Re: [PATCH v2 1/7] s390/kdump: implement is_kdump_kernel()

[David Hildenbrand <david@redhat.com>]

Am 21.10.24 um 14:46 schrieb Alexander Egorenkov:
> Hi David,
> 
> David Hildenbrand <david@redhat.com> writes:
> 
>> Makes sense, so it boils down to either
>>
>> bool is_kdump_kernel(void)
>> {
>>            return oldmem_data.start;
>> }
>>
>> Which means is_kdump_kernel() can be "false" even though /proc/vmcore is
>> available or
>>
>> bool is_kdump_kernel(void)
>> {
>>            return dump_available();
>> }
>>
>> Which means is_kdump_kernel() can never be "false" if /proc/vmcore is
>> available. There is the chance of is_kdump_kernel() being "true" if
>> "elfcorehdr_alloc()" fails with -ENODEV.

Thanks for having another look!

> 
> Do you consider is_kdump_kernel() returning "true" in case of zfcpdump or
> nvme/eckd+ldipl dump (also called NGDump) okay ? Because
> dump_available() would return "true" in such cases too.
> If yes then please explain why, i might have missed a previous
> explanation from you.

I consider it okay because this is the current behavior after elfcorehdr_alloc() 
succeeded and set elfcorehdr_addr.

Not sure if it is the right think to do, though :)

Whatever we do, we should achieve on s390 that the result of is_kdump_kernel() 
is consistent throughout the booting stages, just like on all other architectures.

Right now it goes from false->true when /proc/vmcore gets initialized (and only 
if it gets initialized properly).

> 
> I'm afraid everyone will make wrong assumptions while reading the name
> of is_kdump_kernel() and assuming that it only applies to kdump or
> kdump-alike dumps (like stand-alone kdump), and, therefore, introduce
> bugs because the name of the function conveys the wrong idea to code
> readers. I consider dump_available() as a superset of is_kdump_kernel()
> and, therefore, to me they are not equivalent.
 > > I have the feeling you consider is_kdump_kernel() equivalent to
> "/proc/vmcore" being present and not really saying anything about
> whether kdump is active ?

Yes, but primarily because this is the existing handling.

Staring at the powerpc implementation:

/*
  * Return true only when kexec based kernel dump capturing method is used.
  * This ensures all restritions applied for kdump case are not automatically
  * applied for fadump case.
  */
bool is_kdump_kernel(void)
{
	return !is_fadump_active() && elfcorehdr_addr != ELFCORE_ADDR_MAX;
}
EXPORT_SYMBOL_GPL(is_kdump_kernel);


Which was added by

commit b098f1c32365304633077d73e4ae21c72d4241b3
Author: Hari Bathini <hbathini@linux.ibm.com>
Date:   Tue Sep 12 13:59:50 2023 +0530

     powerpc/fadump: make is_kdump_kernel() return false when fadump is active

     Currently, is_kdump_kernel() returns true in crash dump capture kernel
     for both kdump and fadump crash dump capturing methods, as both these
     methods set elfcorehdr_addr. Some restrictions enforced for crash dump
     capture kernel, based on is_kdump_kernel(), are specifically meant for
     kdump case and not desirable for fadump - eg. IO queues restriction in
     device drivers. So, define is_kdump_kernel() to return false when f/w
     assisted dump is active.


For my purpose (virtio-mem), it's sufficient to only support "kexec triggered 
kdump" either way, so I don't care.

So for me it's good enough to have

bool is_kdump_kernel(void)
{
	return oldmem_data.start;
}

And trying to document the situation in a comment like powerpc does :)

-- 
Cheers,

David / dhildenb