linux-kernel - Re: [RFC PATCH 02/14] sparc64: add new fields to mmu context for shared context support

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Date:   Wed, 21 Dec 2016 19:13:46 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     sparclinux@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Bob Picco <bob.picco@...cle.com>,
        Nitin Gupta <nitin.m.gupta@...cle.com>,
        Vijay Kumar <vijay.ac.kumar@...cle.com>,
        Julian Calaby <julian.calaby@...il.com>,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 02/14] sparc64: add new fields to mmu context for
 shared context support

On Sun, Dec 18, 2016 at 03:45:31PM -0800, Mike Kravetz wrote:
> On 12/16/2016 11:38 PM, Sam Ravnborg wrote:
> > Hi Mike
> > 
> >> diff --git a/arch/sparc/include/asm/mmu_context_64.h b/arch/sparc/include/asm/mmu_context_64.h
> >> index b84be67..d031799 100644
> >> --- a/arch/sparc/include/asm/mmu_context_64.h
> >> +++ b/arch/sparc/include/asm/mmu_context_64.h
> >> @@ -35,15 +35,15 @@ void __tsb_context_switch(unsigned long pgd_pa,
> >>  static inline void tsb_context_switch(struct mm_struct *mm)
> >>  {
> >>  	__tsb_context_switch(__pa(mm->pgd),
> >> -			     &mm->context.tsb_block[0],
> >> +			     &mm->context.tsb_block[MM_TSB_BASE],
> >>  #if defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_TRANSPARENT_HUGEPAGE)
> >> -			     (mm->context.tsb_block[1].tsb ?
> >> -			      &mm->context.tsb_block[1] :
> >> +			     (mm->context.tsb_block[MM_TSB_HUGE].tsb ?
> >> +			      &mm->context.tsb_block[MM_TSB_HUGE] :
> >>  			      NULL)
> >>  #else
> >>  			     NULL
> >>  #endif
> >> -			     , __pa(&mm->context.tsb_descr[0]));
> >> +			     , __pa(&mm->context.tsb_descr[MM_TSB_BASE]));
> >>  }
> >>  
> > This is a nice cleanup that has nothing to do with your series.
> > Could you submit this as a separate patch so we can get it applied.
> > 
> > This is the only place left where the array index for tsb_block
> > and tsb_descr uses hardcoded values. And it would be good to get
> > rid of these.
> 
> Sure, I will submit a separate cleanup patch for this.
> 
> However, do note that in my series if CONFIG_SHARED_MMU_CTX is defined,
> then MM_TSB_HUGE_SHARED is index 0, instead of MM_TSB_BASE being 0 in
> the case where CONFIG_SHARED_MMU_CTX is not defined.  This may seem
> 'strange' and the obvious question would be 'why not put CONFIG_SHARED_MMU_CTX
> at the end of the existing array (index 2)?'.  The reason is that tsb_descr
> array can not have any 'holes' when passed to the hypervisor.  Since there
> will always be a MM_TSB_BASE tsb, with MM_TSB_HUGE_SHARED before and
> MM_TSB_HUGE after MM_TSB_BASE, few tricks are necessary to ensure no holes
> are in the array passed to the hypervisor.
So this is the explanation for the strange changes of the constants.
Add a similar explanation to the code to help the next reader.

	Sam