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Date: Thu, 18 Apr 2024 15:09:20 +0200
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Subject: Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and
 !skeys KVM guests
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To: David Hildenbrand <david@redhat.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Heiko Carstens
 <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Sven Schnelle <svens@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org
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 <a6a4b284-e21b-4a04-88d1-7402eb5a08ef@redhat.com>
From: Christian Borntraeger <borntraeger@linux.ibm.com>
In-Reply-To: <a6a4b284-e21b-4a04-88d1-7402eb5a08ef@redhat.com>
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Am 16.04.24 um 15:41 schrieb David Hildenbrand:
> On 16.04.24 14:02, Christian Borntraeger wrote:
>>
>>
>> Am 16.04.24 um 08:37 schrieb Alexander Gordeev:
>>
>>>> We could piggy-back on vm_fault_to_errno(). We could use
>>>> vm_fault_to_errno(rc, FOLL_HWPOISON), and only continue (retry) if the rc is 0 or
>>>> -EFAULT, otherwise fail with the returned error.
>>>>
>>>> But I'd do that as a follow up, and also use it in break_ksm() in the same fashion.
>>>
>>> @Christian, do you agree with this suggestion?
>>
>> I would need to look into that more closely to give a proper answer. In general I am ok
>> with this but I prefer to have more eyes on that.
>>   From what I can tell we should cover all the normal cases with our CI as soon as it hits
>> next. But maybe we should try to create/change a selftest to trigger these error cases?
> 
> If we find a shared zeropage we expect the next unsharing fault to succeed except:
> 
> (1) OOM, in which case we translate to -ENOMEM.
> 
> (2) Some obscure race with MADV_DONTNEED paired with concurrent truncate(), in which case we get an error, but if we look again, we will find the shared zeropage no longer mapped. (this is what break_ksm() describes)
> 
> (3) MCE while copying the page, which doesn't quite apply here.
> 
> For the time being, we only get shared zeropages in (a) anon mappings (b) MAP_PRIVATE shmem mappings via UFFDIO_ZEROPAGE. So (2) is hard or even impossible to trigger. (1) is hard to test as well, and (3) ...
> 
> No easy way to extend selftests that I can see.

Yes, lets just go forward.
> 
> If we repeatedly find a shared zeropage in a COW mapping and get an error from the unsharing fault, something else would be deeply flawed. So I'm not really worried about that, but I agree that having a more centralized check will make sense.