Re: [PATCH 15/20] Do not disable interrupts in free_page_mlock()

[Mel Gorman]

On Mon, Feb 23, 2009 at 10:19:00AM +0100, Peter Zijlstra wrote:
> On Sun, 2009-02-22 at 23:17 +0000, Mel Gorman wrote:
> > free_page_mlock() tests and clears PG_mlocked. 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. 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@xxxxxxxxx>
> > ---
> >  mm/internal.h   |   10 ++--------
> >  mm/page_alloc.c |    8 +++++++-
> >  2 files changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 478223b..b52bf86 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -155,14 +155,8 @@ 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);
> > -	}
> > +	__dec_zone_page_state(page, NR_MLOCK);
> > +	__count_vm_event(UNEVICTABLE_MLOCKFREED);
> >  }
> 
> Its not actually clearing PG_mlocked anymore, so the name is now a tad
> misleading.
> 

Really? I see the following

#ifdef CONFIG_UNEVICTABLE_LRU
PAGEFLAG(Unevictable, unevictable) __CLEARPAGEFLAG(Unevictable, unevictable)
        TESTCLEARFLAG(Unevictable, unevictable)

#define MLOCK_PAGES 1
PAGEFLAG(Mlocked, mlocked) __CLEARPAGEFLAG(Mlocked, mlocked)
        TESTSCFLAG(Mlocked, mlocked)

#else

#define MLOCK_PAGES 0
PAGEFLAG_FALSE(Mlocked)
        SETPAGEFLAG_NOOP(Mlocked) TESTCLEARFLAG_FALSE(Mlocked)

PAGEFLAG_FALSE(Unevictable) TESTCLEARFLAG_FALSE(Unevictable)
        SETPAGEFLAG_NOOP(Unevictable) CLEARPAGEFLAG_NOOP(Unevictable)
        __CLEARPAGEFLAG_NOOP(Unevictable)
#endif

So there is a PG_mlocked bit once UNEVITABLE_LRU is set which was the
case on the tests I was running. I'm probably missing something silly.

> That said, since we're freeing the page, there ought to not be another
> reference to the page, in which case it appears to me we could safely
> use the unlocked variant of TestClear*().
> 

Regrettably, unlocked variants do not appear to be defined as such but
the following should do the job, right? It applies on top of the current
change.

diff --git a/mm/internal.h b/mm/internal.h
index b52bf86..7f775a1 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -155,6 +155,7 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page)
  */
 static inline void free_page_mlock(struct page *page)
 {
+	__ClearPageMlocked(page);
 	__dec_zone_page_state(page, NR_MLOCK);
 	__count_vm_event(UNEVICTABLE_MLOCKFREED);
 }
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index edac673..8bd0533 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -580,7 +580,7 @@ static void __free_pages_ok(struct page *page, unsigned int order,
 	unsigned long flags;
 	int i;
 	int bad = 0;
-	int clearMlocked = TestClearPageMlocked(page);
+	int clearMlocked = PageMlocked(page);
 
 	for (i = 0 ; i < (1 << order) ; ++i)
 		bad += free_pages_check(page + i);
@@ -1040,7 +1040,7 @@ static void free_pcp_page(struct page *page)
 	struct per_cpu_pages *pcp;
 	unsigned long flags;
 	int migratetype;
-	int clearMlocked = TestClearPageMlocked(page);
+	int clearMlocked = PageMlocked(page);
 
 	if (PageAnon(page))
 		page->mapping = NULL;

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