linux-kernel - Re: [PATCH v2 1/2] mm: fix contiguous memmap assumptions about split page

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Date:   Wed, 30 Mar 2022 15:14:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Chen Wandun <chenwandun@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        willy@...radead.org
Subject: Re: [PATCH v2 1/2] mm: fix contiguous memmap assumptions about split
 page

On 30.03.22 12:25, Chen Wandun wrote:
> It isn't true for only SPARSEMEM configs to assume that a compound page
> has virtually contiguous page structs, so use nth_page to iterate each
> page.

Is this actually a "fix" or rather a preparation for having very large
compound pages (>= MAX_ORDER) that we'd be able to split?

Naive me would think that we'd currently only have order < MAX_ORDER,
and consequently would always fall into a single memory section where
the memmap is contiguous.

> 
> Inspired by:
> https://lore.kernel.org/linux-mm/20220204195852.1751729-8-willy@infradead.org/
> 
> Signed-off-by: Chen Wandun <chenwandun@...wei.com>
> ---
>  mm/compaction.c  | 6 +++---
>  mm/huge_memory.c | 2 +-
>  mm/page_alloc.c  | 2 +-
>  3 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c3e37aa9ff9e..ddff13b968a2 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -87,7 +87,7 @@ static unsigned long release_freepages(struct list_head *freelist)
>  static void split_map_pages(struct list_head *list)
>  {
>  	unsigned int i, order, nr_pages;
> -	struct page *page, *next;
> +	struct page *page, *next, *tmp;
>  	LIST_HEAD(tmp_list);
>  
>  	list_for_each_entry_safe(page, next, list, lru) {
> @@ -101,8 +101,8 @@ static void split_map_pages(struct list_head *list)
>  			split_page(page, order);
>  
>  		for (i = 0; i < nr_pages; i++) {
> -			list_add(&page->lru, &tmp_list);
> -			page++;
> +			tmp = nth_page(page, i);
> +			list_add(&tmp->lru, &tmp_list);
>  		}
>  	}
>  
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2fe38212e07c..d77fc2ad581d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2297,7 +2297,7 @@ static void lru_add_page_tail(struct page *head, struct page *tail,
>  static void __split_huge_page_tail(struct page *head, int tail,
>  		struct lruvec *lruvec, struct list_head *list)
>  {
> -	struct page *page_tail = head + tail;
> +	struct page *page_tail = nth_page(head, tail);
>  
>  	VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f648decfe39d..855211dea13e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3513,7 +3513,7 @@ void split_page(struct page *page, unsigned int order)
>  	VM_BUG_ON_PAGE(!page_count(page), page);
>  
>  	for (i = 1; i < (1 << order); i++)
> -		set_page_refcounted(page + i);
> +		set_page_refcounted(nth_page(page, i));
>  	split_page_owner(page, 1 << order);
>  	split_page_memcg(page, 1 << order);
>  }


-- 
Thanks,

David / dhildenb