[PATCH][v2] arm64: Allocate elfcorehdr & crashkernel mem before any reservation

James Morse james.morse at arm.com
Fri Jan 12 09:58:54 PST 2018

Hi Poonam,

On 08/01/18 04:31, Poonam Aggrwal wrote:
> James Morse wrote:
>> On 04/01/18 15:34, Poonam Aggrwal wrote:
>>> Address for both crashkernel memory and elfcorehdr can be assigned
>>> statically. For crashkernel memory it is via crashkernel=SIZE at ADDRESS
>>> and elfcorehdr_addr via by kexec-utils in dump kernel device tree.
>> There are some crashkernel=SIZE at ADDRESS values that the user can supply that
>> we must reject. The obvious one is if it overlaps with the kernel text. (this patch
>> won't spot this). We need to read the hardware's reserved regions from the DT
>> before we allocate the crashkernel region, for example if the bootloader
>> reserved a chunk of memory for a frame-buffer, I shouldn't be able to use that
>> as crashkernel memory.
>> (you shouldn't need to specify an address, doing so prevents the kernel from
>> picking memory it can use)
>>> So memory should be reserved for both the above areas before any other
>>> memory reservations are done. Otherwise overlaps may happen and memory
>>> allocations will fail leading to failure in booting the dump capture
>>> kernel.

>>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c index
>>> 00e7b90..24ce15d 100644
>>> --- a/arch/arm64/mm/init.c
>>> +++ b/arch/arm64/mm/init.c
>>> @@ -453,6 +453,14 @@ void __init arm64_memblock_init(void)

>>> +	reserve_elfcorehdr();
>> (Moving reserve_elfcorehdr() makes sense, but..)
>>> +	reserve_crashkernel();
>> reserve_crashkernel() does the allocation for the crashkernels reserved memory.
>> I expect this to always fail in the kdump kernel because there isn't enough
>> memory. (fdt_enforce_memory_region() at the top of this function calls
>> memblock_cap_memory_range()).
>> Moving this allocation above the early_init_fdt_scan_reserved_mem() below
>> means we may allocate memory for the crashdump that is in use by
>> firmware/hardware and described as reserved in the DT.

> Yeah, this is a good point. So ideally the address of the crash kernel should
> be diligently provided by the user based on the system.

Even better: the region to store the crash kernel in should be chosen by the kernel.
When using kdump I boot with 'crashkernel=1G', the kernel chooses where to place
the reserved region. Even if I specified a reasonable physical address, the
efistub may relocate the kernel over the top during boot as part of its KASLR work.

(Why does anyone ever need to specify an offset here?)



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