linux-kernel - Re: [RFC PATCH 05/14] sparc64: Add PAGE_SHR

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Date:   Tue, 20 Dec 2016 13:33:59 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mike.kravetz@...cle.com
Cc:     sparclinux@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bob.picco@...cle.com,
        nitin.m.gupta@...cle.com, vijay.ac.kumar@...cle.com,
        julian.calaby@...il.com, adam.buchbinder@...il.com,
        kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 05/14] sparc64: Add PAGE_SHR_CTX flag

From: Mike Kravetz <mike.kravetz@...cle.com>
Date: Sun, 18 Dec 2016 16:42:52 -0800

> On 12/17/2016 07:12 PM, David Miller wrote:
>> From: Mike Kravetz <mike.kravetz@...cle.com>
>> Date: Fri, 16 Dec 2016 10:35:28 -0800
>> 
>>> @@ -166,6 +166,7 @@ bool kern_addr_valid(unsigned long addr);
>>>  #define _PAGE_EXEC_4V	  _AC(0x0000000000000080,UL) /* Executable Page      */
>>>  #define _PAGE_W_4V	  _AC(0x0000000000000040,UL) /* Writable             */
>>>  #define _PAGE_SOFT_4V	  _AC(0x0000000000000030,UL) /* Software bits        */
>>> +#define _PAGE_SHR_CTX_4V  _AC(0x0000000000000020,UL) /* Shared Context       */
>>>  #define _PAGE_PRESENT_4V  _AC(0x0000000000000010,UL) /* Present              */
>>>  #define _PAGE_RESV_4V	  _AC(0x0000000000000008,UL) /* Reserved             */
>>>  #define _PAGE_SZ16GB_4V	  _AC(0x0000000000000007,UL) /* 16GB Page            */
>> 
>> You really don't need this.
>> 
>> The VMA is available, and you can obtain the information you need
>> about whether this is a shared mapping or not from the. It just isn't
>> being passed down into things like set_huge_pte_at().  Simply make it
>> do so.
>> 
> 
> I was more concerned about the page table walk code at tlb/tsb miss time.
> Specifically, the code after tsb_miss_page_table_walk_sun4v_fastpath in
> tsb.S.  AFAICT, the tsb entries should have been created when the pte entries
> were created.  Yet, this code is still walking the page table and creating
> tsb entries.  We do not have a pointer to the vma here, and I thought it
> would be somewhat difficult to get access.  This is the reason why I went
> down the path of a page flag.

You are right, you will need a page flag for that part.