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

Alexander Gordeev
2024-10-17T09:54:13Z

> > 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