Subject: Re: [PATCH RFC 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

From: Michael Ellerman <[email-masked]>
Date: Wed, 06 Mar 2024 23:56:56 +1100

[email-masked] writes:
> From: Peter Xu <[email-masked]>
>
> PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
> constantly returns 0 for hash MMUs.  AFAICT that is fine to be removed,
> because pXd_huge() reflects a hugetlb entry, while it's own hugetlb pgtable
> lookup function (__find_linux_pte() shared by all powerpc code) already use
> pXd_leaf() irrelevant of the MMU type.  It means pXd_leaf() should work all
> fine with hash MMU pgtables or something could already went wrong.

Yes I think that's correct.

4K Hash MMU doesn't support any hugepage size at PMD or PUD level (the
geometry is wrong), so pmd/pud_huge() were written with that in mind,
ie. they are hard coded to return false.

But it should be OK to use pmd/pud_leaf(), they will actually look for
_PAGE_PTE, but it should never be set for 4K Hash.

See eg. arch/powerpc/include/asm/book3s/64/hash-4k.h:

static inline pmd_t hash__pmd_mkhuge(pmd_t pmd)
{
	BUG();
	return pmd;
}

> The goal should be that we will have one API pXd_leaf() to detect all kinds
> of huge mappings.  AFAICT we need to use the pXd_leaf() impl (rather than
> pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true.
>
> This helps to simplify a follow up patch to drop pXd_huge() treewide.
>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Nicholas Piggin <[email-masked]>
> Cc: Christophe Leroy <[email-masked]>
> Cc: "Aneesh Kumar K.V" <[email-masked]>
> Cc: "Naveen N. Rao" <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Peter Xu <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable-4k.h | 14 ++------------
>  1 file changed, 2 insertions(+), 12 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> index 48f21820afe2..92545981bb49 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> @@ -8,22 +8,12 @@
>  #ifdef CONFIG_HUGETLB_PAGE
>  static inline int pmd_huge(pmd_t pmd)
>  {
> -	/*
> -	 * leaf pte for huge page
> -	 */
> -	if (radix_enabled())
> -		return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
> -	return 0;
> +	return pmd_leaf(pmd);
>  }
>  
>  static inline int pud_huge(pud_t pud)
>  {
> -	/*
> -	 * leaf pte for huge page
> -	 */
> -	if (radix_enabled())
> -		return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
> -	return 0;
> +	return pud_leaf(pud);
>  }

This doesn't actually compile though.

  arch/powerpc/include/asm/book3s/64/pgtable-4k.h:11:16: error: implicit declaration of function ‘pmd_leaf’; did you mean ‘pgd_clear’? [-Werror=implicit-function-declaration]

etc.

To make it compile we'd need to relocate the pmd/pud_leaf() definitions:

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index df66dce8306f..fd7180fded75 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
@@ -262,6 +262,18 @@ extern unsigned long __kernel_io_end;

 extern struct page *vmemmap;
 extern unsigned long pci_io_base;
+
+#define pmd_leaf pmd_leaf
+static inline bool pmd_leaf(pmd_t pmd)
+{
+       return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
+}
+
+#define pud_leaf pud_leaf
+static inline bool pud_leaf(pud_t pud)
+{
+       return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
+}
 #endif /* __ASSEMBLY__ */

 #include <asm/book3s/64/hash.h>
@@ -1436,20 +1448,5 @@ static inline bool is_pte_rw_upgrade(unsigned long old_val, unsigned long new_va
        return false;
 }

-/*
- * Like pmd_huge(), but works regardless of config options
- */
-#define pmd_leaf pmd_leaf
-static inline bool pmd_leaf(pmd_t pmd)
-{
-       return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
-}
-
-#define pud_leaf pud_leaf
-static inline bool pud_leaf(pud_t pud)
-{
-       return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
-}
-
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */

cheers