Re: [PATCH for mmotm 1/5] cleanp page_remove_rmap()

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Jun 11, 2009 at 07:26:04PM +0900, KOSAKI Motohiro wrote:
> Subject: [PATCH] cleanp page_remove_rmap()
> 
> page_remove_rmap() has multiple PageAnon() test and it has
> a bit deeply nesting.
> 
> cleanup here.
> 
> note: this patch doesn't have behavior change.
> 
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Cc: Wu Fengguang <fengguang.wu@intel.com> 
> ---
>  mm/rmap.c |   59 ++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 32 insertions(+), 27 deletions(-)
> 
> Index: b/mm/rmap.c
> ===================================================================
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -862,34 +862,39 @@ void page_dup_rmap(struct page *page, st
>   */
>  void page_remove_rmap(struct page *page)
>  {
> -	if (atomic_add_negative(-1, &page->_mapcount)) {
> -		/*
> -		 * Now that the last pte has gone, s390 must transfer dirty
> -		 * flag from storage key to struct page.  We can usually skip
> -		 * this if the page is anon, so about to be freed; but perhaps
> -		 * not if it's in swapcache - there might be another pte slot
> -		 * containing the swap entry, but page not yet written to swap.
> -		 */
> -		if ((!PageAnon(page) || PageSwapCache(page)) &&
> -		    page_test_dirty(page)) {
> -			page_clear_dirty(page);
> -			set_page_dirty(page);
> -		}
> -		if (PageAnon(page))
> -			mem_cgroup_uncharge_page(page);
> -		__dec_zone_page_state(page,
> -			PageAnon(page) ? NR_ANON_PAGES : NR_FILE_MAPPED);
> -		mem_cgroup_update_mapped_file_stat(page, -1);
> -		/*
> -		 * It would be tidy to reset the PageAnon mapping here,
> -		 * but that might overwrite a racing page_add_anon_rmap
> -		 * which increments mapcount after us but sets mapping
> -		 * before us: so leave the reset to free_hot_cold_page,
> -		 * and remember that it's only reliable while mapped.
> -		 * Leaving it set also helps swapoff to reinstate ptes
> -		 * faster for those pages still in swapcache.
> -		 */
> +	if (!atomic_add_negative(-1, &page->_mapcount)) {
> +		/* the page is still mapped another one */
> +		return;
>  	}
> +
> +	/*
> +	 * Now that the last pte has gone, s390 must transfer dirty
> +	 * flag from storage key to struct page.  We can usually skip
> +	 * this if the page is anon, so about to be freed; but perhaps
> +	 * not if it's in swapcache - there might be another pte slot
> +	 * containing the swap entry, but page not yet written to swap.
> +	 */
> +	if ((!PageAnon(page) || PageSwapCache(page)) &&
> +		page_test_dirty(page)) {
> +		page_clear_dirty(page);
> +		set_page_dirty(page);
> +	}

Pure nitpick. It looks like page_test_dirty() can merge with the line
above it now. Then the condition won't be at the same indentation as the
statements.


> +	if (PageAnon(page)) {
> +		mem_cgroup_uncharge_page(page);
> +		__dec_zone_page_state(page, NR_ANON_PAGES);
> +	} else {
> +		__dec_zone_page_state(page, NR_FILE_MAPPED);
> +	}

Ok, first actual change and it looks functionally equivalent and avoids a
second PageAnon test. I suspect it fractionally increases text size but as
PageAnon is an atomic bit opreation, we want to avoid calling that twice too.

> +	mem_cgroup_update_mapped_file_stat(page, -1);
> +	/*
> +	 * It would be tidy to reset the PageAnon mapping here,
> +	 * but that might overwrite a racing page_add_anon_rmap
> +	 * which increments mapcount after us but sets mapping
> +	 * before us: so leave the reset to free_hot_cold_page,
> +	 * and remember that it's only reliable while mapped.
> +	 * Leaving it set also helps swapoff to reinstate ptes
> +	 * faster for those pages still in swapcache.
> +	 */
>  }
>  

Ok, patch looks good to me. I'm not seeing what it has to do with the
zone_reclaim() problem though so you might want to send it separate from
the set for clarity.

Acked-by: Mel Gorman <mel@csn.ul.ie>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab