[PATCH] mm/arm64: Drop dead code for pud special bit handling

[PATCH] mm/arm64: Drop dead code for pud special bit handling

[Peter Xu]

Keith Busch observed some incorrect macros defined in arm64 code [1].

It turns out the two lines should never be needed and won't be exposed to
anyone, because aarch64 doesn't select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD,
hence ARCH_SUPPORTS_PUD_PFNMAP is always N.  The only archs that support
THP PUDs so far are x86 and powerpc.

Instead of fixing the lines (with no way to test it..), remove the two
lines that are in reality dead code, to avoid confusing readers.

Fixes tag is attached to reflect where the wrong macros were introduced,
but explicitly not copying stable, because there's no real issue to be
fixed.  So it's only about removing the dead code so far.

[1] https://lore.kernel.org/all/Z9tDjOk-JdV_fCY4@kbusch-mbp.dhcp.thefacebook.com/#t

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Donald Dutile <ddutile@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Fixes: 3e509c9b03f9 ("mm/arm64: support large pfn mappings")
Reported-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/arm64/include/asm/pgtable.h | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0b2a2ad1b9e8..15211f74b035 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -620,11 +620,6 @@ static inline pmd_t pmd_mkspecial(pmd_t pmd)
 #define pud_pfn(pud)		((__pud_to_phys(pud) & PUD_MASK) >> PAGE_SHIFT)
 #define pfn_pud(pfn,prot)	__pud(__phys_to_pud_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
 
-#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
-#define pud_special(pte)	pte_special(pud_pte(pud))
-#define pud_mkspecial(pte)	pte_pud(pte_mkspecial(pud_pte(pud)))
-#endif
-
 #define pmd_pgprot pmd_pgprot
 static inline pgprot_t pmd_pgprot(pmd_t pmd)
 {
-- 
2.48.1

Re: [PATCH] mm/arm64: Drop dead code for pud special bit handling

[Donald Dutile]

Peter,

Thanks for the cc, as I have the original commit in a recent, proposed backport that now needs an edit...
or, since harmless, maybe this patch to additionally backport, once committed. :-)

On 3/20/25 2:34 PM, Peter Xu wrote:
> Keith Busch observed some incorrect macros defined in arm64 code [1].
> 
> It turns out the two lines should never be needed and won't be exposed to
> anyone, because aarch64 doesn't select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD,
> hence ARCH_SUPPORTS_PUD_PFNMAP is always N.  The only archs that support
> THP PUDs so far are x86 and powerpc.
> 
> Instead of fixing the lines (with no way to test it..), remove the two
> lines that are in reality dead code, to avoid confusing readers.
> 
> Fixes tag is attached to reflect where the wrong macros were introduced,
> but explicitly not copying stable, because there's no real issue to be
> fixed.  So it's only about removing the dead code so far.
> 
> [1] https://lore.kernel.org/all/Z9tDjOk-JdV_fCY4@kbusch-mbp.dhcp.thefacebook.com/#t
> 
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Donald Dutile <ddutile@redhat.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Fixes: 3e509c9b03f9 ("mm/arm64: support large pfn mappings")
> Reported-by: Keith Busch <kbusch@kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   arch/arm64/include/asm/pgtable.h | 5 -----
>   1 file changed, 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 0b2a2ad1b9e8..15211f74b035 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -620,11 +620,6 @@ static inline pmd_t pmd_mkspecial(pmd_t pmd)
>   #define pud_pfn(pud)		((__pud_to_phys(pud) & PUD_MASK) >> PAGE_SHIFT)
>   #define pfn_pud(pfn,prot)	__pud(__phys_to_pud_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
>   
> -#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
> -#define pud_special(pte)	pte_special(pud_pte(pud))
> -#define pud_mkspecial(pte)	pte_pud(pte_mkspecial(pud_pte(pud)))
> -#endif
> -
>   #define pmd_pgprot pmd_pgprot
>   static inline pgprot_t pmd_pgprot(pmd_t pmd)
>   {

Reviewed-by: Donald Dutile <ddutile@redhat.com>

Re: [PATCH] mm/arm64: Drop dead code for pud special bit handling

[Peter Xu]

On Thu, Mar 20, 2025 at 05:03:05PM -0400, Donald Dutile wrote:
> Peter,

Hello, Don,

> 
> Thanks for the cc, as I have the original commit in a recent, proposed backport that now needs an edit...
> or, since harmless, maybe this patch to additionally backport, once committed. :-)

I would suggest not having the backport blocked by this. :)

Thanks for the quick review!

-- 
Peter Xu

Re: [PATCH] mm/arm64: Drop dead code for pud special bit handling

[Anshuman Khandual]

On 3/21/25 00:04, Peter Xu wrote:
> Keith Busch observed some incorrect macros defined in arm64 code [1].
> 
> It turns out the two lines should never be needed and won't be exposed to
> anyone, because aarch64 doesn't select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD,
> hence ARCH_SUPPORTS_PUD_PFNMAP is always N.  The only archs that support
> THP PUDs so far are x86 and powerpc.

Correct.

> 
> Instead of fixing the lines (with no way to test it..), remove the two
> lines that are in reality dead code, to avoid confusing readers.

Agreed. Because ARCH_SUPPORTS_PUD_PFNMAP is always N and hence these lines
are never reachable. So removing these lines will be preferred than fixing.

> 
> Fixes tag is attached to reflect where the wrong macros were introduced,
> but explicitly not copying stable, because there's no real issue to be
> fixed.  So it's only about removing the dead code so far.

Makes sense.

> 
> [1] https://lore.kernel.org/all/Z9tDjOk-JdV_fCY4@kbusch-mbp.dhcp.thefacebook.com/#t
> 
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Donald Dutile <ddutile@redhat.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Fixes: 3e509c9b03f9 ("mm/arm64: support large pfn mappings")

The commit ID checks out.

> Reported-by: Keith Busch <kbusch@kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 0b2a2ad1b9e8..15211f74b035 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -620,11 +620,6 @@ static inline pmd_t pmd_mkspecial(pmd_t pmd)
>  #define pud_pfn(pud)		((__pud_to_phys(pud) & PUD_MASK) >> PAGE_SHIFT)
>  #define pfn_pud(pfn,prot)	__pud(__phys_to_pud_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
>  
> -#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
> -#define pud_special(pte)	pte_special(pud_pte(pud))
> -#define pud_mkspecial(pte)	pte_pud(pte_mkspecial(pud_pte(pud)))
> -#endif
> -
>  #define pmd_pgprot pmd_pgprot
>  static inline pgprot_t pmd_pgprot(pmd_t pmd)
>  {

LGTM

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

Re: [PATCH] mm/arm64: Drop dead code for pud special bit handling

[Catalin Marinas]

On Thu, 20 Mar 2025 14:34:05 -0400, Peter Xu wrote:
> Keith Busch observed some incorrect macros defined in arm64 code [1].
> 
> It turns out the two lines should never be needed and won't be exposed to
> anyone, because aarch64 doesn't select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD,
> hence ARCH_SUPPORTS_PUD_PFNMAP is always N.  The only archs that support
> THP PUDs so far are x86 and powerpc.
> 
> [...]

Applied to arm64 (for-next/core), thanks!

[1/1] mm/arm64: Drop dead code for pud special bit handling
      https://git.kernel.org/arm64/c/0fff2aa96f6b

-- 
Catalin