linux-kernel - Re: [PATCH v6 1/4] MIPS: Do not flush tlb page when updating PTE entry

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Message-Id: <20200525144241.f0e55183a846607cdbb0d819@linux-foundation.org>
Date:   Mon, 25 May 2020 14:42:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Bibo Mao <maobibo@...ngson.cn>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhc@...ote.com>,
        Paul Burton <paulburton@...nel.org>,
        Dmitry Korotin <dkorotin@...ecomp.com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>,
        Stafford Horne <shorne@...il.com>,
        Steven Price <steven.price@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "Maciej W. Rozycki" <macro@....com>, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v6 1/4] MIPS: Do not flush tlb page when updating PTE
 entry

On Mon, 25 May 2020 10:52:37 +0800 Bibo Mao <maobibo@...ngson.cn> wrote:

> It is not necessary to flush tlb page on all CPUs if suitable PTE
> entry exists already during page fault handling, just updating
> TLB is fine.
> 
> Here redefine flush_tlb_fix_spurious_fault as empty on MIPS system.
> 
> ...
>
> --- a/arch/mips/include/asm/pgtable.h
> +++ b/arch/mips/include/asm/pgtable.h
> @@ -478,6 +478,8 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
>  	return __pgprot(prot);
>  }
>  
> +#define flush_tlb_fix_spurious_fault(vma, address) do { } while (0)
> +

static inline C would be preferred, if that works.  For a number of reasons:

- looks nicer

- more likely to get a code comment (for some reason)

- adds typechecking.  So right now a MIPS developer could do

	struct wibble a;
	struct wobble b;

	flush_tlb_fix_spurious_fault(&a, &b);

  and there would be no compiler warning.  Then the code gets merged
  upstream and in come the embarrassing emails!

- avoids unused-var warnings

	foo()
	{
		struct address_space a;
		struct vm_area_struct v;

		flush_tlb_fix_spurious_fault(&v, &a);
	}

will generate unused-variable warnings if
flush_tlb_fix_spurious_fault() is a macro.  Making
flush_tlb_fix_spurious_fault() inlined C prevents this.