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

[Baoquan He <bhe@redhat.com>]

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. 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
============


======= config items extracted from this patchset====
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

config VIRTIO_MEM
	depends on X86_64 || ARM64 || RISCV
         ~~~~~ I don't get why VIRTIO_MEM dones't depend on S390 if
               s390 need PROC_VMCORE_DEVICE_RAM. 
        ......
        select PROVIDE_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE

config S390
        select NEED_PROC_VMCORE_DEVICE_RAM if PROC_VMCORE
=================================================