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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH] mm/tlb: Fix use_mm() vs TLB invalidate
Date:   Fri, 21 Feb 2020 11:22:16 -0800
Message-Id: <6A09F721-0AD9-4B86-AB3E-563A1CF5ABDE@amacapital.net>
References: <CAHk-=wi4uO+Djqr4Jc1TnCofwxUTuXHtgkgwnVX86q06UGV6DA@mail.gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Will Deacon <will@kernel.org>
In-Reply-To: <CAHk-=wi4uO+Djqr4Jc1TnCofwxUTuXHtgkgwnVX86q06UGV6DA@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Feb 21, 2020, at 11:19 AM, Linus Torvalds <torvalds@linux-foundation.or=
g> wrote:
>=20
> =EF=BB=BFOn Fri, Feb 21, 2020 at 3:11 AM Peter Zijlstra <peterz@infradead.=
org> wrote:
>>=20
>> +       BUG_ON(!(tsk->flags & PF_KTHREAD));
>> +       BUG_ON(tsk->mm !=3D NULL);
>=20
> Stop this craziness.
>=20
> There is absolutely ZERO excuse for this kind of garbage.
>=20
> Making this a BUG_ON() will just cause all the possible debugging info
> to be thrown away and lost, and you often have a dead machine.
>=20
> For absolutely no good reason.
>=20
> Make it a WARN_ON_ONCE(). If it triggers, everything works the way it
> always did, but we get notified.
>=20
> Stop with the stupid crazy BUG_ON() crap already. It is actively _bad_
> for debugging.
>=20
> =20

In this particular case, if we actually flub this, we are very likely to cau=
se data corruption =E2=80=94 we=E2=80=99re about to do user access with the w=
rong mm.

So I suppose we could switch to init_mm and carry on. *Something* will crash=
, but it probably won=E2=80=99t corrupt data or take down the machine.=