Re: [PATCH 1/6] mm/compaction: split freepages without holding the zone lock

[Vlastimil Babka]

On 05/03/2016 07:22 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> We don't need to split freepages with holding the zone lock. It will cause
> more contention on zone lock so not desirable.

Fair enough, I just worry about the same thing as Hugh pointed out
recently [1] in that it increases the amount of tricky stuff in
compaction.c doing similar but not quite the same stuff as page/alloc.c,
and which will be forgotten to be updated once somebody updates
prep_new_page with e.g. a new debugging check. Can you perhaps think of
a more robust solution here?

Thanks

[1] http://marc.info/?i=alpine.LSU.2.11.1604270029350.7066%40eggly.anvils%3E

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   include/linux/mm.h |  1 -
>   mm/compaction.c    | 42 ++++++++++++++++++++++++++++++------------
>   mm/page_alloc.c    | 27 ---------------------------
>   3 files changed, 30 insertions(+), 40 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7b52750..9608f33 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -523,7 +523,6 @@ void __put_page(struct page *page);
>   void put_pages_list(struct list_head *pages);
>   
>   void split_page(struct page *page, unsigned int order);
> -int split_free_page(struct page *page);
>   
>   /*
>    * Compound pages have a destructor function.  Provide a
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c9a95c1..ecf0252 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -65,13 +65,31 @@ static unsigned long release_freepages(struct list_head *freelist)
>   
>   static void map_pages(struct list_head *list)
>   {
> -	struct page *page;
> +	unsigned int i, order, nr_pages;
> +	struct page *page, *next;
> +	LIST_HEAD(tmp_list);
> +
> +	list_for_each_entry_safe(page, next, list, lru) {
> +		list_del(&page->lru);
> +
> +		order = page_private(page);
> +		nr_pages = 1 << order;
> +		set_page_private(page, 0);
> +		set_page_refcounted(page);
> +
> +		arch_alloc_page(page, order);
> +		kernel_map_pages(page, nr_pages, 1);
> +		kasan_alloc_pages(page, order);
> +		if (order)
> +			split_page(page, order);
>   
> -	list_for_each_entry(page, list, lru) {
> -		arch_alloc_page(page, 0);
> -		kernel_map_pages(page, 1, 1);
> -		kasan_alloc_pages(page, 0);
> +		for (i = 0; i < nr_pages; i++) {
> +			list_add(&page->lru, &tmp_list);
> +			page++;
> +		}
>   	}
> +
> +	list_splice(&tmp_list, list);
>   }
>   
>   static inline bool migrate_async_suitable(int migratetype)
> @@ -368,12 +386,13 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>   	unsigned long flags = 0;
>   	bool locked = false;
>   	unsigned long blockpfn = *start_pfn;
> +	unsigned int order;
>   
>   	cursor = pfn_to_page(blockpfn);
>   
>   	/* Isolate free pages. */
>   	for (; blockpfn < end_pfn; blockpfn++, cursor++) {
> -		int isolated, i;
> +		int isolated;
>   		struct page *page = cursor;
>   
>   		/*
> @@ -439,13 +458,12 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>   				goto isolate_fail;
>   		}
>   
> -		/* Found a free page, break it into order-0 pages */
> -		isolated = split_free_page(page);
> +		/* Found a free page, will break it into order-0 pages */
> +		order = page_order(page);
> +		isolated = __isolate_free_page(page, page_order(page));
> +		set_page_private(page, order);
>   		total_isolated += isolated;
> -		for (i = 0; i < isolated; i++) {
> -			list_add(&page->lru, freelist);
> -			page++;
> -		}
> +		list_add_tail(&page->lru, freelist);
>   
>   		/* If a page was split, advance to the end of it */
>   		if (isolated) {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5dd65d9..60d7f10 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2532,33 +2532,6 @@ int __isolate_free_page(struct page *page, unsigned int order)
>   }
>   
>   /*
> - * Similar to split_page except the page is already free. As this is only
> - * being used for migration, the migratetype of the block also changes.
> - * As this is called with interrupts disabled, the caller is responsible
> - * for calling arch_alloc_page() and kernel_map_page() after interrupts
> - * are enabled.
> - *
> - * Note: this is probably too low level an operation for use in drivers.
> - * Please consult with lkml before using this in your driver.
> - */
> -int split_free_page(struct page *page)
> -{
> -	unsigned int order;
> -	int nr_pages;
> -
> -	order = page_order(page);
> -
> -	nr_pages = __isolate_free_page(page, order);
> -	if (!nr_pages)
> -		return 0;
> -
> -	/* Split into individual pages */
> -	set_page_refcounted(page);
> -	split_page(page, order);
> -	return nr_pages;
> -}
> -
> -/*
>    * Update NUMA hit/miss statistics
>    *
>    * Must be called with interrupts disabled.
>