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Subject: Re: [PATCH 18/25] Do not disable interrupts in free_page_mlock()
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Lin Ming <ming.m.lin@intel.com>,
       Zhang Yanmin <yanmin_zhang@linux.intel.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>
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References: <1240266011-11140-1-git-send-email-mel@csn.ul.ie>
	 <1240266011-11140-19-git-send-email-mel@csn.ul.ie>
Date: Tue, 21 Apr 2009 10:55:07 +0300
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On Mon, 2009-04-20 at 23:20 +0100, Mel Gorman 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.
> 
> 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>

> ---
>  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);
> -	}

Maybe add a VM_BUG_ON(!PageMlocked(page))?

> +	__ClearPageMlocked(page);
> +	__dec_zone_page_state(page, NR_MLOCK);
> +	__count_vm_event(UNEVICTABLE_MLOCKFREED);
>  }
>  
>  #else /* CONFIG_HAVE_MLOCKED_PAGE_BIT */