linux-kernel - Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests

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Date: Tue, 16 Apr 2024 09:05:12 +0200
From: David Hildenbrand <david@...hat.com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>, Heiko Carstens
 <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
 Sven Schnelle <svens@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and
 !skeys KVM guests

On 16.04.24 08:37, Alexander Gordeev wrote:
> On Mon, Apr 15, 2024 at 09:14:03PM +0200, David Hildenbrand wrote:
>>>> +retry:
>>>> +		rc = walk_page_range_vma(vma, addr, vma->vm_end,
>>>> +					 &find_zeropage_ops, &addr);
>>>> +		if (rc <= 0)
>>>> +			continue;
>>>
>>> So in case an error is returned for the last vma, __s390_unshare_zeropage()
>>> finishes with that error. By contrast, the error for a non-last vma would
>>> be ignored?
>>
>> Right, it looks a bit off. walk_page_range_vma() shouldn't fail
>> unless find_zeropage_pte_entry() would fail -- which would also be
>> very unexpected.
>>
>> To handle it cleanly in case we would ever get a weird zeropage where we
>> don't expect it, we should probably just exit early.
>>
>> Something like the following (not compiled, addressing the comment below):
> 
>> @@ -2618,7 +2618,8 @@ static int __s390_unshare_zeropages(struct mm_struct *mm)
>>   	struct vm_area_struct *vma;
>>   	VMA_ITERATOR(vmi, mm, 0);
>>   	unsigned long addr;
>> -	int rc;
>> +	vm_fault_t rc;
>> +	int zero_page;
> 
> I would use "fault" for mm faults (just like everywhere else handle_mm_fault() is
> called) and leave rc as is:
> 
> 	vm_fault_t fault;
> 	int rc;

Sure, let me know once discussion here stopped whether you want a v4 or 
can fix that up.

-- 
Cheers,

David / dhildenb