "kswapd is always bound to
> > specific node. So allocate shrinker_map from the related NUMA node to
> > respect its NUMA locality." There is no kswapd for offlined node, so
> > just allocate shrinker info on node 0. This is also what
> > alloc_mem_cgroup_per_node_info() does.
>
> Yes, that's what I refer to as fixing it in the caller -- similar to
> [1]. Michals point is to not require such node_online() checks at all,
> neither in the caller nor in the buddy.
>
> I see 2 options short-term
>
> 1) What we have in [1].
> 2) What I proposed in [2], fixing it for all such instances until we
> have something better.
>
> Long term I tend to agree that what Michal proposes is better.
>
> Short term I tend to like [2], because it avoids having to mess with all
> such instances to eventually get it right and the temporary overhead
> until we have the code reworked should be really negligible ...

Thanks, David. Basically either option looks fine to me. But I'm a
little bit concerned about [2]. It silently changes the node requested
by the callers. It actually papers over potential bugs? And what if
the callers specify __GFP_THISNODE (I didn't search if such callers
really exist in the current code)?

How's about a helper function, for example, called
kvmalloc_best_node()? It does:

void * kvmalloc_best_node(unsigned long size, int flag, int nid)
{
    bool onlined = node_online(nid);

    WARN_ON_ONCE((flag & __GFP_THISNODE) && !onlined);

    if (!onlined)
        nid = -1;

    return kvmalloc_node(size, GFP_xxx, nid);
}

>
>
>
> [1] https://lkml.kernel.org/r/20211108202325.20304-1-amakhalov@vmware.com
> [2]
> https://lkml.kernel.org/r/51c65635-1dae-6ba4-daf9-db9df0ec35d8@redhat.com
>
> --
> Thanks,
>
> David / dhildenb
>