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

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>
---
I noticed this when I was looking at adding kfence support a while back.
I've put that work aside and jpn has since gotten further than me, but I
think this is a fix worth considering.

  arch/powerpc/include/asm/book3s/64/hash.h |  2 ++
  arch/powerpc/mm/book3s64/hash_utils.c     |  2 +-
  arch/powerpc/mm/book3s64/pgtable.c        | 12 ++++++++++++
  3 files changed, 15 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/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index c145776d3ae5..cfd45245d009 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c

@@ -1988,7 +1988,7 @@ static void kernel_unmap_linear_page(unsigned long vaddr, unsigned long lmi)
  				     mmu_kernel_ssize, 0);
  }
  
-void __kernel_map_pages(struct page *page, int numpages, int enable)
+void hash__kernel_map_pages(struct page *page, int numpages, int enable)
  {
  	unsigned long flags, vaddr, lmi;
  	int i;

diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 9e16c7b1a6c5..0aefc272cd03 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c

@@ -526,3 +526,15 @@ static int __init pgtable_debugfs_setup(void)
  	return 0;
  }
  arch_initcall(pgtable_debugfs_setup);
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+void __kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (radix_enabled()) {
+		pr_warn_once("DEBUG_PAGEALLOC not supported in radix mode\n");
+		return;
+	}
+
+	hash__kernel_map_pages(page, numpages, enable);
+}