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

From: Yu Zhao <[email-masked]>
Date: Mon, 18 Feb 2019 13:48:42 -0700

On Mon, Feb 18, 2019 at 12:49:38PM -0700, Yu Zhao wrote:
> 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().

I'm sorry. I've missed that we call apply_to_page_range() on efi_mm.
The function does require the ctor. So we actually can't remove it.
Though pgtable_page_ctor() also does the work adequately for pmd in
terms of giving apply_to_page_range() what it requires, it would be
more appropriate to use pgtable_pmd_page_ctor() instead (and not
calling the ctor at all on pud).

I could add this change prior to this patch, if it makes sense to
you. Thanks.

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