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Subject: Re: [RFC V2 3/3] s390/mm: Define arch_get_mappable_range()
To:     David Hildenbrand <david@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, Vasily Gorbik <gor@linux.ibm.com>
References: <1606706992-26656-1-git-send-email-anshuman.khandual@arm.com>
 <1606706992-26656-4-git-send-email-anshuman.khandual@arm.com>
 <20201202203233.GB11274@osiris>
 <24905c32-f6c1-97a0-000f-f822b9870ea5@arm.com> <20201203115133.GB9994@osiris>
 <4d6c9ec4-f1be-46b9-5d67-5c53f5afedc5@redhat.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <62c60c9e-20d6-25bd-94d0-78bfed0f2476@arm.com>
Date:   Mon, 7 Dec 2020 10:08:49 +0530
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On 12/3/20 5:31 PM, David Hildenbrand wrote:
> On 03.12.20 12:51, Heiko Carstens wrote:
>> On Thu, Dec 03, 2020 at 06:03:00AM +0530, Anshuman Khandual wrote:
>>>>> diff --git a/arch/s390/mm/extmem.c b/arch/s390/mm/extmem.c
>>>>> index 5060956b8e7d..cc055a78f7b6 100644
>>>>> --- a/arch/s390/mm/extmem.c
>>>>> +++ b/arch/s390/mm/extmem.c
>>>>> @@ -337,6 +337,11 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long
>>>>>  		goto out_free_resource;
>>>>>  	}
>>>>>  
>>>>> +	if (seg->end + 1 > VMEM_MAX_PHYS || seg->end + 1 < seg->start_addr) {
>>>>> +		rc = -ERANGE;
>>>>> +		goto out_resource;
>>>>> +	}
>>>>> +
>>>>>  	rc = vmem_add_mapping(seg->start_addr, seg->end - seg->start_addr + 1);
>>>>>  	if (rc)
>>>>>  		goto out_resource;
>>>>> diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c
>>>>> index b239f2ba93b0..06dddcc0ce06 100644
>>>>> --- a/arch/s390/mm/vmem.c
>>>>> +++ b/arch/s390/mm/vmem.c
>>>>> @@ -532,14 +532,19 @@ void vmem_remove_mapping(unsigned long start, unsigned long size)
>>>>>  	mutex_unlock(&vmem_mutex);
>>>>>  }
>>>>>  
>>>>> +struct range arch_get_mappable_range(void)
>>>>> +{
>>>>> +	struct range memhp_range;
>>>>> +
>>>>> +	memhp_range.start = 0;
>>>>> +	memhp_range.end =  VMEM_MAX_PHYS;
>>>>> +	return memhp_range;
>>>>> +}
>>>>> +
>>>>>  int vmem_add_mapping(unsigned long start, unsigned long size)
>>>>>  {
>>>>>  	int ret;
>>>>>  
>>>>> -	if (start + size > VMEM_MAX_PHYS ||
>>>>> -	    start + size < start)
>>>>> -		return -ERANGE;
>>>>> -
>>>>
>>>> I really fail to see how this could be considered an improvement for
>>>> s390. Especially I do not like that the (central) range check is now
>>>> moved to the caller (__segment_load). Which would mean potential
>>>> additional future callers would have to duplicate that code as well.
>>>
>>> The physical range check is being moved to the generic hotplug code
>>> via arch_get_mappable_range() instead, making the existing check in
>>> vmem_add_mapping() redundant. Dropping the check there necessitates
>>> adding back a similar check in __segment_load(). Otherwise there
>>> will be a loss of functionality in terms of range check.
>>>
>>> May be we could just keep this existing check in vmem_add_mapping()
>>> as well in order avoid this movement but then it would be redundant
>>> check in every hotplug path.
>>>
>>> So I guess the choice is to either have redundant range checks in
>>> all hotplug paths or future internal callers of vmem_add_mapping()
>>> take care of the range check.
>>
>> The problem I have with this current approach from an architecture
>> perspective: we end up having two completely different methods which
>> are doing the same and must be kept in sync. This might be obvious
>> looking at this patch, but I'm sure this will go out-of-sync (aka
>> broken) sooner or later.
> 
> Exactly, there should be one function only that was the whole idea of
> arch_get_mappable_range().
> 
>>
>> Therefore I would really like to see a single method to do the range
>> checking. Maybe you could add a callback into architecture code, so
>> that such an architecture specific function could also be used
>> elsewhere. Dunno.
>>
> 
> I think we can just switch to using "memhp_range_allowed()" here then
> after implementing arch_get_mappable_range().
> 
> Doesn't hurt to double check in vmem_add_mapping() - especially to keep
> extmem working without changes. At least for callers of memory hotplug
> it's then clear which values actually won't fail deep down in arch code.

But there is a small problem here. memhp_range_allowed() is now defined
and available with CONFIG_MEMORY_HOTPLUG where as vmem_add_mapping() and
__segment_load() are generally available without any config dependency.
So if CONFIG_MEMORY_HOTPLUG is not enabled there will be a build failure
in vmem_add_mapping() for memhp_range_allowed() symbol.

We could just move VM_BUG_ON(!memhp_range_allowed(start, size, 1)) check
from vmem_add_mapping() to arch_add_memory() like on arm64 platform. But
then __segment_load() would need that additional new check to compensate
as proposed earlier.

Also leaving vmem_add_mapping() and __segment_load() unchanged will cause
the address range check to be called three times on the hotplug path i.e

1. register_memory_resource()
2. arch_add_memory()
3. vmem_add_mapping()

Moving memhp_range_allowed() check inside arch_add_memory() seems better
and consistent with arm64. Also in the future, any platform which choose
to override arch_get_mappable() will have this additional VM_BUG_ON() in
their arch_add_memory().