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    Subject[PATCH v2 9/9] zsmalloc: replace get_cpu_var with local_lock
    Date
    From: Mike Galbraith <umgwanakikbuti@gmail.com>

    The usage of get_cpu_var() in zs_map_object() is problematic because
    it disables preemption and makes it impossible to acquire any sleeping
    lock on PREEMPT_RT such as a spinlock_t.
    Replace the get_cpu_var() usage with a local_lock_t which is embedded
    struct mapping_area. It ensures that the access the struct is
    synchronized against all users on the same CPU.

    Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    [minchan: remove the bit_spin_lock part and change the title]
    Signed-off-by: Minchan Kim <minchan@kernel.org>
    ---
    mm/zsmalloc.c | 11 ++++++++---
    1 file changed, 8 insertions(+), 3 deletions(-)

    diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
    index 5d4c4d254679..7e03cc9363bb 100644
    --- a/mm/zsmalloc.c
    +++ b/mm/zsmalloc.c
    @@ -65,6 +65,7 @@
    #include <linux/wait.h>
    #include <linux/pagemap.h>
    #include <linux/fs.h>
    +#include <linux/local_lock.h>

    #define ZSPAGE_MAGIC 0x58

    @@ -276,6 +277,7 @@ struct zspage {
    };

    struct mapping_area {
    + local_lock_t lock;
    char *vm_buf; /* copy buffer for objects that span pages */
    char *vm_addr; /* address of kmap_atomic()'ed pages */
    enum zs_mapmode vm_mm; /* mapping mode */
    @@ -451,7 +453,9 @@ MODULE_ALIAS("zpool-zsmalloc");
    #endif /* CONFIG_ZPOOL */

    /* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
    -static DEFINE_PER_CPU(struct mapping_area, zs_map_area);
    +static DEFINE_PER_CPU(struct mapping_area, zs_map_area) = {
    + .lock = INIT_LOCAL_LOCK(lock),
    +};

    static __maybe_unused int is_first_page(struct page *page)
    {
    @@ -1269,7 +1273,8 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
    class = zspage_class(pool, zspage);
    off = (class->size * obj_idx) & ~PAGE_MASK;

    - area = &get_cpu_var(zs_map_area);
    + local_lock(&zs_map_area.lock);
    + area = this_cpu_ptr(&zs_map_area);
    area->vm_mm = mm;
    if (off + class->size <= PAGE_SIZE) {
    /* this object is contained entirely within a page */
    @@ -1320,7 +1325,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)

    __zs_unmap_object(area, pages, off, class->size);
    }
    - put_cpu_var(zs_map_area);
    + local_unlock(&zs_map_area.lock);

    migrate_read_unlock(zspage);
    }
    --
    2.34.0.rc1.387.gb447b232ab-goog
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