Re: [PATCH] mm: move buffer_heads_over_limit check up

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Feb 2012 16:00:14 -0800 (PST)
Hugh Dickins <hughd@google.com> wrote:

> Not a functional change, just a minor internal cleanup: move the
> buffer_heads_over_limit processing up from move_active_pages_to_lru()
> (where it has to drop lock and reloop) to its caller shrink_active_list().
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  mm/vmscan.c |   19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> --- mmotm.orig/mm/vmscan.c	2012-02-07 16:59:13.000000000 -0800
> +++ mmotm/mm/vmscan.c	2012-02-07 17:07:23.800524771 -0800
> @@ -1641,18 +1641,6 @@ static void move_active_pages_to_lru(str
>  	unsigned long pgmoved = 0;
>  	struct page *page;
>  
> -	if (buffer_heads_over_limit) {
> -		spin_unlock_irq(&zone->lru_lock);
> -		list_for_each_entry(page, list, lru) {
> -			if (page_has_private(page) && trylock_page(page)) {
> -				if (page_has_private(page))
> -					try_to_release_page(page, 0);
> -				unlock_page(page);
> -			}
> -		}
> -		spin_lock_irq(&zone->lru_lock);
> -	}
> -
>  	while (!list_empty(list)) {
>  		struct lruvec *lruvec;
>  
> @@ -1734,6 +1722,13 @@ static void shrink_active_list(unsigned
>  			continue;
>  		}
>  
> +		if (buffer_heads_over_limit &&
> +		    page_has_private(page) && trylock_page(page)) {
> +			if (page_has_private(page))
> +				try_to_release_page(page, 0);
> +			unlock_page(page);
> +		}
> +
>  		if (page_referenced(page, 0, mz->mem_cgroup, &vm_flags)) {
>  			nr_rotated += hpage_nr_pages(page);
>  			/*

yup.

I don't think there's a lot of point in trying to micro-optimise the
buffer_heads_over_limit==true case, either.  But I suppose that
pointlessly locking 1000000 anon pages is indeed pointless.  Hopefully
there _is_ a point in micro-optimising the actual test for
buffer_heads_over_limit==true.  So...

--- a/mm/vmscan.c~mm-vmscan-forcibly-scan-highmem-if-there-are-too-many-buffer_heads-pinning-highmem-fix-fix
+++ a/mm/vmscan.c
@@ -1723,11 +1723,12 @@ static void shrink_active_list(unsigned 
 			continue;
 		}
 
-		if (buffer_heads_over_limit &&
-		    page_has_private(page) && trylock_page(page)) {
-			if (page_has_private(page))
-				try_to_release_page(page, 0);
-			unlock_page(page);
+		if (unlikely(buffer_heads_over_limit)) {
+			if (page_has_private(page) && trylock_page(page)) {
+				if (page_has_private(page))
+					try_to_release_page(page, 0);
+				unlock_page(page);
+			}
 		}
 
 		if (page_referenced(page, 0, mz->mem_cgroup, &vm_flags)) {
_

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