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    Subject[PATCH v25 04/13] mm/idle_page_tracking: Make PG_idle reusable
    Date
    From: SeongJae Park <sjpark@amazon.de>

    PG_idle and PG_young allow the two PTE Accessed bit users, Idle Page
    Tracking and the reclaim logic concurrently work while don't interfere
    each other. That is, when they need to clear the Accessed bit, they set
    PG_young to represent the previous state of the bit, respectively. And
    when they need to read the bit, if the bit is cleared, they further read
    the PG_young to know whether the other has cleared the bit meanwhile or
    not.

    We could add another page flag and extend the mechanism to use the flag
    if we need to add another concurrent PTE Accessed bit user subsystem.
    However, the space is limited. Meanwhile, if the new subsystem is
    mutually exclusive with IDLE_PAGE_TRACKING or interfering with it is not
    a real problem, it would be ok to simply reuse the PG_idle flag.
    However, it's impossible because the flags are dependent on
    IDLE_PAGE_TRACKING.

    To allow such reuse of the flags, this commit separates the PG_young and
    PG_idle flag logic from IDLE_PAGE_TRACKING and introduces new kernel
    config, 'PAGE_IDLE_FLAG'. Hence, a new subsystem would be able to reuse
    PG_idle without depending on IDLE_PAGE_TRACKING.

    In the next commit, DAMON's reference implementation of the virtual
    memory address space monitoring primitives will use it.

    Signed-off-by: SeongJae Park <sjpark@amazon.de>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    ---
    include/linux/page-flags.h | 4 ++--
    include/linux/page_ext.h | 2 +-
    include/linux/page_idle.h | 6 +++---
    include/trace/events/mmflags.h | 2 +-
    mm/Kconfig | 8 ++++++++
    mm/page_ext.c | 12 +++++++++++-
    mm/page_idle.c | 10 ----------
    7 files changed, 26 insertions(+), 18 deletions(-)

    diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
    index 04a34c08e0a6..6be2c1e2fb48 100644
    --- a/include/linux/page-flags.h
    +++ b/include/linux/page-flags.h
    @@ -131,7 +131,7 @@ enum pageflags {
    #ifdef CONFIG_MEMORY_FAILURE
    PG_hwpoison, /* hardware poisoned page. Don't touch */
    #endif
    -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
    +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
    PG_young,
    PG_idle,
    #endif
    @@ -436,7 +436,7 @@ PAGEFLAG_FALSE(HWPoison)
    #define __PG_HWPOISON 0
    #endif

    -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
    +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
    TESTPAGEFLAG(Young, young, PF_ANY)
    SETPAGEFLAG(Young, young, PF_ANY)
    TESTCLEARFLAG(Young, young, PF_ANY)
    diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
    index aff81ba31bd8..fabb2e1e087f 100644
    --- a/include/linux/page_ext.h
    +++ b/include/linux/page_ext.h
    @@ -19,7 +19,7 @@ struct page_ext_operations {
    enum page_ext_flags {
    PAGE_EXT_OWNER,
    PAGE_EXT_OWNER_ALLOCATED,
    -#if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
    +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
    PAGE_EXT_YOUNG,
    PAGE_EXT_IDLE,
    #endif
    diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
    index 1e894d34bdce..d8a6aecf99cb 100644
    --- a/include/linux/page_idle.h
    +++ b/include/linux/page_idle.h
    @@ -6,7 +6,7 @@
    #include <linux/page-flags.h>
    #include <linux/page_ext.h>

    -#ifdef CONFIG_IDLE_PAGE_TRACKING
    +#ifdef CONFIG_PAGE_IDLE_FLAG

    #ifdef CONFIG_64BIT
    static inline bool page_is_young(struct page *page)
    @@ -106,7 +106,7 @@ static inline void clear_page_idle(struct page *page)
    }
    #endif /* CONFIG_64BIT */

    -#else /* !CONFIG_IDLE_PAGE_TRACKING */
    +#else /* !CONFIG_PAGE_IDLE_FLAG */

    static inline bool page_is_young(struct page *page)
    {
    @@ -135,6 +135,6 @@ static inline void clear_page_idle(struct page *page)
    {
    }

    -#endif /* CONFIG_IDLE_PAGE_TRACKING */
    +#endif /* CONFIG_PAGE_IDLE_FLAG */

    #endif /* _LINUX_MM_PAGE_IDLE_H */
    diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
    index 629c7a0eaff2..ea434bbc2d2b 100644
    --- a/include/trace/events/mmflags.h
    +++ b/include/trace/events/mmflags.h
    @@ -73,7 +73,7 @@
    #define IF_HAVE_PG_HWPOISON(flag,string)
    #endif

    -#if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
    +#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
    #define IF_HAVE_PG_IDLE(flag,string) ,{1UL << flag, string}
    #else
    #define IF_HAVE_PG_IDLE(flag,string)
    diff --git a/mm/Kconfig b/mm/Kconfig
    index fa8c95a54346..19e8f8284d00 100644
    --- a/mm/Kconfig
    +++ b/mm/Kconfig
    @@ -765,10 +765,18 @@ config DEFERRED_STRUCT_PAGE_INIT
    lifetime of the system until these kthreads finish the
    initialisation.

    +config PAGE_IDLE_FLAG
    + bool "Add PG_idle and PG_young flags"
    + help
    + This feature adds PG_idle and PG_young flags in 'struct page'. PTE
    + Accessed bit writers can set the state of the bit in the flags to let
    + other PTE Accessed bit readers don't disturbed.
    +
    config IDLE_PAGE_TRACKING
    bool "Enable idle page tracking"
    depends on SYSFS && MMU
    select PAGE_EXTENSION if !64BIT
    + select PAGE_IDLE_FLAG
    help
    This feature allows to estimate the amount of user pages that have
    not been touched during a given period of time. This information can
    diff --git a/mm/page_ext.c b/mm/page_ext.c
    index df6f74aac8e1..8e59da0f4367 100644
    --- a/mm/page_ext.c
    +++ b/mm/page_ext.c
    @@ -58,11 +58,21 @@
    * can utilize this callback to initialize the state of it correctly.
    */

    +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
    +static bool need_page_idle(void)
    +{
    + return true;
    +}
    +struct page_ext_operations page_idle_ops = {
    + .need = need_page_idle,
    +};
    +#endif
    +
    static struct page_ext_operations *page_ext_ops[] = {
    #ifdef CONFIG_PAGE_OWNER
    &page_owner_ops,
    #endif
    -#if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
    +#if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
    &page_idle_ops,
    #endif
    };
    diff --git a/mm/page_idle.c b/mm/page_idle.c
    index 64e5344a992c..edead6a8a5f9 100644
    --- a/mm/page_idle.c
    +++ b/mm/page_idle.c
    @@ -207,16 +207,6 @@ static const struct attribute_group page_idle_attr_group = {
    .name = "page_idle",
    };

    -#ifndef CONFIG_64BIT
    -static bool need_page_idle(void)
    -{
    - return true;
    -}
    -struct page_ext_operations page_idle_ops = {
    - .need = need_page_idle,
    -};
    -#endif
    -
    static int __init page_idle_init(void)
    {
    int err;
    --
    2.17.1
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