linux-kernel - Re: Crash in MM code in v4.4.y, v4.9.y with TRANSPARENT

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Date:   Fri, 17 Aug 2018 17:44:08 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        the arch/x86 maintainers <x86@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Crash in MM code in v4.4.y, v4.9.y with TRANSPARENT_HUGEPAGE
 enabled

On 08/17/2018 05:25 PM, Linus Torvalds wrote:
> On Fri, Aug 17, 2018 at 3:27 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> [    6.649970] random: crng init done
>> [    6.689002] BUG: unable to handle kernel paging request at ffffeafffa1a0020
> 
> Hmm. Lots of bits set.
> 
>> [    6.689082] RIP: 0010:[<ffffffff8116ba10>]  [<ffffffff8116ba10>] page_remove_rmap+0x10/0x230
>> [    6.689082] RSP: 0018:ffffc900007abc18  EFLAGS: 00000296
>> [    6.689082] RAX: ffffea0005e58000 RBX: ffffeafffa1a0000 RCX: 0000000020200000
>> [    6.689082] RDX: 00003fffffe00000 RSI: 0000000000000001 RDI: ffffeafffa1a0000
> 
> Is that RDX value the same value as PHYSICAL_PMD_PAGE_MASK?
> 
> If I did my math right, it would be, if your CPU has 46 bits of
> physical memory. Might that be the case?
> 
Yes.

> The reason I mention that is because we had the bug with spurious
> inversion of the zero pte/pmd, fixed by
> 
>    f19f5c49bbc3 ("x86/speculation/l1tf: Exempt zeroed PTEs from inversion")
> 
I applied that patch, but it didn't help. I get exactly the same crash and
traceback.

> and that would make a zeroed pmd entry be inverted by
> PHYSICAL_PMD_PAGE_MASK, and then you get odd garbage page pointers
> etc.
> 
> Maybe. I could have gotten the math wrong too, but it sounds like the
> register contents _potentially_ might match up with something like
> this, and then we'd zap a bogus hugepage because of some confusion.
> 
> Although then I'd have expected the bisection to hit
> "x86/speculation/l1tf: Invert all not present mappings" instead of the
> one you hit, so I don't know.
> 
> Plus I'd have expected the problem to have been in mainline too, and
> apparently it's just the 4.4 and 4.9 backports.
> 
Personally I suspect that something went wrong or is missing in the backport
from 4.14 to 4.9. 5-level paging was introduced in between, and thp support
was extended to support additional architectures. With all those changes,
it is easy to miss something. Only I have no idea what that might be.

Guenter