linux-kernel - Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker

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Date:   Tue, 7 Dec 2021 17:26:32 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Nico Pache <npache@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>, raquini@...hat.com,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker_info on
 offlined nodes

On Tue, Dec 7, 2021 at 5:23 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Tue, Dec 7, 2021 at 4:33 PM Nico Pache <npache@...hat.com> wrote:
> >
> >
> >
> > On 12/7/21 19:26, Yang Shi wrote:
> > > On Tue, Dec 7, 2021 at 3:44 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > >>
> > >> On Tue,  7 Dec 2021 17:40:13 -0500 Nico Pache <npache@...hat.com> wrote:
> > >>
> > >>> We have run into a panic caused by a shrinker allocation being attempted
> > >>> on an offlined node.
> > >>>
> > >>> Our crash analysis has determined that the issue originates from trying
> > >>> to allocate pages on an offlined node in expand_one_shrinker_info. This
> > >>> function makes the incorrect assumption that we can allocate on any node.
> > >>> To correct this we make sure the node is online before tempting an
> > >>> allocation. If it is not online choose the closest node.
> > >>
> > >> This isn't fully accurate, is it?  We could allocate on a node which is
> > >> presently offline but which was previously onlined, by testing
> > >> NODE_DATA(nid).
> > >>
> > >> It isn't entirely clear to me from the v1 discussion why this approach
> > >> isn't being taken?
> > >>
> > >> AFAICT the proposed patch is *already* taking this approach, by having
> > >> no protection against a concurrent or subsequent node offlining?
> > >
> > > AFAICT, we have not reached agreement on how to fix it yet. I saw 3
> > > proposals at least:
> > >
> > > 1. From Michal, allocate node data for all possible nodes.
> > > https://lore.kernel.org/all/Ya89aqij6nMwJrIZ@dhcp22.suse.cz/T/#u
> > >
> > > 2. What this patch does. Proposed originally from
> > > https://lore.kernel.org/all/20211108202325.20304-1-amakhalov@vmware.com/T/#u
> >
> > Correct me if im wrong, but isn't that a different caller? This patch fixes the
> > issue in expand_one_shrinker_info.
>
> Yes, different caller, but same approach. The cons with this approach

And the same underlying problem.

> is we have to fix all the places. It seems Michal and David are not
> fans for this approach IIRC.
>
> >
> > > 3. From David, fix in node_zonelist().
> > > https://lore.kernel.org/all/51c65635-1dae-6ba4-daf9-db9df0ec35d8@redhat.com/T/#u
> > >
> > >>
> > >>> --- a/mm/vmscan.c
> > >>> +++ b/mm/vmscan.c
> > >>> @@ -222,13 +222,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> > >>>       int size = map_size + defer_size;
> > >>>
> > >>>       for_each_node(nid) {
> > >>> +             int tmp = nid;
> > >>
> > >> Not `tmp', please.  Better to use an identifier which explains the
> > >> variable's use.  target_nid?
> > >>
> > >> And a newline after defining locals, please.
> > >>
> > >>>               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))
> > >>
> > >> s/if(/if (/
> > >>
> > >>> +                     tmp = numa_mem_id();
> > >>> +             new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, tmp);
> > >>>               if (!new)
> > >>>                       return -ENOMEM;
> > >>>
> > >>
> > >> And a code comment fully explaining what's going on here?
> > >
> >