Re: [PATCH 15/22] Do not disable interrupts in free_page_mlock()

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

On Wed, 22 Apr 2009 14:53:20 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> free_page_mlock() tests and clears PG_mlocked using locked versions of the
> bit operations. If set, it disables interrupts to update counters and this
> happens on every page free even though interrupts are disabled very shortly
> afterwards a second time.  This is wasteful.

Well.  It's only wasteful if the page was mlocked, which is rare.

> This patch splits what free_page_mlock() does. The bit check is still
> made. However, the update of counters is delayed until the interrupts are
> disabled and the non-lock version for clearing the bit is used. One potential
> weirdness with this split is that the counters do not get updated if the
> bad_page() check is triggered but a system showing bad pages is getting
> screwed already.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
> Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
>  mm/internal.h   |   11 +++--------
>  mm/page_alloc.c |    8 +++++++-
>  2 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 987bb03..58ec1bc 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -157,14 +157,9 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page)
>   */
>  static inline void free_page_mlock(struct page *page)
>  {
> -	if (unlikely(TestClearPageMlocked(page))) {
> -		unsigned long flags;
> -
> -		local_irq_save(flags);
> -		__dec_zone_page_state(page, NR_MLOCK);
> -		__count_vm_event(UNEVICTABLE_MLOCKFREED);
> -		local_irq_restore(flags);
> -	}
> +	__ClearPageMlocked(page);
> +	__dec_zone_page_state(page, NR_MLOCK);
> +	__count_vm_event(UNEVICTABLE_MLOCKFREED);
>  }

The conscientuous reviewer runs around and checks for free_page_mlock()
callers in other .c files which might be affected.

Only there are no such callers.

The reviewer's job would be reduced if free_page_mlock() wasn't
needlessly placed in a header file!

>  #else /* CONFIG_HAVE_MLOCKED_PAGE_BIT */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 67cafd0..7f45de1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -499,7 +499,6 @@ static inline void __free_one_page(struct page *page,
>  
>  static inline int free_pages_check(struct page *page)
>  {
> -	free_page_mlock(page);
>  	if (unlikely(page_mapcount(page) |
>  		(page->mapping != NULL)  |
>  		(page_count(page) != 0)  |
> @@ -556,6 +555,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
>  	unsigned long flags;
>  	int i;
>  	int bad = 0;
> +	int clearMlocked = PageMlocked(page);
>  
>  	for (i = 0 ; i < (1 << order) ; ++i)
>  		bad += free_pages_check(page + i);
> @@ -571,6 +571,8 @@ static void __free_pages_ok(struct page *page, unsigned int order)
>  	kernel_map_pages(page, 1 << order, 0);
>  
>  	local_irq_save(flags);
> +	if (unlikely(clearMlocked))
> +		free_page_mlock(page);

I wonder what the compiler does in the case
CONFIG_HAVE_MLOCKED_PAGE_BIT=n.  If it is dumb, this patch would cause
additional code generation.

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