From: Christian Borntraeger <borntraeger@linux.ibm.com>
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
Subject: Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests
Date: Thu, 18 Apr 2024 15:09:20 +0200 [thread overview]
Message-ID: <453afb13-c7e3-4156-9dbb-c6317503c715@linux.ibm.com> (raw)
In-Reply-To: <a6a4b284-e21b-4a04-88d1-7402eb5a08ef@redhat.com>
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.
next prev parent reply other threads:[~2024-04-18 13:09 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-11 16:14 [PATCH v3 0/2] s390/mm: shared zeropage + KVM fixes David Hildenbrand
2024-04-11 16:14 ` [PATCH v3 1/2] mm/userfaultfd: don't place zeropages when zeropages are disallowed David Hildenbrand
2024-04-11 16:14 ` [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests David Hildenbrand
2024-04-11 16:37 ` Alexander Gordeev
2024-04-11 21:09 ` David Hildenbrand
2024-04-15 11:49 ` Christian Borntraeger
2024-04-15 13:14 ` Alexander Gordeev
2024-04-15 18:24 ` Alexander Gordeev
2024-04-15 19:14 ` David Hildenbrand
2024-04-16 6:37 ` Alexander Gordeev
2024-04-16 7:05 ` David Hildenbrand
2024-04-16 12:02 ` Christian Borntraeger
2024-04-16 13:41 ` David Hildenbrand
2024-04-18 13:09 ` Christian Borntraeger [this message]
2024-04-11 21:28 ` [PATCH v3 0/2] s390/mm: shared zeropage + KVM fixes Andrew Morton
2024-04-11 21:56 ` David Hildenbrand
2024-04-12 13:25 ` Alexander Gordeev
2024-04-17 12:46 ` Alexander Gordeev
2024-04-17 12:47 ` David Hildenbrand
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