linux-kernel - Re: [PATCH 18/25] Do not disable interrupts in free_page

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Date:	Wed, 22 Apr 2009 10:43:00 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 18/25] Do not disable interrupts in free_page_mlock()

On Wed, 2009-04-22 at 09:13 +0900, KOSAKI Motohiro wrote:
> (cc to Lee)
> 
> > 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.
> 
> Looks good. thanks good improvement.
> I hope Lee also ack this.
> 
> 	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> 
> 
>   -kosaki
> 

Yes, this looks OK.  If we're racing on a page that is being freed or,
as Mel notes, some other condition triggers bad_page(), we got bigger
problems than inaccurate statistics...

	Acked-by: Lee Schermerhorn <lee.schermerhorn@...com>

> 
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > Reviewed-by: Christoph Lameter <cl@...ux-foundation.org>
> > ---
> >  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);
> >  }
> >  
> >  #else /* CONFIG_HAVE_MLOCKED_PAGE_BIT */
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index a1ca038..bf4b8d9 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);
> >  	__count_vm_events(PGFREE, 1 << order);
> >  	free_one_page(page_zone(page), page, order,
> >  					get_pageblock_migratetype(page));
> > @@ -1018,6 +1020,7 @@ static void free_hot_cold_page(struct page *page, int cold)
> >  	struct zone *zone = page_zone(page);
> >  	struct per_cpu_pages *pcp;
> >  	unsigned long flags;
> > +	int clearMlocked = PageMlocked(page);
> >  
> >  	if (PageAnon(page))
> >  		page->mapping = NULL;
> > @@ -1033,7 +1036,10 @@ static void free_hot_cold_page(struct page *page, int cold)
> >  
> >  	pcp = &zone_pcp(zone, get_cpu())->pcp;
> >  	local_irq_save(flags);
> > +	if (unlikely(clearMlocked))
> > +		free_page_mlock(page);
> >  	__count_vm_event(PGFREE);
> > +
> >  	if (cold)
> >  		list_add_tail(&page->lru, &pcp->list);
> >  	else
> > -- 
> > 1.5.6.5
> > 
> 
> 
> 

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