linux-kernel - Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory devices

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Message-ID: <04d5169f-3289-4aac-abca-90b20ad4e9c9@redhat.com>
Date: Thu, 17 Oct 2024 10:19:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-s390@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-doc@...r.kernel.org, kvm@...r.kernel.org,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Thomas Huth <thuth@...hat.com>,
 Cornelia Huck <cohuck@...hat.com>, Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>, Mario Casquero <mcasquer@...hat.com>
Subject: Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to
 support QEMU/KVM memory devices

On 17.10.24 09:36, Alexander Gordeev wrote:
> On Mon, Oct 14, 2024 at 04:46:16PM +0200, David Hildenbrand wrote:
> 
> Hi David!

Hi Alexander!

> 
>> @@ -157,7 +189,9 @@ unsigned long detect_max_physmem_end(void)
>>   {
>>   	unsigned long max_physmem_end = 0;
>>   
>> -	if (!sclp_early_get_memsize(&max_physmem_end)) {
>> +	if (!diag500_storage_limit(&max_physmem_end)) {
>> +		physmem_info.info_source = MEM_DETECT_DIAG500_STOR_LIMIT;
>> +	} else if (!sclp_early_get_memsize(&max_physmem_end)) {
>>   		physmem_info.info_source = MEM_DETECT_SCLP_READ_INFO;
>>   	} else {
>>   		max_physmem_end = search_mem_end();
>> @@ -170,11 +204,17 @@ void detect_physmem_online_ranges(unsigned long max_physmem_end)
>>   {
>>   	if (!sclp_early_read_storage_info()) {
>>   		physmem_info.info_source = MEM_DETECT_SCLP_STOR_INFO;
>> +		return;
>>   	} else if (!diag260()) {
>>   		physmem_info.info_source = MEM_DETECT_DIAG260;
>> -	} else if (max_physmem_end) {
>> -		add_physmem_online_range(0, max_physmem_end);
>> +		return;
>> +	} else if (physmem_info.info_source == MEM_DETECT_DIAG500_STOR_LIMIT) {
>> +		max_physmem_end = 0;
>> +		if (!sclp_early_get_memsize(&max_physmem_end))
>> +			physmem_info.info_source = MEM_DETECT_SCLP_READ_INFO;
> 
> Why search_mem_end() is not tried in case sclp_early_get_memsize() failed?

Patch #3 documents that:

+    The storage limit does not indicate currently usable storage, it may
+    include holes, standby storage and areas reserved for other means, such
+    as memory hotplug or virtio-mem devices. Other interfaces for detecting
+    actually usable storage, such as SCLP, must be used in conjunction with
+    this subfunction.

If SCLP would fail, something would be seriously wrong and we should just crash
instead of trying to fallback to the legacy way of scanning.

> 
>>   	}
>> +	if (max_physmem_end)
>> +		add_physmem_online_range(0, max_physmem_end);
>>   }
>>   
>>   void physmem_set_usable_limit(unsigned long limit)
>> diff --git a/arch/s390/include/asm/physmem_info.h b/arch/s390/include/asm/physmem_info.h
>> index f45cfc8bc233..51b68a43e195 100644
>> --- a/arch/s390/include/asm/physmem_info.h
>> +++ b/arch/s390/include/asm/physmem_info.h
>> @@ -9,6 +9,7 @@ enum physmem_info_source {
>>   	MEM_DETECT_NONE = 0,
>>   	MEM_DETECT_SCLP_STOR_INFO,
>>   	MEM_DETECT_DIAG260,
>> +	MEM_DETECT_DIAG500_STOR_LIMIT,
>>   	MEM_DETECT_SCLP_READ_INFO,
>>   	MEM_DETECT_BIN_SEARCH
>>   };
>> @@ -107,6 +108,8 @@ static inline const char *get_physmem_info_source(void)
>>   		return "sclp storage info";
>>   	case MEM_DETECT_DIAG260:
>>   		return "diag260";
>> +	case MEM_DETECT_DIAG500_STOR_LIMIT:
>> +		return "diag500 storage limit";
> 
> AFAIU you want to always override MEM_DETECT_DIAG500_STOR_LIMIT method
> with an online memory detection method. In that case this code is dead.

Not in the above case, pathological case above where something went wrong
during sclp_early_get_memsize(). In that scenario, die_oom() would indicate
that there are no memory ranges but that "diag500 storage limit" worked.

Does that make sense?

Thanks for the review!

-- 
Cheers,

David / dhildenb