Re: [PATCH v2] powerpc/s64: Clarify that radix lacks DEBUG_PAGEALLOC

Christophe Leroy [off-list ref] writes:

Le 13/10/2021 à 23:34, Joel Stanley a écrit :

quoted

The page_alloc.c code will call into __kernel_map_pages when
DEBUG_PAGEALLOC is configured and enabled.

As the implementation assumes hash, this should crash spectacularly if
not for a bit of luck in __kernel_map_pages. In this function
linear_map_hash_count is always zero, the for loop exits without doing
any damage.

There are no other platforms that determine if they support
debug_pagealloc at runtime. Instead of adding code to mm/page_alloc.c to
do that, this change turns the map/unmap into a noop when in radix
mode and prints a warning once.

Signed-off-by: Joel Stanley <joel@jms.id.au>
---
v2: Put __kernel_map_pages in pgtable.h

  arch/powerpc/include/asm/book3s/64/hash.h    |  2 ++
  arch/powerpc/include/asm/book3s/64/pgtable.h | 11 +++++++++++
  arch/powerpc/include/asm/book3s/64/radix.h   |  3 +++
  arch/powerpc/mm/book3s64/hash_utils.c        |  2 +-
  arch/powerpc/mm/book3s64/radix_pgtable.c     |  7 +++++++
  5 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
index d959b0195ad9..674fe0e890dc 100644
--- a/arch/powerpc/include/asm/book3s/64/hash.h
+++ b/arch/powerpc/include/asm/book3s/64/hash.h

@@ -255,6 +255,8 @@ int hash__create_section_mapping(unsigned long start, unsigned long end,
  				 int nid, pgprot_t prot);
  int hash__remove_section_mapping(unsigned long start, unsigned long end);
  
+void hash__kernel_map_pages(struct page *page, int numpages, int enable);
+
  #endif /* !__ASSEMBLY__ */
  #endif /* __KERNEL__ */
  #endif /* _ASM_POWERPC_BOOK3S_64_HASH_H */

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index 5d34a8646f08..265661ded238 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -1101,6 +1101,17 @@ static inline void vmemmap_remove_mapping(unsigned long start,
  }
  #endif
  
+#ifdef CONFIG_DEBUG_PAGEALLOC
+static inline void __kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (radix_enabled()) {
+		radix__kernel_map_pages(page, numpages, enable);
+		return;
+	}
+	hash__kernel_map_pages(page, numpages, enable);

I'd have prefered something like below

	if (radix_enabled())
		radix__kernel_map_pages(page, numpages, enable);
	else
		hash__kernel_map_pages(page, numpages, enable);

I did that when applying.

cheers