linux-kernel - Re: [PATCH] arm64/mm: Fix mapping_dirty

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Date:   Fri, 2 Apr 2021 12:26:57 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Zack Rusin <zackr@...are.com>
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64/mm: Fix mapping_dirty_helpers with arm64

On Wed, Mar 31, 2021 at 02:23:44PM -0400, Zack Rusin wrote:
> The pagetable walk callbacks in mm/mapping_dirty_helpers.c depend
> on a set of helpers from which pud_dirty(pud) was missing. I'm
> assuming mapping_dirty_helpers weren't used on ARM64 before
> because the missing pud_dirty is causing a compilation error.
> 
> The drivers/gpu/drm/vmwgfx code uses mapping_dirty_helpers and
> has been ported to ARM64 but it depends on this code getting in
> first in order to compile/work.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Zack Rusin <zackr@...are.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 47027796c2f9..ecd80f87a996 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -451,6 +451,7 @@ static inline pmd_t pmd_mkdevmap(pmd_t pmd)
>  #define pfn_pmd(pfn,prot)	__pmd(__phys_to_pmd_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
>  #define mk_pmd(page,prot)	pfn_pmd(page_to_pfn(page),prot)
>  
> +#define pud_dirty(pud)		pte_dirty(pud_pte(pud))
>  #define pud_young(pud)		pte_young(pud_pte(pud))
>  #define pud_mkyoung(pud)	pte_pud(pte_mkyoung(pud_pte(pud)))
>  #define pud_write(pud)		pte_write(pud_pte(pud))

I think pud_dirty() should only be defined if
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD which is not the case on
arm64. Well, the pud_young/mkyoung/write should probably also be removed
until we have pud pages on arm64.

The wp_clean_pud_entry() function bails out early since
pud_trans_unstable() returns 0 on arm64. So we either bracket this
function by some #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD or we
make some generic definitions for pud_* accessors when we don't have
this config.

-- 
Catalin