Re: [PATCH v1 00/11] fs/proc/vmcore: kdump support for virtio-mem on s390

[David Hildenbrand <david@redhat.com>]

On 15.11.24 09:46, Baoquan He wrote:
> On 10/25/24 at 05:11pm, David Hildenbrand wrote:
>> This is based on "[PATCH v3 0/7] virtio-mem: s390 support" [1], which adds
>> virtio-mem support on s390.
>>
>> The only "different than everything else" thing about virtio-mem on s390
>> is kdump: The crash (2nd) kernel allocates+prepares the elfcore hdr
>> during fs_init()->vmcore_init()->elfcorehdr_alloc(). Consequently, the
>> crash kernel must detect memory ranges of the crashed/panicked kernel to
>> include via PT_LOAD in the vmcore.
>>
>> On other architectures, all RAM regions (boot + hotplugged) can easily be
>> observed on the old (to crash) kernel (e.g., using /proc/iomem) to create
>> the elfcore hdr.
>>
>> On s390, information about "ordinary" memory (heh, "storage") can be
>> obtained by querying the hypervisor/ultravisor via SCLP/diag260, and
>> that information is stored early during boot in the "physmem" memblock
>> data structure.
>>
>> But virtio-mem memory is always detected by as device driver, which is
>> usually build as a module. So in the crash kernel, this memory can only be
>                                         ~~~~~~~~~~~
>                                         Is it 1st kernel or 2nd kernel?
> Usually we call the 1st kernel as panicked kernel, crashed kernel, the
> 2nd kernel as kdump kernel.

It should have been called "kdump (2nd) kernel" here indeed.

>> properly detected once the virtio-mem driver started up.
>>
>> The virtio-mem driver already supports the "kdump mode", where it won't
>> hotplug any memory but instead queries the device to implement the
>> pfn_is_ram() callback, to avoid reading unplugged memory holes when reading
>> the vmcore.
>>
>> With this series, if the virtio-mem driver is included in the kdump
>> initrd -- which dracut already takes care of under Fedora/RHEL -- it will
>> now detect the device RAM ranges on s390 once it probes the devices, to add
>> them to the vmcore using the same callback mechanism we already have for
>> pfn_is_ram().
> 
> Do you mean on s390 virtio-mem memory region will be detected and added
> to vmcore in kdump kernel when virtio-mem driver is initialized? Not
> sure if I understand it correctly.

Yes exactly. In the kdump kernel, the driver gets probed and registers 
the vmcore callbacks. From there, we detect and add the device regions.

Thanks!

-- 
Cheers,

David / dhildenb