makedumpfile: support for newer kernels [v4.9 onwards]

[Atsushi Kumagai]

Hello Abhishek,

Thanks for your investigation.

>I was finally able to root cause the issue that I was facing in the
>kdump compressed output
>generated by makedumpfile.
>
>While debugging "write_kdump_pages_and_bitmap_cyclic"; I came to know that
>somehow "info->max_mapnr" was modified/ reduced.
>
>Seeing further, I could see that the "get_mem_map" function at the end
>[if (!is_xen_memory())]
>is attempting to change "info->max_mapnr" value based on "mem=" option.
>-- in particular commit ebe2fa3a5c1e2bfac3778c0fbeca4287a934cc58.
>And in my kernel command line, "mem=2G" was present, as I intended to
>take dump of 2G RAM,
>resulting into reduced "info->max_mapnr".

Basically, this adjusting info->max_mapnr must be harmless since
it just intends to ignore unused memory regions.
Did you find out why adjusting info->max_mapnr causes the problem ?

>So, I commented out this portion and confirmed that "info->max_mapnr"
>remains same
>throughout. And I was able to get kdump compressed vmcore process-able
>by crash utility.

This sounds like there are some valid data beyond the adjusted info->max_mapnr,
is that true ?

Thanks,
Atsushi Kumagai

>Just to summarize:
>Following was the problem statement: crash was giving "seek error"
>while makedumpfile generated kdump-compressed vmcore was getting analyzed.
>Details:
>> The output generated by makedumpfile gets processed by crash utility
>> if I use following command:
>> makedumpfile -E /proc/vmcore vmcore
>>
>> But, the output generated by following command (which I used while
>> reporting the issue) gives the "seek error" while being processed by
>> crash.
>> makedumpfile /proc/vmcore vmcore
>>
>> (As a result?), output from following command gives "seek error" as
>> well while being processed by crash:
>> makedumpfile -c /proc/vmcore vmcore
>
>Following is my setup details:
>>>> Kernel config:
>>>> ARM64_VA_BITS_48 [=y]
>>>> ARM64_PAGE_SHIFT [=12]
>>>> RANDOMIZE_BASE [=n]
>>>> ARM64_4K_PAGES [=y]
>
>> kernel: 4.12- rc5
>> yocto based rootfs: Pyro[2.3] which has kexec 2.0.14 and makedumpfile 1.6.1.
>> crash: 7.1.9++ [Head: 4d517ad28acd845fe6e91360e645cf0446a4757b]
>
>
>Thanks,
>Abhishek