linux-kernel - Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to flush_pmd_tlb

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Message-ID: <fe12101e-8efe-22ad-0258-e6aeafc798cc@huawei.com>
Date:   Wed, 1 Apr 2020 16:51:15 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mark.rutland@....com>, <will@...nel.org>,
        <catalin.marinas@....com>, <aneesh.kumar@...ux.ibm.com>,
        <akpm@...ux-foundation.org>, <npiggin@...il.com>, <arnd@...db.de>,
        <rostedt@...dmis.org>, <maz@...nel.org>, <suzuki.poulose@....com>,
        <tglx@...utronix.de>, <yuzhao@...gle.com>, <Dave.Martin@....com>,
        <steven.price@....com>, <broonie@...nel.org>,
        <guohanjun@...wei.com>, <corbet@....net>, <vgupta@...opsys.com>,
        <tony.luck@...el.com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>,
        <prime.zeng@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <kuhn.chenqun@...wei.com>
Subject: Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to
 flush_pmd_tlb_range

Hi Peter,

On 2020/3/31 23:13, Peter Zijlstra wrote:
> On Tue, Mar 31, 2020 at 10:29:23PM +0800, Zhenyu Ye wrote:
>> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
>> index e2e2bef07dd2..32d4661e5a56 100644
>> --- a/include/asm-generic/pgtable.h
>> +++ b/include/asm-generic/pgtable.h
>> @@ -1160,10 +1160,10 @@ static inline int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
>>   * invalidate the entire TLB which is not desitable.
>>   * e.g. see arch/arc: flush_pmd_tlb_range
>>   */
>> -#define flush_pmd_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>> +#define flush_pmd_tlb_range(tlb, vma, addr, end)	flush_tlb_range(vma, addr, end)
>>  #define flush_pud_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>>  #else
>> -#define flush_pmd_tlb_range(vma, addr, end)	BUILD_BUG()
>> +#define flush_pmd_tlb_range(tlb, vma, addr, end)	BUILD_BUG()
>>  #define flush_pud_tlb_range(vma, addr, end)	BUILD_BUG()
>>  #endif
>>  #endif
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 3d7c01e76efc..96c9cf77bfb5 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -109,8 +109,14 @@ int pmdp_set_access_flags(struct vm_area_struct *vma,
>>  	int changed = !pmd_same(*pmdp, entry);
>>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>>  	if (changed) {
>> +		struct mmu_gather tlb;
>> +		unsigned long tlb_start = address;
>> +		unsigned long tlb_end = address + HPAGE_PMD_SIZE;
>>  		set_pmd_at(vma->vm_mm, address, pmdp, entry);
>> -		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>> +		tlb_gather_mmu(&tlb, vma->vm_mm, tlb_start, tlb_end);
>> +		tlb.cleared_pmds = 1;
>> +		flush_pmd_tlb_range(&tlb, vma, tlb_start, tlb_end);
>> +		tlb_finish_mmu(&tlb, tlb_start, tlb_end);
>>  	}
>>  	return changed;
>>  }
> 
> This is madness. Please, carefully consider what you just wrote and what
> it will do in the generic case.
> 
> Instead of trying to retro-fit flush_*tlb_range() to take an mmu_gather
> parameter, please replace them out-right.
> 

I'm sorry that I'm not sure what "replace them out-right" means.  Do you
mean that I should define flush_*_tlb_range like this?

#define flush_pmd_tlb_range(vma, addr, end)				\
	do {								\
		struct mmu_gather tlb;					\
		tlb_gather_mmu(&tlb, (vma)->vm_mm, addr, end);		\
		tlba.cleared_pmds = 1;					\
		flush_tlb_range(&tlb, vma, addr, end);			\
		tlb_finish_mmu(&tlb, addr, end);			\
	} while (0)


Thanks,
Zhenyu