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    Subject[PATCH] mm/page_ext: Do not allocate space for page_ext->flags if not needed
    Date
    There is 8 byte page_ext->flags field allocated per page whenever
    CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
    uses flags. Therefore, check whether flags is needed at least by one
    user and if so allocate space for it.

    For example when page_table_check is enabled, on a machine with 128G
    of memory before the fix:

    [ 2.244288] allocated 536870912 bytes of page_ext
    after the fix:
    [ 2.160154] allocated 268435456 bytes of page_ext

    Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
    ---
    include/linux/page_ext.h | 7 +++++++
    mm/page_ext.c | 12 +++++++++++-
    mm/page_owner.c | 1 +
    mm/page_table_check.c | 1 +
    4 files changed, 20 insertions(+), 1 deletion(-)

    diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
    index 22be4582faae..29909e135052 100644
    --- a/include/linux/page_ext.h
    +++ b/include/linux/page_ext.h
    @@ -12,10 +12,14 @@ struct page_ext_operations {
    size_t size;
    bool (*need)(void);
    void (*init)(void);
    + bool using_shared_ext_flags;
    };

    #ifdef CONFIG_PAGE_EXTENSION

    +/*
    + * The page_ext_flags users must set using_shared_ext_flags to true.
    + */
    enum page_ext_flags {
    PAGE_EXT_OWNER,
    PAGE_EXT_OWNER_ALLOCATED,
    @@ -31,6 +35,9 @@ enum page_ext_flags {
    * page_ext helps us add more information about the page.
    * All page_ext are allocated at boot or memory hotplug event,
    * then the page_ext for pfn always exists.
    + * Note: that if none of the users of page_ext is setting
    + * using_shared_ext_flags to true, the field cannot be used,
    + * as it will be overlapped with another data.
    */
    struct page_ext {
    unsigned long flags;
    diff --git a/mm/page_ext.c b/mm/page_ext.c
    index 4ee522fd381c..7443a797b96b 100644
    --- a/mm/page_ext.c
    +++ b/mm/page_ext.c
    @@ -71,6 +71,7 @@ static bool need_page_idle(void)
    }
    static struct page_ext_operations page_idle_ops __initdata = {
    .need = need_page_idle,
    + .using_shared_ext_flags = true,
    };
    #endif

    @@ -86,7 +87,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
    #endif
    };

    -unsigned long page_ext_size = sizeof(struct page_ext);
    +unsigned long page_ext_size;

    static unsigned long total_usage;
    static struct page_ext *lookup_page_ext(const struct page *page);
    @@ -105,6 +106,15 @@ static bool __init invoke_need_callbacks(void)
    int entries = ARRAY_SIZE(page_ext_ops);
    bool need = false;

    + for (i = 0; i < entries; i++) {
    + if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
    + if (page_ext_ops[i]->using_shared_ext_flags) {
    + page_ext_size = sizeof(struct page_ext);
    + break;
    + }
    + }
    + }
    +
    for (i = 0; i < entries; i++) {
    if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
    page_ext_ops[i]->offset = page_ext_size;
    diff --git a/mm/page_owner.c b/mm/page_owner.c
    index 2d27f532df4c..612a988373cc 100644
    --- a/mm/page_owner.c
    +++ b/mm/page_owner.c
    @@ -99,6 +99,7 @@ struct page_ext_operations page_owner_ops = {
    .size = sizeof(struct page_owner),
    .need = need_page_owner,
    .init = init_page_owner,
    + .using_shared_flags = true,
    };

    static inline struct page_owner *get_page_owner(struct page_ext *page_ext)
    diff --git a/mm/page_table_check.c b/mm/page_table_check.c
    index 93e633c1d587..74cfe89d5f53 100644
    --- a/mm/page_table_check.c
    +++ b/mm/page_table_check.c
    @@ -45,6 +45,7 @@ struct page_ext_operations page_table_check_ops = {
    .size = sizeof(struct page_table_check),
    .need = need_page_table_check,
    .init = init_page_table_check,
    + .using_shared_ext_flags = false,
    };

    static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
    --
    2.39.0.314.g84b9a713c41-goog
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