linux-kernel - Re: [PATCH 2/5] mm/huge_memory.c: use page->deferred

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Date:   Wed, 28 Apr 2021 16:23:21 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        <akpm@...ux-foundation.org>
CC:     <ziy@...dia.com>, <william.kucharski@...cle.com>,
        <willy@...radead.org>, <yang.shi@...ux.alibaba.com>,
        <aneesh.kumar@...ux.ibm.com>, <rcampbell@...dia.com>,
        <songliubraving@...com>, <kirill.shutemov@...ux.intel.com>,
        <riel@...riel.com>, <hannes@...xchg.org>, <minchan@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/5] mm/huge_memory.c: use page->deferred_list

On 2021/4/28 11:07, Anshuman Khandual wrote:
> 
> On 4/27/21 7:02 PM, Miaohe Lin wrote:
>> Now that we can represent the location of ->deferred_list instead of
>> ->mapping + ->index, make use of it to improve readability.
> 
> Could you please explain how page->deferred_list and page->mapping
> are interchangeable here ?

It's because there is a union in struct page:

union {
		struct {
			struct list_head lru;
			struct address_space *mapping;
			pgoff_t index;
			unsigned long private;
		};
		struct {
			unsigned long _compound_pad_1;
			atomic_t hpage_pinned_refcount;
			struct list_head *deferred_list*;
		};
};

And initially there is no deferred_list and it's added via commit 66a6ffd2af42 ("mm: combine first
three unions in struct page"). Also commit fa3015b7eed5 ("mm: use page->deferred_list") did the similar
cleanup. Am I expected to add these to the commit log?

Many thanks.

> 
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/huge_memory.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 63ed6b25deaa..76ca1eb2a223 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2868,7 +2868,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>  	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>  	/* Take pin on all head pages to avoid freeing them under us */
>>  	list_for_each_safe(pos, next, &ds_queue->split_queue) {
>> -		page = list_entry((void *)pos, struct page, mapping);
>> +		page = list_entry((void *)pos, struct page, deferred_list);
>>  		page = compound_head(page);
>>  		if (get_page_unless_zero(page)) {
>>  			list_move(page_deferred_list(page), &list);
>> @@ -2883,7 +2883,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>  	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>>  
>>  	list_for_each_safe(pos, next, &list) {
>> -		page = list_entry((void *)pos, struct page, mapping);
>> +		page = list_entry((void *)pos, struct page, deferred_list);
>>  		if (!trylock_page(page))
>>  			goto next;
>>  		/* split_huge_page() removes page from list on success */
>>
> .
>