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

From: Mel Gorman
Date: Thu Jun 11 2009 - 07:02:08 EST


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@xxxxxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Cc: Mel Gorman <mel@xxxxxxxxx>
> Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> 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@xxxxxxxxx>

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