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

[Christophe Leroy]

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) <ritesh.list@xxxxxxxxx>
> ---
>   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;


> +		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 ?

>   }
>   #endif