Re: [PATCH v2 00/12] fs/proc/vmcore: kdump support for virtio-mem on s390

David Hildenbrand
2025-01-08T12:14:33Z

On 08.01.25 13:10, Heiko Carstens wrote:
> On Wed, Jan 08, 2025 at 07:04:23AM -0500, Michael S. Tsirkin wrote:
>> On Wed, Dec 04, 2024 at 01:54:31PM +0100, David Hildenbrand wrote:
>>> 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
>>> kdump kernel must detect memory ranges of the crashed 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
>>> 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().
>>>
>>> To add these device RAM ranges to the vmcore ("patch the vmcore"), we will
>>> add new PT_LOAD entries that describe these memory ranges, and update
>>> all offsets vmcore size so it is all consistent.
>>>
>>> My testing when creating+analyzing crash dumps with hotplugged virtio-mem
>>> memory (incl. holes) did not reveal any surprises.
>>>
>>> Patch #1 -- #7 are vmcore preparations and cleanups
>>> Patch #8 adds the infrastructure for drivers to report device RAM
>>> Patch #9 + #10 are virtio-mem preparations
>>> Patch #11 implements virtio-mem support to report device RAM
>>> Patch #12 activates it for s390, implementing a new function to fill
>>> PT_LOAD entry for device RAM
>>
>> Who is merging this?
>> virtio parts:
>>
>> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>
> I guess this series should go via Andrew Morton. Andrew?
>
> Acked-by: Heiko Carstens <hca@linux.ibm.com> # s390
>

Yes, it's in mm-unstable already for quite a while.

Thanks for the acks!

--
Cheers,

David / dhildenb