linux-kernel - Re: [PATCH] arm64/mm: Drop redundant BUG_ON(!pgtable

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Message-ID: <Y3doGGaOX+9NB2my@FVFF77S0Q05N>
Date:   Fri, 18 Nov 2022 11:10:16 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Drop redundant BUG_ON(!pgtable_alloc)

On Fri, Nov 18, 2022 at 11:01:02AM +0530, Anshuman Khandual wrote:
> __create_pgd_mapping_locked() expects a page allocator used while mapping a
> virtual range. This page allocator function propagates down the call chain,
> while building intermediate levels in the page table. Passed page allocator
> is a necessary ingredient required to build the page table but its presence
> can be asserted just once in the very beginning rather than in all the down
> stream functions. This consolidates BUG_ON(!pgtable_alloc) checks just in a
> single place i.e __create_pgd_mapping_locked().
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>

I don't have strong feelings either way, so FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.

> ---
> This applies on v6.1-rc5
> 
>  arch/arm64/mm/mmu.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 5a19950e7289..97ca82001089 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -207,7 +207,6 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>  
>  		if (flags & NO_EXEC_MAPPINGS)
>  			pmdval |= PMD_TABLE_PXN;
> -		BUG_ON(!pgtable_alloc);
>  		pte_phys = pgtable_alloc(PAGE_SHIFT);
>  		__pmd_populate(pmdp, pte_phys, pmdval);
>  		pmd = READ_ONCE(*pmdp);
> @@ -285,7 +284,6 @@ static void alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
>  
>  		if (flags & NO_EXEC_MAPPINGS)
>  			pudval |= PUD_TABLE_PXN;
> -		BUG_ON(!pgtable_alloc);
>  		pmd_phys = pgtable_alloc(PMD_SHIFT);
>  		__pud_populate(pudp, pmd_phys, pudval);
>  		pud = READ_ONCE(*pudp);
> @@ -324,7 +322,6 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
>  
>  		if (flags & NO_EXEC_MAPPINGS)
>  			p4dval |= P4D_TABLE_PXN;
> -		BUG_ON(!pgtable_alloc);
>  		pud_phys = pgtable_alloc(PUD_SHIFT);
>  		__p4d_populate(p4dp, pud_phys, p4dval);
>  		p4d = READ_ONCE(*p4dp);
> @@ -383,6 +380,7 @@ static void __create_pgd_mapping_locked(pgd_t *pgdir, phys_addr_t phys,
>  	phys &= PAGE_MASK;
>  	addr = virt & PAGE_MASK;
>  	end = PAGE_ALIGN(virt + size);
> +	BUG_ON(!pgtable_alloc);
>  
>  	do {
>  		next = pgd_addr_end(addr, end);
> -- 
> 2.25.1
>