linux-kernel - Re: [PATCH] mm: avoid unnecessary page table walk for __get_user

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Message-ID: <afa4392a-01e1-5daf-4cdb-e92d708b85e1@nvidia.com>
Date:   Wed, 7 Sep 2022 18:58:52 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Chen Wandun <chenwandun@...wei.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] mm: avoid unnecessary page table walk for
 __get_user_pages

On 9/7/22 16:34, Andrew Morton wrote:
> On Tue, 6 Sep 2022 10:44:01 +0800 Chen Wandun <chenwandun@...wei.com> wrote:
> 
>> It is no need to walk page table and find pages if faultin_page success
>> and __get_user_pages does't care pages at all, so directly handle next
>> page.
>>
>> ...
>>
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1198,7 +1198,10 @@ static long __get_user_pages(struct mm_struct *mm,
>>   					   PTR_ERR(page) == -EMLINK, locked);
>>   			switch (ret) {
>>   			case 0:
>> -				goto retry;
>> +				if (pages)
>> +					goto retry;
>> +				else
>> +					goto next_page;

Wow, good point. And it's been doing that for a *long* time!

>>   			case -EBUSY:
>>   			case -EAGAIN:
>>   				ret = 0;
> 
> Looks right to me.  I redid the changelog somewhat:
> 
> : There is no need to walk page tables and find pages if faultin_page)_
> : succeeds and __get_user_pages)_ doesn't care about populating the pages[]
> : array.  So directly go on to handle the next page.
> 
> 

With that re-worded commit description, please feel to add

Reviewed-by: John Hubbard <jhubbard@...dia.com>


thanks,
-- 
John Hubbard
NVIDIA