linux-kernel - Re: [PATCH -v4 RESEND 8/9] mm, THP, swap: Support to split THP in swap cache

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Message-ID: <871syx71gc.fsf@yhuang-dev.intel.com>
Date:   Mon, 31 Oct 2016 09:26:59 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Hillf Danton <hillf.zj@...baba-inc.com>
Cc:     "'Huang\, Ying'" <ying.huang@...el.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        <tim.c.chen@...el.com>, <dave.hansen@...el.com>,
        <andi.kleen@...el.com>, <aaron.lu@...el.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        'Andrea Arcangeli' <aarcange@...hat.com>,
        "'Kirill A . Shutemov'" <kirill.shutemov@...ux.intel.com>,
        'Ebru Akagunduz' <ebru.akagunduz@...il.com>
Subject: Re: [PATCH -v4 RESEND 8/9] mm, THP, swap: Support to split THP in swap cache

Hillf Danton <hillf.zj@...baba-inc.com> writes:

> On Friday, October 28, 2016 1:56 PM Huang, Ying wrote: 
>> @@ -2016,10 +2021,12 @@ int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
>>  /* Racy check whether the huge page can be split */
>>  bool can_split_huge_page(struct page *page)
>>  {
>> -	int extra_pins = 0;
>> +	int extra_pins;
>> 
>>  	/* Additional pins from radix tree */
>> -	if (!PageAnon(page))
>> +	if (PageAnon(page))
>> +		extra_pins = PageSwapCache(page) ? HPAGE_PMD_NR : 0;
>> +	else
>>  		extra_pins = HPAGE_PMD_NR;
>
> extra_pins is computed in this newly added helper.
>
>>  	return total_mapcount(page) == page_count(page) - extra_pins - 1;
>>  }
>> @@ -2072,7 +2079,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>  			ret = -EBUSY;
>>  			goto out;
>>  		}
>> -		extra_pins = 0;
>> +		extra_pins = PageSwapCache(head) ? HPAGE_PMD_NR : 0;
>
> It is also computed at the call site, so can we fold them into one?

Sounds reasonable.  I will add another argument to can_split_huge_page()
to return extra_pins, so we can avoid duplicated code and calculation.

Best Regards,
Huang, Ying

>>  		mapping = NULL;
>>  		anon_vma_lock_write(anon_vma);
>>  	} else {
>> --
>> 2.9.3