netdev - Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf

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Date:   Thu, 09 Mar 2017 19:31:03 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
        ast@...com, the arch/x86 maintainers <x86@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request
 at 0000a7cf

On 03/09/2017 07:15 PM, Linus Torvalds wrote:
> On Thu, Mar 9, 2017 at 10:10 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Very odd. We should always have PGE (0x0080) set in cr4 (if the CPU
>> supports it).
>
> Daniel, do you see the code in probe_page_size_mask() triggering?
>
>          /* Enable PGE if available */
>          if (boot_cpu_has(X86_FEATURE_PGE)) {
>                  cr4_set_bits_and_update_boot(X86_CR4_PGE);
>                  __supported_pte_mask |= _PAGE_GLOBAL;

We do have boot_cpu_has(X86_FEATURE_PGE) and go indeed into this
branch here. So it seems something must be clearing it later, hmm.

>          } else
>                  __supported_pte_mask &= ~_PAGE_GLOBAL;
>
> but maybe there's something wrong with the percpu cr4 caching?
>
>                 Linus
>