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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 21 Jul 2018 14:06:52 -0700
Message-ID: <CA+55aFyjFjdeAwAOu-WsE=yk_+cAaaOvp4-DSEHDaWG+1g_BSA@mail.gmail.com>
Subject: Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Joerg Roedel <jroedel@suse.de>,
        Andrew Lutomirski <luto@kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
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On Fri, Jul 20, 2018 at 3:20 PM Andy Lutomirski <luto@amacapital.net> wrote=
:
> Thanks for digging!  The problem was presumably that vmalloc_fault() will=
 IRET and re-enable NMIs on the way out.
>  But we=E2=80=99ve supported page faults on user memory in NMI handlers o=
n 32-bit and 64-bit for quite a while, and it=E2=80=99s fine now.
>
> I would remove the warning, re-test, and revert the other patch.

Agreed. I don't think we have any issues with page faults during NMI
any more.  Afaik the kprobe people depend on it.

That said, 64-bit mode has that scary PV-op case
(arch_flush_lazy_mmu_mode). Being PV mode, I can't find it in myself
to worry about it, I'm assuming it's ok.

                Linus