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

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Message-ID: <d9d8703a-df24-47e3-bd0d-2ff5a6eae184@csgroup.eu>
Date: Thu, 19 Sep 2024 07:20:04 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 linuxppc-dev@...ts.ozlabs.org
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin
 <npiggin@...il.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Hari Bathini <hbathini@...ux.ibm.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Donet Tom <donettom@...ux.vnet.ibm.com>,
 Pavithra Prakash <pavrampu@...ux.vnet.ibm.com>,
 Nirjhar Roy <nirjhar@...ux.ibm.com>, LKML <linux-kernel@...r.kernel.org>,
 kasan-dev@...glegroups.com
Subject: Re: [RFC v2 03/13] book3s64/hash: Remove kfence support temporarily

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@...il.com>
> ---
>   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