Re: [PATCH 1/1] x86/mm: Forbid the zero page once it has uncorrectable errors

From: David Hildenbrand <david@redhat.com>
To: "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>,
	"Hansen, Dave" <dave.hansen@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"Luck, Tony" <tony.luck@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Lutomirski, Andy" <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Cc: "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
	"open list:HWPOISON MEMORY FAILURE HANDLING" <linux-mm@kvack.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] x86/mm: Forbid the zero page once it has uncorrectable errors
Date: Thu, 21 Apr 2022 10:50:44 +0200	[thread overview]
Message-ID: <022fe906-054d-43b4-14d4-a4c1cb7527af@redhat.com> (raw)
In-Reply-To: <DM8PR11MB566941C063EA44929147035E89F49@DM8PR11MB5669.namprd11.prod.outlook.com>

On 21.04.22 09:53, Zhuo, Qiuxu wrote:
>> From: Hansen, Dave <dave.hansen@intel.com>
>> ...
>> Subject: Re: [PATCH 1/1] x86/mm: Forbid the zero page once it has
>> uncorrectable errors
>> ...
>> There are lots of pages which are entirely fatal if they have uncorrectable errors.
>> On my laptop, if there were an error, there is a 0.00000596% chance it will be in
>> the zero page.
>>
>> Why is this worth special casing this one page?
> 
> Hi Dave,
> 
>    Yes, this is a rare problem. Just feel that the fix is simple, so post it here to see whether you'll consider it 😊.

Just some background information.

mm_forbids_zeropage() exists for the sole purpose of s390x/kvm not being
able to use the shared zeropage for a KVM guest because the storage keys
associated with the shared zeropage could result in trouble. So
s390x/kvm has to make sure that no shared zeropage
is mapped into the process.

See fa41ba0d08de ("s390/mm: avoid empty zero pages for KVM guests to
avoid postcopy hangs") for details.

@Christian

a) with keyless guests we could actually use the shared zeropage because
the guest cannot possibly enable storage keys, correct?

b) Why is there no mm_forbids_zeropage() check in mfill_zeropage_pte()?
Maybe I'm missing something, but looks like we can still place the
shared zeropage into a KVM guest via uffd.

In general, there are more place that will use the shared zeropage, most
notably, fs/dax.c  will place the shared zeropage for holes and would
still use it on x86-64. IIRC, s390x doesn't use it.

/proc/vmcore will map the zeropage to user space for areas that are not
RAM, so you could still stumble over it there and trigger a MCE.

Last but not least, the huge shared zeropage would suffer from similar
problems.

Also, I wonder if the generic code change in mm/memory-failure.c is
correct as it touches common code and you only mess with the x86
zeropage. But I did not look into the details.

So the code here at least isn't complete. So I'm not convinced this
change is worth it.

-- 
Thanks,

David / dhildenb