Re: [PATCH v1 07/11] fs/proc/vmcore: introduce PROC_VMCORE_DEVICE_RAM to detect device RAM ranges in 2nd kernel

[David Hildenbrand <david@redhat.com>]

On 20.11.24 15:05, Baoquan He wrote:
> On 11/20/24 at 11:48am, David Hildenbrand wrote:
>> On 20.11.24 11:13, Baoquan He wrote:
>>> On 10/25/24 at 05:11pm, David Hildenbrand wrote:
>>>> s390 allocates+prepares the elfcore hdr in the dump (2nd) kernel, not in
>>>> the crashed kernel.
>>>>
>>>> RAM provided by memory devices such as virtio-mem can only be detected
>>>> using the device driver; when vmcore_init() is called, these device
>>>> drivers are usually not loaded yet, or the devices did not get probed
>>>> yet. Consequently, on s390 these RAM ranges will not be included in
>>>> the crash dump, which makes the dump partially corrupt and is
>>>> unfortunate.
>>>>
>>>> Instead of deferring the vmcore_init() call, to an (unclear?) later point,
>>>> let's reuse the vmcore_cb infrastructure to obtain device RAM ranges as
>>>> the device drivers probe the device and get access to this information.
>>>>
>>>> Then, we'll add these ranges to the vmcore, adding more PT_LOAD
>>>> entries and updating the offsets+vmcore size.
>>>>
>>>> Use Kconfig tricks to include this code automatically only if (a) there is
>>>> a device driver compiled that implements the callback
>>>> (PROVIDE_PROC_VMCORE_DEVICE_RAM) and; (b) the architecture actually needs
>>>> this information (NEED_PROC_VMCORE_DEVICE_RAM).
>>>>
>>>> The current target use case is s390, which only creates an elf64
>>>> elfcore, so focusing on elf64 is sufficient.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>    fs/proc/Kconfig            |  25 ++++++
>>>>    fs/proc/vmcore.c           | 156 +++++++++++++++++++++++++++++++++++++
>>>>    include/linux/crash_dump.h |   9 +++
>>>>    3 files changed, 190 insertions(+)
>>>>
>>>> diff --git a/fs/proc/Kconfig b/fs/proc/Kconfig
>>>> index d80a1431ef7b..1e11de5f9380 100644
>>>> --- a/fs/proc/Kconfig
>>>> +++ b/fs/proc/Kconfig
>>>> @@ -61,6 +61,31 @@ config PROC_VMCORE_DEVICE_DUMP
>>>>    	  as ELF notes to /proc/vmcore. You can still disable device
>>>>    	  dump using the kernel command line option 'novmcoredd'.
>>>> +config PROVIDE_PROC_VMCORE_DEVICE_RAM
>>>> +	def_bool n
>>>> +
>>>> +config NEED_PROC_VMCORE_DEVICE_RAM
>>>> +	def_bool n
>>>> +
>>>> +config PROC_VMCORE_DEVICE_RAM
>>>> +	def_bool y
>>>> +	depends on PROC_VMCORE
>>>> +	depends on NEED_PROC_VMCORE_DEVICE_RAM
>>>> +	depends on PROVIDE_PROC_VMCORE_DEVICE_RAM
>>>
>>> Kconfig item is always a thing I need learn to master.
>>
>> Yes, it's usually a struggle to get it right. It took me a couple of
>> iterations to get to this point :)
>>
>>> When I checked
>>> this part, I have to write them down to deliberate. I am wondering if
>>> below 'simple version' works too and more understandable. Please help
>>> point out what I have missed.
>>>
>>> ===========simple version======
>>> config PROC_VMCORE_DEVICE_RAM
>>>           def_bool y
>>>           depends on PROC_VMCORE && VIRTIO_MEM
>>>           depends on NEED_PROC_VMCORE_DEVICE_RAM
>>>
>>> config S390
>>>           select NEED_PROC_VMCORE_DEVICE_RAM
>>> ============
> 
> Sorry, things written down didn't correctly reflect them in my mind.
> 
> ===========simple version======
> fs/proc/Kconfig:
> config PROC_VMCORE_DEVICE_RAM
>          def_bool y
>          depends on PROC_VMCORE && VIRTIO_MEM
>          depends on NEED_PROC_VMCORE_DEVICE_RAM
> 
> arch/s390/Kconfig:
> config NEED_PROC_VMCORE_DEVICE_RAM
>          def y
> ==================================

That would work, but I don't completely like it.

(a) I want s390x to select NEED_PROC_VMCORE_DEVICE_RAM instead. Staring 
at a bunch of similar cases (git grep "config NEED" | grep Kconfig, git 
grep "config ARCH_WANTS" | grep Kconfig), "select" is the common way to 
do it.

So unless there is a pretty good reason, I'll keep 
NEED_PROC_VMCORE_DEVICE_RAM as is.


(b) In the context of this patch, "depends on VIRTIO_MEM" does not make 
sense. We could have an intermediate:

config PROC_VMCORE_DEVICE_RAM
          def_bool n
          depends on PROC_VMCORE
          depends on NEED_PROC_VMCORE_DEVICE_RAM

And change that with VIRTIO_MEM support in the relevant patch.


I faintly remember that we try avoiding such dependencies and prefer 
selecting Kconfigs instead. Just look at the SPLIT_PTE_PTLOCKS mess we 
still have to clean up. But as we don't expect that many providers for 
now, I don't care.

Thanks!

-- 
Cheers,

David / dhildenb