Subject: Re: [RFC v2 03/13] book3s64/hash: Remove kfence support temporarily

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Thu, 19 Sep 2024 11:53:15 +0530

Christophe Leroy <[email-masked]> writes:

> Le 19/09/2024 à 04:56, Ritesh Harjani (IBM) a écrit :
>> Kfence on book3s Hash on pseries is anyways broken. It fails to boot
>> due to RMA size limitation. That is because, kfence with Hash uses
>> debug_pagealloc infrastructure. debug_pagealloc allocates linear map
>> for entire dram size instead of just kfence relevant objects.
>> This means for 16TB of DRAM it will require (16TB >> PAGE_SHIFT)
>> which is 256MB which is half of RMA region on P8.
>> crash kernel reserves 256MB and we also need 2048 * 16KB * 3 for
>> emergency stack and some more for paca allocations.
>> That means there is not enough memory for reserving the full linear map
>> in the RMA region, if the DRAM size is too big (>=16TB)
>> (The issue is seen above 8TB with crash kernel 256 MB reservation).
>> 
>> Now Kfence does not require linear memory map for entire DRAM.
>> It only needs for kfence objects. So this patch temporarily removes the
>> kfence functionality since debug_pagealloc code needs some refactoring.
>> We will bring in kfence on Hash support in later patches.
>> 
>> Signed-off-by: Ritesh Harjani (IBM) <[email-masked]>
>> ---
>>   arch/powerpc/include/asm/kfence.h     |  5 +++++
>>   arch/powerpc/mm/book3s64/hash_utils.c | 16 +++++++++++-----
>>   2 files changed, 16 insertions(+), 5 deletions(-)
>> 
>> diff --git a/arch/powerpc/include/asm/kfence.h b/arch/powerpc/include/asm/kfence.h
>> index fab124ada1c7..f3a9476a71b3 100644
>> --- a/arch/powerpc/include/asm/kfence.h
>> +++ b/arch/powerpc/include/asm/kfence.h
>> @@ -10,6 +10,7 @@
>>   
>>   #include <linux/mm.h>
>>   #include <asm/pgtable.h>
>> +#include <asm/mmu.h>
>>   
>>   #ifdef CONFIG_PPC64_ELF_ABI_V1
>>   #define ARCH_FUNC_PREFIX "."
>> @@ -25,6 +26,10 @@ static inline void disable_kfence(void)
>>   
>>   static inline bool arch_kfence_init_pool(void)
>>   {
>> +#ifdef CONFIG_PPC64
>> +	if (!radix_enabled())
>
> No need for a #ifdef here, you can just do:
>
> 	if (IS_ENABLED(CONFIG_PPC64) && !radix_enabled())
> 		return false;
>
>

This special radix handling is anyway dropped in later pacthes. 
So I didn't bother changing it here.

>> +		return false;
>> +#endif
>>   	return !kfence_disabled;
>
> But why not just set kfence_disabled to true by calling disable_kfence() 
> from one of the powerpc init functions ?
>

This patch is only temporarily disabling kfence support for only Hash.
This special Hash handling gets removed in patch-10 which brings back
kfence support.

-ritesh