linux-kernel - Re: [RFC PATCH 2/2] mm/vmscan.c: Prevent allocating shrinker

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Date:   Tue, 7 Dec 2021 11:55:17 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>,
        Nico Pache <npache@...hat.com>,
        Rafael Aquini <aquini@...hat.com>
Cc:     Yang Shi <shy828301@...il.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>, raquini@...hat.com
Subject: Re: [RFC PATCH 2/2] mm/vmscan.c: Prevent allocating shrinker_info on
 offlined nodes

On Mon 06-12-21 20:01:34, David Hildenbrand wrote:
[...]
> 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 ...

I do dislike both but if I were to chose which to chose between the two
then 2 is surely more targeted. We really do not want to spread this
into bulk/pcp or whatever other allocator there is. The problem is that
somebody might still try to access NODE_DATA (e.g. via a helper that
hides that fact).

Anyway, I am not sure whether authors of the patch can reproduce the
problem and whether they can run a testing code on their machine. If yes
it would be great to try with http://lkml.kernel.org/r/Ya89aqij6nMwJrIZ@dhcp22.suse.cz
that I have just sent.

> [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

-- 
Michal Hocko
SUSE Labs