linux-kernel - Re: [PATCH-resend] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

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Message-Id: <20170817150942.017f87537b6cbb48e9cfc082@linux-foundation.org>
Date:   Thu, 17 Aug 2017 15:09:42 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...e.de>, Dave Hansen <dave.hansen@...el.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Elliott, Robert (Persistent Memory)" <elliott@....com>,
        x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH-resend] mm/hwpoison: Clear PRESENT bit for kernel 1:1
 mappings of poison pages

On Wed, 16 Aug 2017 10:18:03 -0700 "Luck, Tony" <tony.luck@...el.com> wrote:

> Speculative processor accesses may reference any memory that has a
> valid page table entry.  While a speculative access won't generate
> a machine check, it will log the error in a machine check bank. That
> could cause escalation of a subsequent error since the overflow bit
> will be then set in the machine check bank status register.
> 
> Code has to be double-plus-tricky to avoid mentioning the 1:1 virtual
> address of the page we want to map out otherwise we may trigger the
> very problem we are trying to avoid.  We use a non-canonical address
> that passes through the usual Linux table walking code to get to the
> same "pte".
> 
> Thanks to Dave Hansen for reviewing several iterations of this.

It's unclear (to lil ole me) what the end-user-visible effects of this
are.

Could we please have a description of that?  So a) people can
understand your decision to cc:stable and b) people whose kernels are
misbehaving can use your description to decide whether your patch might
fix the issue their users are reporting.

Thanks.