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

Mel Gorman
2009-02-23T12:23:44Z

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@csn.ul.ie>
> > ---
> > 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