linux-kernel - Re: [GIT PULL] x86/mm changes for v5.9

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Date:   Thu, 6 Aug 2020 12:42:23 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [GIT PULL] x86/mm changes for v5.9

On Thu, Aug 6, 2020 at 12:23 PM Joerg Roedel <jroedel@...e.de> wrote:
>
> Yes, that's the best for now. My gut feeling is that the fault Jason is
> seeing didn't happen on a vmalloc address, but I can't prove that yet.

No, it's definitely fairly high in the vmalloc space. Look at the
faulting address:

   BUG: unable to handle page fault for address: ffffe8ffffd00608

and the code sequence is this:

>   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

that admittedly odd sequence is get_work_pwq(work)

And then the faulting instruction is:

>   2a:* 49 8b 46 08          mov    0x8(%r14),%rax <-- trapping instruction

and this is the "->wq" dereference.

So it's the pwq->wq that traps, with 'pwq' being the trapping base
pointer, and clearly being in the vmalloc space.

I think pwq may a percpu allocation, so not _directly_ vmalloc().
Adding Tejun to the cc in case he can clarify ("No, silly Linus, it's
allocated here..").

                Linus