linux-kernel - Re: [PATCH v2 1/3] arm64: mm: use appropriate ctors for page tables

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Message-ID: <20190226151230.GA20230@lakrids.cambridge.arm.com>
Date:   Tue, 26 Feb 2019 15:12:31 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jun Yao <yaojun8558363@...il.com>,
        Laura Abbott <labbott@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] arm64: mm: use appropriate ctors for page tables

Hi,

On Mon, Feb 18, 2019 at 04:13:17PM -0700, Yu Zhao wrote:
> For pte page, use pgtable_page_ctor(); for pmd page, use
> pgtable_pmd_page_ctor() if not folded; and for the rest (pud,
> p4d and pgd), don't use any.
> 
> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> ---
>  arch/arm64/mm/mmu.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)

[...]

> -static phys_addr_t pgd_pgtable_alloc(void)
> +static phys_addr_t pgd_pgtable_alloc(int shift)
>  {
>  	void *ptr = (void *)__get_free_page(PGALLOC_GFP);
> -	if (!ptr || !pgtable_page_ctor(virt_to_page(ptr)))
> -		BUG();
> +	BUG_ON(!ptr);
> +
> +	/*
> +	 * Initialize page table locks in case later we need to
> +	 * call core mm functions like apply_to_page_range() on
> +	 * this pre-allocated page table.
> +	 */
> +	if (shift == PAGE_SHIFT)
> +		BUG_ON(!pgtable_page_ctor(virt_to_page(ptr)));
> +	else if (shift == PMD_SHIFT && PMD_SHIFT != PUD_SHIFT)
> +		BUG_ON(!pgtable_pmd_page_ctor(virt_to_page(ptr)));

IIUC, this is for nopmd kernels, where we only have real PGD and PTE
levels of table. From my PoV, that would be clearer if we did:

	else if (shift == PMD_SHIFT && !is_defined(__PAGETABLE_PMD_FOLDED))

... though IMO it would be a bit nicer if the generic
pgtable_pmd_page_ctor() were nop'd out for __PAGETABLE_PMD_FOLDED
builds, so that callers don't have to be aware of folding.

I couldn't think of a nicer way of distinguishing levels of table, and
having separate function pointers for each level seems over-the-top, so
otehr than that this looks good to me.

Assuming you're happy with the above change:

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

Thanks,
Mark.