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

Mel Gorman
2009-04-24T11:19:08Z

On Thu, Apr 23, 2009 at 03:59:51PM -0700, Andrew Morton wrote:
> 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.
>

True. mlocked pages are only going to be torn down during process exit or
munmap, both of which you'd expect to be rare when mlock is involved.

s/This is wasteful/While rare, this is unnecessary./

?

> > 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.
>

The patch author should have done the same thing :/

> 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.
>

PageMlocked becomes

static inline int PageMlocked(page)
{ return 0; }

so the compiler should be fit to spot that clearMlocked will always be 0. Even
if it didn't, it should have at least spotted that free_page_mlock(page)
is an empty inline function in this case.

Still double checked and I did not see the branch with
CONFIG_HAVE_MLOCKED_PAGE_BIT=n

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