linux-kernel - Re: [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end

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Date:   Tue, 02 Feb 2021 16:41:22 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-csky@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Nadav Amit <namit@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        Yu Zhao <yuzhao@...gle.com>
Subject: Re: [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end_vma()

Excerpts from Peter Zijlstra's message of February 1, 2021 10:09 pm:
> On Sat, Jan 30, 2021 at 04:11:23PM -0800, Nadav Amit wrote:
> 
>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>> index 427bfcc6cdec..b97136b7010b 100644
>> --- a/include/asm-generic/tlb.h
>> +++ b/include/asm-generic/tlb.h
>> @@ -334,8 +334,8 @@ static inline void __tlb_reset_range(struct mmu_gather *tlb)
>>  
>>  #ifdef CONFIG_MMU_GATHER_NO_RANGE
>>  
>> -#if defined(tlb_flush) || defined(tlb_start_vma) || defined(tlb_end_vma)
>> -#error MMU_GATHER_NO_RANGE relies on default tlb_flush(), tlb_start_vma() and tlb_end_vma()
>> +#if defined(tlb_flush)
>> +#error MMU_GATHER_NO_RANGE relies on default tlb_flush()
>>  #endif
>>  
>>  /*
>> @@ -362,10 +362,6 @@ static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vm
>>  
>>  #ifndef tlb_flush
>>  
>> -#if defined(tlb_start_vma) || defined(tlb_end_vma)
>> -#error Default tlb_flush() relies on default tlb_start_vma() and tlb_end_vma()
>> -#endif
> 
> #ifdef CONFIG_ARCH_WANT_AGGRESSIVE_TLB_FLUSH_BATCHING
> #error ....
> #endif
> 
> goes here...
> 
> 
>>  static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
>>  {
>>  	if (tlb->fullmm)
>>  		return;
>>  
>> +	if (IS_ENABLED(CONFIG_ARCH_WANT_AGGRESSIVE_TLB_FLUSH_BATCHING))
>> +		return;
> 
> Also, can you please stick to the CONFIG_MMU_GATHER_* namespace?
> 
> I also don't think AGRESSIVE_FLUSH_BATCHING quite captures what it does.
> How about:
> 
> 	CONFIG_MMU_GATHER_NO_PER_VMA_FLUSH

Yes please, have to have descriptive names.

I didn't quite see why this was much of an improvement though. Maybe 
follow up patches take advantage of it? I didn't see how they all fit 
together.

Thanks,
Nick