linux-kernel - Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset

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Date:   Mon, 13 May 2019 16:01:09 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Will Deacon <will.deacon@....com>
Cc:     jstancek@...hat.com, peterz@...radead.org, namit@...are.com,
        minchan@...nel.org, mgorman@...e.de, stable@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush



On 5/13/19 9:38 AM, Will Deacon wrote:
> On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>> index 99740e1..469492d 100644
>> --- a/mm/mmu_gather.c
>> +++ b/mm/mmu_gather.c
>> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>>   {
>>   	/*
>>   	 * If there are parallel threads are doing PTE changes on same range
>> -	 * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
>> -	 * flush by batching, a thread has stable TLB entry can fail to flush
>> -	 * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
>> -	 * forcefully if we detect parallel PTE batching threads.
>> +	 * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
>> +	 * flush by batching, one thread may end up seeing inconsistent PTEs
>> +	 * and result in having stale TLB entries.  So flush TLB forcefully
>> +	 * if we detect parallel PTE batching threads.
>> +	 *
>> +	 * However, some syscalls, e.g. munmap(), may free page tables, this
>> +	 * needs force flush everything in the given range. Otherwise this
>> +	 * may result in having stale TLB entries for some architectures,
>> +	 * e.g. aarch64, that could specify flush what level TLB.
>>   	 */
>> -	if (mm_tlb_flush_nested(tlb->mm)) {
>> -		__tlb_reset_range(tlb);
>> -		__tlb_adjust_range(tlb, start, end - start);
>> +	if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
>> +		/*
>> +		 * Since we can't tell what we actually should have
>> +		 * flushed, flush everything in the given range.
>> +		 */
>> +		tlb->freed_tables = 1;
>> +		tlb->cleared_ptes = 1;
>> +		tlb->cleared_pmds = 1;
>> +		tlb->cleared_puds = 1;
>> +		tlb->cleared_p4ds = 1;
>> +
>> +		/*
>> +		 * Some architectures, e.g. ARM, that have range invalidation
>> +		 * and care about VM_EXEC for I-Cache invalidation, need force
>> +		 * vma_exec set.
>> +		 */
>> +		tlb->vma_exec = 1;
>> +
>> +		/* Force vma_huge clear to guarantee safer flush */
>> +		tlb->vma_huge = 0;
>> +
>> +		tlb->start = start;
>> +		tlb->end = end;
>>   	}
> Whilst I think this is correct, it would be interesting to see whether
> or not it's actually faster than just nuking the whole mm, as I mentioned
> before.
>
> At least in terms of getting a short-term fix, I'd prefer the diff below
> if it's not measurably worse.

I did a quick test with ebizzy (96 threads with 5 iterations) on my x86 
VM, it shows slightly slowdown on records/s but much more sys time spent 
with fullmm flush, the below is the data.

                                     nofullmm                 fullmm
ops (records/s)              225606                  225119
sys (s)                            0.69                        1.14

It looks the slight reduction of records/s is caused by the increase of 
sys time.

>
> Will
>
> --->8
>
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index 99740e1dd273..cc251422d307 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>   	 * forcefully if we detect parallel PTE batching threads.
>   	 */
>   	if (mm_tlb_flush_nested(tlb->mm)) {
> +		tlb->fullmm = 1;
>   		__tlb_reset_range(tlb);
> -		__tlb_adjust_range(tlb, start, end - start);
> +		tlb->freed_tables = 1;
>   	}
>   
>   	tlb_flush_mmu(tlb);