root/include/linux/shrinker.h

INCLUDED FROM

#ifndef _LINUX_SHRINKER_H
#define _LINUX_SHRINKER_H

/*
 * This struct is used to pass information from page reclaim to the shrinkers.
 * We consolidate the values for easier extention later.
 *
 * The 'gfpmask' refers to the allocation we are currently trying to
 * fulfil.
 */
struct shrink_control {
        gfp_t gfp_mask;

        /*
         * How many objects scan_objects should scan and try to reclaim.
         * This is reset before every call, so it is safe for callees
         * to modify.
         */
        unsigned long nr_to_scan;

        /* shrink from these nodes */
        nodemask_t nodes_to_scan;
        /* current node being shrunk (for NUMA aware shrinkers) */
        int nid;
};

#define SHRINK_STOP (~0UL)
/*
 * A callback you can register to apply pressure to ageable caches.
 *
 * @count_objects should return the number of freeable items in the cache. If
 * there are no objects to free or the number of freeable items cannot be
 * determined, it should return 0. No deadlock checks should be done during the
 * count callback - the shrinker relies on aggregating scan counts that couldn't
 * be executed due to potential deadlocks to be run at a later call when the
 * deadlock condition is no longer pending.
 *
 * @scan_objects will only be called if @count_objects returned a non-zero
 * value for the number of freeable objects. The callout should scan the cache
 * and attempt to free items from the cache. It should then return the number
 * of objects freed during the scan, or SHRINK_STOP if progress cannot be made
 * due to potential deadlocks. If SHRINK_STOP is returned, then no further
 * attempts to call the @scan_objects will be made from the current reclaim
 * context.
 *
 * @flags determine the shrinker abilities, like numa awareness
 */
struct shrinker {
        unsigned long (*count_objects)(struct shrinker *,
                                       struct shrink_control *sc);
        unsigned long (*scan_objects)(struct shrinker *,
                                      struct shrink_control *sc);

        int seeks;      /* seeks to recreate an obj */
        long batch;     /* reclaim batch size, 0 = default */
        unsigned long flags;

        /* These are for internal use */
        struct list_head list;
        /* objs pending delete, per node */
        atomic_long_t *nr_deferred;
};
#define DEFAULT_SEEKS 2 /* A good number if you don't know better. */

/* Flags */
#define SHRINKER_NUMA_AWARE (1 << 0)

extern int register_shrinker(struct shrinker *);
extern void unregister_shrinker(struct shrinker *);
#endif