[PATCH] arm64: Force NO_BLOCK_MAPPINGS if crashkernel reservation is required

[Anshuman Khandual]

On 11/20/20 2:53 PM, Catalin Marinas wrote:
> On Fri, Nov 20, 2020 at 09:25:22AM +0530, Anshuman Khandual wrote:
>> On 11/19/20 11:25 PM, Catalin Marinas wrote:
>>> mem_init() currently relies on knowing the boundaries of the crashkernel
>>> reservation to map such region with page granularity for later
>>> unmapping via set_memory_valid(..., 0). If the crashkernel reservation
>>> is deferred, such boundaries are not known when the linear mapping is
>>> created. Simply parse the command line for "crashkernel" and, if found,
>>> create the linear map with NO_BLOCK_MAPPINGS.
>>>
>>> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
>>> Cc: James Morse <james.morse at arm.com>
>>> Cc: Nicolas Saenz Julienne <nsaenzjulienne at suse.de>
>>> ---
>>>
>>> Following the online (and offline) discussion with James and Nicolas,
>>> this aims to avoid issues with moving the reserve_crashkernel() call to
>>> after the memory map has been created (the ZONE_DMA patches).
>>>
>>> https://lore.kernel.org/r/e60d643e-4879-3fc3-737d-2c145332a6d7@arm.com
>>>
>>>  arch/arm64/mm/mmu.c | 37 ++++++++++++++++---------------------
>>>  1 file changed, 16 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index d7fe72ee678a..dd214157a026 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -469,6 +469,21 @@ void __init mark_linear_text_alias_ro(void)
>>>  			    PAGE_KERNEL_RO);
>>>  }
>>>  
>>> +static bool crash_mem_map __initdata;
>>> +
>>> +static int __init enable_crash_mem_map(char *arg)
>>> +{
>>> +	/*
>>> +	 * Proper parameter parsing is done by reserve_crashkernel(). We only
>>> +	 * need to know if the linear map has to avoid block mappings so that
>>> +	 * the crashkernel reservations can be unmapped later.
>>> +	 */
>>> +	crash_mem_map = true;
>>> +
>>> +	return 0;
>>> +}
>>> +early_param("crashkernel", enable_crash_mem_map);
>>
>> Should not the crash kernel cmdline parameter gets parsed enough, just to
>> ensure it is atleast a valid one ? Otherwise an invalid cmdline request
>> can prevent the block mapping. In that case, the kernel will neither have
>> a crash kernel nor the block mapping.
> 
> Does it actually matter? If people pass random strings on the command
> line, they should expect some side-effects. That's about the intention
> to have a crashkernel. We can't fully validate the crashkernel parameter
> until we know the zones layout.

Makes sense.

> 
> Also note that with defconfig, we don't get block mappings anyway
> (rodata_full).
>

Sure, seems alight then.