include/linux/shrinker.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_SHRINKER_H
#define _LINUX_SHRINKER_H

#include <linux/atomic.h>
#include <linux/types.h>
#include <linux/refcount.h>
#include <linux/completion.h>

#define SHRINKER_UNIT_BITS	BITS_PER_LONG

/*
 * Bitmap and deferred work of shrinker::id corresponding to memcg-aware
 * shrinkers, which have elements charged to the memcg.
 */
struct shrinker_info_unit {
	atomic_long_t nr_deferred[SHRINKER_UNIT_BITS];
	DECLARE_BITMAP(map, SHRINKER_UNIT_BITS);
};

struct shrinker_info {
	struct rcu_head rcu;
	int map_nr_max;
	struct shrinker_info_unit *unit[];
};

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

	/* current node being shrunk (for NUMA aware shrinkers) */
	int nid;

	/*
	 * 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;

	/*
	 * How many objects did scan_objects process?
	 * This defaults to nr_to_scan before every call, but the callee
	 * should track its actual progress.
	 */
	unsigned long nr_scanned;

	/* current memcg being shrunk (for memcg aware shrinkers) */
	struct mem_cgroup *memcg;
};

#define SHRINK_STOP (~0UL)
#define SHRINK_EMPTY (~0UL - 1)
/*
 * 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, it should return SHRINK_EMPTY, while 0 is
 * returned in cases of the number of freeable items cannot be determined
 * or shrinker should skip this cache for this time (e.g., their number
 * is below shrinkable limit). 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);

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

	/*
	 * The reference count of this shrinker. Registered shrinker have an
	 * initial refcount of 1, then the lookup operations are now allowed
	 * to use it via shrinker_try_get(). Later in the unregistration step,
	 * the initial refcount will be discarded, and will free the shrinker
	 * asynchronously via RCU after its refcount reaches 0.
	 */
	refcount_t refcount;
	struct completion done;	/* use to wait for refcount to reach 0 */
	struct rcu_head rcu;

	void *private_data;

	/* These are for internal use */
	struct list_head list;
#ifdef CONFIG_MEMCG
	/* ID in shrinker_idr */
	int id;
#endif
#ifdef CONFIG_SHRINKER_DEBUG
	int debugfs_id;
	const char *name;
	struct dentry *debugfs_entry;
#endif
	/* objs pending delete, per node */
	atomic_long_t *nr_deferred;
};
#define DEFAULT_SEEKS 2 /* A good number if you don't know better. */

/* Internal flags */
#define SHRINKER_REGISTERED	BIT(0)
#define SHRINKER_ALLOCATED	BIT(1)

/* Flags for users to use */
#define SHRINKER_NUMA_AWARE	BIT(2)
#define SHRINKER_MEMCG_AWARE	BIT(3)
/*
 * It just makes sense when the shrinker is also MEMCG_AWARE for now,
 * non-MEMCG_AWARE shrinker should not have this flag set.
 */
#define SHRINKER_NONSLAB	BIT(4)

__printf(2, 3)
struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
void shrinker_register(struct shrinker *shrinker);
void shrinker_free(struct shrinker *shrinker);

static inline bool shrinker_try_get(struct shrinker *shrinker)
{
	return refcount_inc_not_zero(&shrinker->refcount);
}

static inline void shrinker_put(struct shrinker *shrinker)
{
	if (refcount_dec_and_test(&shrinker->refcount))
		complete(&shrinker->done);
}

#ifdef CONFIG_SHRINKER_DEBUG
extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
						  const char *fmt, ...);
#else /* CONFIG_SHRINKER_DEBUG */
static inline __printf(2, 3)
int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
{
	return 0;
}
#endif /* CONFIG_SHRINKER_DEBUG */
#endif /* _LINUX_SHRINKER_H */