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    SubjectRe: [PATCH] mm: Warn once when a page is freed with PG_mlocked set V2
    Hello Christoph,

    On Wed, Jul 15, 2009 at 10:31:54AM -0400, Christoph Lameter wrote:
    > On Wed, 15 Jul 2009, Mel Gorman wrote:
    >
    > > -static inline int free_pages_check(struct page *page)
    > > +static inline int free_pages_check(struct page *page, int wasMlocked)
    > > {
    > > + WARN_ONCE(wasMlocked, KERN_WARNING
    > > + "Page flag mlocked set for process %s at pfn:%05lx\n"
    > > + "page:%p flags:0x%lX\n",
    > > + current->comm, page_to_pfn(page),
    > > + page, page->flags|__PG_MLOCKED);
    > > +
    > > if (unlikely(page_mapcount(page) |
    >
    > There is already a free_page_mlocked() that is only called if the mlock
    > bit is set. Move it into there to avoid having to run two checks in the
    > hot codee path?
    >
    > Also __free_pages_ok() now has a TestClearMlocked in the hot code path.
    > Would it be possible to get rid of the unconditional use of an atomic
    > operation? Just check the bit and clear it later in free_page_mlocked()?

    That was initially done, but free_pages_check() checks for that flag
    and did bad_page() on those mlocked ones. Now, one idea was to not
    check the mlocked flag at all in free_pages_check() as we handle it
    differently anyway. But I think we might still want to check for it
    in tail-pages of higher order blocks.

    And if you move that warning after free_pages_check(), the interesting
    bits for the warning have been wiped already.

    But we can get rid of the locked test-and-clear despite all the other
    issues, patch below.

    Hannes

    From eee677ddea61b1331a3bd8e402a0d02437fe872a Mon Sep 17 00:00:00 2001
    From: Johannes Weiner <hannes@cmpxchg.org>
    Date: Wed, 15 Jul 2009 23:40:28 +0200
    Subject: [patch] mm: non-atomic test-clear of PG_mlocked on free

    By the time PG_mlocked is cleared in the page freeing path, nobody
    else is looking at our page->flags anymore.

    It is thus safe to make the test-and-clear non-atomic and thereby
    removing an unnecessary and expensive operation from a hotpath.

    Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    ---
    include/linux/page-flags.h | 12 +++++++++---
    mm/page_alloc.c | 4 ++--
    2 files changed, 11 insertions(+), 5 deletions(-)

    diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
    index e2e5ce5..10e6011 100644
    --- a/include/linux/page-flags.h
    +++ b/include/linux/page-flags.h
    @@ -158,6 +158,9 @@ static inline int TestSetPage##uname(struct page *page) \
    static inline int TestClearPage##uname(struct page *page) \
    { return test_and_clear_bit(PG_##lname, &page->flags); }

    +#define __TESTCLEARFLAG(uname, lname) \
    +static inline int __TestClearPage##uname(struct page *page) \
    + { return __test_and_clear_bit(PG_##lname, &page->flags); }

    #define PAGEFLAG(uname, lname) TESTPAGEFLAG(uname, lname) \
    SETPAGEFLAG(uname, lname) CLEARPAGEFLAG(uname, lname)
    @@ -184,6 +187,9 @@ static inline void __ClearPage##uname(struct page *page) { }
    #define TESTCLEARFLAG_FALSE(uname) \
    static inline int TestClearPage##uname(struct page *page) { return 0; }

    +#define __TESTCLEARFLAG_FALSE(uname) \
    +static inline int __TestClearPage##uname(struct page *page) { return 0; }
    +
    struct page; /* forward declaration */

    TESTPAGEFLAG(Locked, locked) TESTSETFLAG(Locked, locked)
    @@ -250,11 +256,11 @@ PAGEFLAG(Unevictable, unevictable) __CLEARPAGEFLAG(Unevictable, unevictable)
    #ifdef CONFIG_HAVE_MLOCKED_PAGE_BIT
    #define MLOCK_PAGES 1
    PAGEFLAG(Mlocked, mlocked) __CLEARPAGEFLAG(Mlocked, mlocked)
    - TESTSCFLAG(Mlocked, mlocked)
    + TESTSCFLAG(Mlocked, mlocked) __TESTCLEARFLAG(Mlocked, mlocked)
    #else
    #define MLOCK_PAGES 0
    -PAGEFLAG_FALSE(Mlocked)
    - SETPAGEFLAG_NOOP(Mlocked) TESTCLEARFLAG_FALSE(Mlocked)
    +PAGEFLAG_FALSE(Mlocked) SETPAGEFLAG_NOOP(Mlocked)
    + TESTCLEARFLAG_FALSE(Mlocked) __TESTCLEARFLAG_FALSE(Mlocked)
    #endif

    #ifdef CONFIG_IA64_UNCACHED_ALLOCATOR
    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index caa9268..b0c8758 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -557,7 +557,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
    unsigned long flags;
    int i;
    int bad = 0;
    - int wasMlocked = TestClearPageMlocked(page);
    + int wasMlocked = __TestClearPageMlocked(page);

    kmemcheck_free_shadow(page, order);

    @@ -1021,7 +1021,7 @@ 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 wasMlocked = TestClearPageMlocked(page);
    + int wasMlocked = __TestClearPageMlocked(page);

    kmemcheck_free_shadow(page, 0);

    --
    1.6.3


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