linux-kernel - Re: [PATCH v2] mm: huge_memory: convert split_huge_pages

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Date:   Fri, 6 Jan 2023 14:23:25 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <willy@...radead.org>
Subject: Re: [PATCH v2] mm: huge_memory: convert split_huge_pages_all() to use
 a folio



On 2023/1/6 11:06, Andrew Morton wrote:
> On Tue, 3 Jan 2023 09:58:36 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:
> 
>>
>>
>> On 2022/12/31 5:45, Andrew Morton wrote:
>>> On Fri, 30 Dec 2022 17:30:20 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:
>>>
>>>> Straightforwardly convert split_huge_pages_all() to use a folio.
>>>>
>>>> ...
>>>>
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -2932,6 +2932,7 @@ static void split_huge_pages_all(void)
>>>>    {
>>>>    	struct zone *zone;
>>>>    	struct page *page;
>>>> +	struct folio *folio;
>>>>    	unsigned long pfn, max_zone_pfn;
>>>>    	unsigned long total = 0, split = 0;
>>>>    
>>>> @@ -2944,24 +2945,32 @@ static void split_huge_pages_all(void)
>>>>    			int nr_pages;
>>>>    
>>>>    			page = pfn_to_online_page(pfn);
>>>> -			if (!page || !get_page_unless_zero(page))
>>>> +			if (!page || PageTail(page))
>>>> +				continue;
>>>> +			folio = page_folio(page);
>>>> +			if (!folio_try_get(folio))
>>>>    				continue;
>>>>    
>>>> -			if (zone != page_zone(page))
>>>> +			if (unlikely(page_folio(page) != folio))
>>>> +				goto next;
>>>> +
>>>> +			if (zone != folio_zone(folio))
>>>>    				goto next;
>>>
>>> I'm still not understanding the above hunk.  Why is the
>>> "page_folio(page) != folio" check added?  Should it be commented?
>>
>> There is a comment in try_get_folio(), is it enough?
> 
> Nobody would think to look at a private function in gup.c.

Sorry，I mean that comment is to explain why adding a new recheck.

> 
> I suggest this folio_try_get() rule be documented at the
> folio_try_get() implementation site.
> 
> Most callers do perform this check, but not the ones in vmscan.c ?

Hope Matthew could give some advise, thanks