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Date:   Thu, 6 Aug 2020 15:10:34 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>
Subject: Re: [GIT PULL] x86/mm changes for v5.9
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Aug 5, 2020 at 4:03 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > The commit 8bb9bf242d1f ("x86/mm/64: Do not sync vmalloc/ioremap
> > mappings") causes the OOPS below, in Linus' tree and in linux-next,
> > unearthed by my CI on <https://www.wireguard.com/build-status/>.
> > Bisecting reveals 8bb9bf242d1f, and reverting this makes the OOPS go
> > away.
> 
> The oops happens early in the function, and the "Code:" line actually
> gets almost the whole function prologue in it (missing first two bytes
> are probably "push %rbp"):
> 
>    0: 41 56                push   %r14
>    2: 41 55                push   %r13
>    4: 41 54                push   %r12
>    6: 55                    push   %rbp
>    7: 48 89 f5              mov    %rsi,%rbp
>    a: 53                    push   %rbx
>    b: 48 89 fb              mov    %rdi,%rbx
>    e: 48 83 ec 08          sub    $0x8,%rsp
>   12: 48 8b 06              mov    (%rsi),%rax
>   15: 4c 8b 67 40          mov    0x40(%rdi),%r12
>   19: 49 89 c6              mov    %rax,%r14
>   1c: 45 30 f6              xor    %r14b,%r14b
>   1f: a8 04                test   $0x4,%al
>   21: b8 00 00 00 00        mov    $0x0,%eax
>   26: 4c 0f 44 f0          cmove  %rax,%r14
>   2a:* 49 8b 46 08          mov    0x8(%r14),%rax <-- trapping instruction
> 
> 
> > BUG: unable to handle page fault for address: ffffe8ffffd00608
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> 
> Yeah, missing page table because it wasn't copied.
> 
> Presumably because that kthread is using the active_mm of some random
> user space process that didn't get sync'ed.
> 
> And the sync_global_pgds() may have ended up being sufficient
> synchronization with whoever allocated thigns, even if it wasn't about
> the TLB contents themselves.
> 
> So apparently the "the page-table pages are all pre-allocated now" is
> simply not true. Joerg?
> 
> Unless somebody can figure this out fairly quickly, I think it should
> just be reverted.

Agreed. Joerg?

Thanks,

	Ingo