linux-kernel - Re: [PATCH v2] mm/mempolicy: do not allow illegal MPOL_F_NUMA_BALANCING

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Date:   Sat, 2 Oct 2021 18:15:58 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        syzbot <syzkaller@...glegroups.com>,
        "Huang, Ying" <ying.huang@...el.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2] mm/mempolicy: do not allow illegal
 MPOL_F_NUMA_BALANCING | MPOL_LOCAL in mbind()

On Fri, Oct 01, 2021 at 04:37:40PM -0700, Eric Dumazet wrote:
> > > Issue came with commit bda420b98505 ("numa balancing: migrate on
> > > fault among multiple bound nodes")
> >
> > No cc:stable?  What's the worst-case user-visible impact here?
> 
> I added the more precise tag  :  Fixes: bda420b98505 ("numa balancing:
> migrate on fault among multiple bound nodes")
> I only put Fixes: tag, so that stable teams can use their automation just fine.
> 
> worst-case impact, I am not sure if any application ever used this
> undocumented combinations of flags ?
> Also, it is generally advised that accessing garbage values has
> undocumented behavior.
> A host could for example crash (it certainly does with KMSAN)

mm has special stable rules; fixes only get backported if explicitly
requested instead of automatically like most of the rest of the kernel.