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: <ZxDetq73hETPMjln@li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com>
Date: Thu, 17 Oct 2024 11:53:58 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: David Hildenbrand <david@...hat.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

> > 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.

Yes, I read this and that exactly what causes my confusion. In this wording it
sounds like SCLP *or* other methods are fine to use. But then you use SCLP or
DIAGNOSE 260, but not memory scanning. So I am still confused ;)

> 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.

But what is wrong with the legacy way of scanning?

> > > +	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?

Yes, I get your approach.

> Thanks for the review!

Thanks!

> -- 
> Cheers,
> 
> David / dhildenb