Christophe Leroy [off-list ref] writes:

Le 19/09/2024 à 04:56, Ritesh Harjani (IBM) a écrit :

quoted

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@gmail.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;

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

quoted

+		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