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

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Message-ID: <20190309040153.GB214016@google.com>
Date:   Fri, 8 Mar 2019 21:01:53 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Mark Rutland <mark.rutland@....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

On Tue, Feb 26, 2019 at 03:12:31PM +0000, Mark Rutland wrote:
> 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.

Agreed. Will make pgtable_pmd_page_ctor() nop when pmd is folded.

> 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.