linux-kernel - Re: [PATCH 2/3] mm/memory.c: Update local TLB if PTE entry exists

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Date:   Sat, 16 May 2020 17:42:55 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Andrew Morton <akpm@...ux-foundation.org>
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
Subject: Re: [PATCH 2/3] mm/memory.c: Update local TLB if PTE entry exists



On 05/16/2020 04:40 AM, Andrew Morton wrote:
> On Fri, 15 May 2020 12:10:08 +0800 Bibo Mao <maobibo@...ngson.cn> wrote:
> 
>> If there are two threads hitting page fault at the same page,
>> one thread updates PTE entry and local TLB, the other can
>> update local tlb also, rather than give up and do page fault
>> again.
>>
>> ...
>>
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1770,8 +1770,8 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>>  			}
>>  			entry = pte_mkyoung(*pte);
>>  			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>> -			if (ptep_set_access_flags(vma, addr, pte, entry, 1))
>> -				update_mmu_cache(vma, addr, pte);
>> +			ptep_set_access_flags(vma, addr, pte, entry, 1);
>> +			update_mmu_cache(vma, addr, pte);
> 
> Presumably these changes mean that other architectures will run
> update_mmu_cache() more frequently than they used to.  How much more
> frequently, and what will be the impact of this change?  (Please fully
> explain all this in the changelog).
> 
It is only useful for those architects where software can update tlb, if the  function update_mmu_cache is used for other reason, it will bring out somewhat impact, and I will explain it in the changelog.

>>  		}
>>  		goto out_unlock;
>>  	}
>>
>> ...
>>
>> @@ -2463,7 +2462,8 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>>  		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
>>  		locked = true;
>>  		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>> -			/* The PTE changed under us. Retry page fault. */
>> +			/* The PTE changed under us, update local tlb */
>> +			pdate_mmu_cache(vma, addr, vmf->pte);
> 
> Missing a 'u' there.  Which tells me this patch isn't the one which you
> tested!
> 
Sorry about it, I will refresh the patch and add modification about this obvious typo

regards
bibo, mao
>>  			ret = false;
>>  			goto pte_unlock;
>>  		}
>>
>> ...
>>