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


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