Re: [PATCH 1/3 v2] mm: Batch unmapping of file mapped pages in shrink_page_list

[Minchan Kim <minchan@kernel.org>]

Hi Tim,

On Mon, Sep 10, 2012 at 09:19:25AM -0700, Tim Chen wrote:
> We gather the pages that need to be unmapped in shrink_page_list.  We batch
> the unmap to reduce the frequency of acquisition of
> the tree lock protecting the mapping's radix tree. This is
> possible as successive pages likely share the same mapping in 
> __remove_mapping_batch routine.  This avoids excessive cache bouncing of
> the tree lock when page reclamations are occurring simultaneously.
> 
> Tim
> ---
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> --- 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index aac5672..d4ab646 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -600,6 +600,85 @@ cannot_free:
>  	return 0;
>  }
>  
> +/* Same as __remove_mapping, but batching operations to minimize locking */
> +/* Pages to be unmapped should be locked first */

Use below comment style.

/*
 * blah, blah,
 */

> +static int __remove_mapping_batch(struct list_head *unmap_pages,
> +				  struct list_head *ret_pages,
> +				  struct list_head *free_pages)
> +{
> +	struct address_space *mapping, *next;
> +	LIST_HEAD(swap_pages);
> +	swp_entry_t swap;
> +	struct page *page;
> +	int nr_reclaimed;

Use unsigned long instead of int.

> +
> +	mapping = NULL;
> +	nr_reclaimed = 0;
> +	while (!list_empty(unmap_pages)) {
> +
> +		page = lru_to_page(unmap_pages);
> +		BUG_ON(!PageLocked(page));
> +
> +		list_del(&page->lru);
> +		next = page_mapping(page);
> +		if (mapping != next) {
> +			if (mapping)
> +				spin_unlock_irq(&mapping->tree_lock);
> +			mapping = next;
> +			spin_lock_irq(&mapping->tree_lock);
> +		}
> +
> +		if (!page_freeze_refs(page, 2))
> +			goto cannot_free;
> +		if (unlikely(PageDirty(page))) {
> +			page_unfreeze_refs(page, 2);
> +			goto cannot_free;
> +		}
> +
> +		if (PageSwapCache(page)) {
> +			__delete_from_swap_cache(page);
> +			/* swapcache_free need to be called without tree_lock */
> +			list_add(&page->lru, &swap_pages);
> +		} else {
> +			void (*freepage)(struct page *);
> +
> +			freepage = mapping->a_ops->freepage;
> +
> +			__delete_from_page_cache(page);
> +			mem_cgroup_uncharge_cache_page(page);
> +
> +			if (freepage != NULL)
> +				freepage(page);
> +
> +			unlock_page(page);

If you use unlock_page, you removes a978d6f5.
You can use __clear_page_locked.


> +			nr_reclaimed++;
> +			list_add(&page->lru, free_pages);
> +		}
> +		continue;
> +cannot_free:
> +		unlock_page(page);
> +		list_add(&page->lru, ret_pages);
> +		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> +
> +	}
> +
> +	if (mapping)
> +		spin_unlock_irq(&mapping->tree_lock);
> +
> +	while (!list_empty(&swap_pages)) {
> +		page = lru_to_page(&swap_pages);
> +		list_del(&page->lru);
> +
> +		swap.val = page_private(page);
> +		swapcache_free(swap, page);
> +
> +		unlock_page(page);

Ditto.

> +		nr_reclaimed++;
> +		list_add(&page->lru, free_pages);
> +	}
> +
> +	return nr_reclaimed;
> +}
>  /*
>   * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
>   * someone else has a ref on the page, abort and return 0.  If it was
> @@ -771,6 +850,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
> +	LIST_HEAD(unmap_pages);
>  	int pgactivate = 0;
>  	unsigned long nr_dirty = 0;
>  	unsigned long nr_congested = 0;
> @@ -969,17 +1049,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			}
>  		}
>  
> -		if (!mapping || !__remove_mapping(mapping, page))
> +		if (!mapping)
>  			goto keep_locked;
>  
> -		/*
> -		 * At this point, we have no other references and there is
> -		 * no way to pick any more up (removed from LRU, removed
> -		 * from pagecache). Can use non-atomic bitops now (and
> -		 * we obviously don't have to worry about waking up a process
> -		 * waiting on the page lock, because there are no references.
> -		 */

Please don't remove this description and move it into your function with
__clear_page_locked.

> -		__clear_page_locked(page);
> +		/* remove pages from mapping in batch at end of loop */
> +		list_add(&page->lru, &unmap_pages);
> +		continue;
> +
>  free_it:
>  		nr_reclaimed++;
>  
> @@ -1014,6 +1090,9 @@ keep_lumpy:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}
>  
> +	nr_reclaimed += __remove_mapping_batch(&unmap_pages, &ret_pages,
> +					       &free_pages);
> +
>  	/*
>  	 * Tag a zone as congested if all the dirty pages encountered were
>  	 * backed by a congested BDI. In this case, reclaimers should just

-- 
Kind regards,
Minchan Kim

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