Subject: Re: [PATCH] arm64: mm: enable per pmd page table lock

From: Yu Zhao <[email-masked]>
Date: Mon, 18 Feb 2019 12:49:38 -0700

On Mon, Feb 18, 2019 at 03:12:23PM +0000, Will Deacon wrote:
> [+Mark]
> 
> On Thu, Feb 14, 2019 at 02:16:42PM -0700, Yu Zhao wrote:
> > Switch from per mm_struct to per pmd page table lock by enabling
> > ARCH_ENABLE_SPLIT_PMD_PTLOCK. This provides better granularity for
> > large system.
> > 
> > I'm not sure if there is contention on mm->page_table_lock. Given
> > the option comes at no cost (apart from initializing more spin
> > locks), why not enable it now.
> > 
> > Signed-off-by: Yu Zhao <[email-masked]>
> > ---
> >  arch/arm64/Kconfig               |  3 +++
> >  arch/arm64/include/asm/pgalloc.h | 12 +++++++++++-
> >  arch/arm64/include/asm/tlb.h     |  5 ++++-
> >  3 files changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index a4168d366127..104325a1ffc3 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -872,6 +872,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
> >  config ARCH_HAS_CACHE_LINE_SIZE
> >  	def_bool y
> >  
> > +config ARCH_ENABLE_SPLIT_PMD_PTLOCK
> > +	def_bool y
> > +
> >  config SECCOMP
> >  	bool "Enable seccomp to safely compute untrusted bytecode"
> >  	---help---
> > diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
> > index 52fa47c73bf0..dabba4b2c61f 100644
> > --- a/arch/arm64/include/asm/pgalloc.h
> > +++ b/arch/arm64/include/asm/pgalloc.h
> > @@ -33,12 +33,22 @@
> >  
> >  static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
> >  {
> > -	return (pmd_t *)__get_free_page(PGALLOC_GFP);
> > +	struct page *page;
> > +
> > +	page = alloc_page(PGALLOC_GFP);
> > +	if (!page)
> > +		return NULL;
> > +	if (!pgtable_pmd_page_ctor(page)) {
> > +		__free_page(page);
> > +		return NULL;
> > +	}
> > +	return page_address(page);
> 
> I'm a bit worried as to how this interacts with the page-table code in
> arch/arm64/mm/mmu.c when pgd_pgtable_alloc is used as the allocator. It
> looks like that currently always calls pgtable_page_ctor(), regardless of
> level. Do we now need a separate allocator function for the PMD level?

Thanks for reminding me, I never noticed this. The short answer is
no.

I guess pgtable_page_ctor() is used on all pud/pmd/pte entries
there because it's also compatible with pud, and pmd too without
this patch. So your concern is valid. Thanks again.

Why my answer is no? Because I don't think the ctor matters for
pgd_pgtable_alloc(). The ctor is only required for userspace page
tables, and that's why we don't have it in pte_alloc_one_kernel().
AFAICT, none of the pgds (efi_mm.pgd, tramp_pg_dir and init_mm.pgd)
pre-populated by pgd_pgtable_alloc() is. (I doubt we pre-populate
userspace page tables in any other arch).

So to avoid future confusion, we might just remove the ctor from
pgd_pgtable_alloc().

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