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

From: Andrew Morton <akpm@linux-foundation.org>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: Borislav Petkov <bp@suse.de>, Dave Hansen <dave.hansen@intel.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	"Elliott, Robert (Persistent Memory)" <elliott@hpe.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH-resend] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages
Date: Thu, 17 Aug 2017 15:09:42 -0700	[thread overview]
Message-ID: <20170817150942.017f87537b6cbb48e9cfc082@linux-foundation.org> (raw)
In-Reply-To: <20170816171803.28342-1-tony.luck@intel.com>

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