linux-kernel - Re: [PATCH RESEND v1 2/5] crash: let arch decide crash memory export to iomem

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Message-ID: <622d9d6d-3dfc-4bdb-b251-4abf9299fc6a@linux.ibm.com>
Date: Wed, 8 Jan 2025 17:32:19 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: Baoquan he <bhe@...hat.com>
Cc: linuxppc-dev@...ts.ozlabs.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v1 2/5] crash: let arch decide crash memory export
 to iomem_resource

Hello Baoquan

On 08/01/25 16:55, Baoquan he wrote:
> Hi,
>
> On 01/08/25 at 03:44pm, Sourabh Jain wrote:
>> insert_crashkernel_resources() adds crash memory to iomem_resource if
>> generic crashkernel reservation is enabled on an architecture.
>>
>> On PowerPC, system RAM is added to iomem_resource. See commit
>> c40dd2f766440 ("powerpc: Add System RAM to /proc/iomem").
>>
>> Enabling generic crashkernel reservation on PowerPC leads to a conflict
>> when system RAM is added to iomem_resource because a part of the system
>> RAM, the crashkernel memory, has already been added to iomem_resource.
>>
>> The next commit in the series "powerpc/crash: use generic crashkernel
>> reservation" enables generic crashkernel reservation on PowerPC. If the
>> crashkernel is added to iomem_resource, the kernel fails to add
>> system RAM to /proc/iomem and prints the following traces:
>>
>> CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.13.0-rc2+
>> snip...
>> NIP [c000000002016b3c] add_system_ram_resources+0xf0/0x15c
>> LR [c000000002016b34] add_system_ram_resources+0xe8/0x15c
>> Call Trace:
>> [c00000000484bbc0] [c000000002016b34] add_system_ram_resources+0xe8/0x15c
>> [c00000000484bc20] [c000000000010a4c] do_one_initcall+0x7c/0x39c
>> [c00000000484bd00] [c000000002005418] do_initcalls+0x144/0x18c
>> [c00000000484bd90] [c000000002005714] kernel_init_freeable+0x21c/0x290
>> [c00000000484bdf0] [c0000000000110f4] kernel_init+0x2c/0x1b8
>> [c00000000484be50] [c00000000000dd3c] ret_from_kernel_user_thread+0x14/0x1c
>>
>> To avoid this, an architecture hook is added in
>> insert_crashkernel_resources(), allowing the architecture to decide
>> whether crashkernel memory should be added to iomem_resource.
> Have you tried defining HAVE_ARCH_ADD_CRASH_RES_TO_IOMEM_EARLY in ppc to
> add crashkernel region to iomem early?


I didn’t try, but I think even if I do, the kernel will run into the 
same issue
because as soon as crashkernel is added to iomem, there will be a resource
conflict when the PowerPC code tries to add system RAM to iomem. Which
happens during subsys_initcall.

Regardless, I will give it a try.

> Now there are two branches in the
> existing code, adding a hook will make three ways.

I agree. I can try updating powerpc code to not consider crashkernel 
memory as
iomem conflict.

Thanks for the review.

- Sourabh Jain