text: /* drivers/misc/lowmemorykiller.c
   *
   * The lowmemorykiller driver lets user-space specify a set of memory thresholds
   * where processes with a range of oom_score_adj values will get killed. Specify
   * the minimum oom_score_adj values in
   * /sys/module/lowmemorykiller/parameters/adj and the number of free pages in
   * /sys/module/lowmemorykiller/parameters/minfree. Both files take a comma
   * separated list of numbers in ascending order.
   *
   * For example, write "0,8" to /sys/module/lowmemorykiller/parameters/adj and
   * "1024,4096" to /sys/module/lowmemorykiller/parameters/minfree to kill
   * processes with a oom_score_adj value of 8 or higher when the free memory
   * drops below 4096 pages and kill processes with a oom_score_adj value of 0 or
   * higher when the free memory drops below 1024 pages.
   *
   * The driver considers memory used for caches to be free, but if a large
   * percentage of the cached memory is locked this can be very inaccurate
   * and processes may not get killed until the normal oom killer is triggered.
   *
   * Copyright (C) 2007-2008 Google, Inc.
   *
   * This software is licensed under the terms of the GNU General Public
   * License version 2, as published by the Free Software Foundation, and
   * may be copied, distributed, and modified under those terms.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   */
  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include <linux/init.h>
  #include <linux/moduleparam.h>
  #include <linux/kernel.h>
  #include <linux/mm.h>
  #include <linux/oom.h>
  #include <linux/sched.h>
  #include <linux/swap.h>
  #include <linux/rcupdate.h>
  #include <linux/profile.h>
  #include <linux/notifier.h>
  
  static u32 lowmem_debug_level = 1;
  static short lowmem_adj[6] = {
  	0,
  	1,
  	6,
  	12,
  };
  
  static int lowmem_adj_size = 4;
  static int lowmem_minfree[6] = {
  	3 * 512,	/* 6MB */
  	2 * 1024,	/* 8MB */
  	4 * 1024,	/* 16MB */
  	16 * 1024,	/* 64MB */
  };
  
  static int lowmem_minfree_size = 4;
  
  static unsigned long lowmem_deathpending_timeout;
  
  #define lowmem_print(level, x...)			\
  	do {						\
  		if (lowmem_debug_level >= (level))	\
  			pr_info(x);			\
  	} while (0)
  
  static unsigned long lowmem_count(struct shrinker *s,
  				  struct shrink_control *sc)
  {
  	return global_page_state(NR_ACTIVE_ANON) +
  		global_page_state(NR_ACTIVE_FILE) +
  		global_page_state(NR_INACTIVE_ANON) +
  		global_page_state(NR_INACTIVE_FILE);
  }
  
  static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
  {
  	struct task_struct *tsk;
  	struct task_struct *selected = NULL;
  	unsigned long rem = 0;
  	int tasksize;
  	int i;
  	short min_score_adj = OOM_SCORE_ADJ_MAX + 1;
  	int minfree = 0;
  	int selected_tasksize = 0;
  	short selected_oom_score_adj;
  	int array_size = ARRAY_SIZE(lowmem_adj);
  	int other_free = global_page_state(NR_FREE_PAGES) - totalreserve_pages;
  	int other_file = global_page_state(NR_FILE_PAGES) -
  						global_page_state(NR_SHMEM) -
  						total_swapcache_pages();
  
  	if (lowmem_adj_size < array_size)
  		array_size = lowmem_adj_size;
  	if (lowmem_minfree_size < array_size)
  		array_size = lowmem_minfree_size;
  	for (i = 0; i < array_size; i++) {
  		minfree = lowmem_minfree[i];
  		if (other_free < minfree && other_file < minfree) {
  			min_score_adj = lowmem_adj[i];
  			break;
  		}
  	}
  
  	lowmem_print(3, "lowmem_scan %lu, %x, ofree %d %d, ma %hd\n",
  		     sc->nr_to_scan, sc->gfp_mask, other_free,
  		     other_file, min_score_adj);
  
  	if (min_score_adj == OOM_SCORE_ADJ_MAX + 1) {
  		lowmem_print(5, "lowmem_scan %lu, %x, return 0\n",
  			     sc->nr_to_scan, sc->gfp_mask);
  		return 0;
  	}
  
  	selected_oom_score_adj = min_score_adj;
  
  	rcu_read_lock();
  	for_each_process(tsk) {
  		struct task_struct *p;
  		short oom_score_adj;
  
  		if (tsk->flags & PF_KTHREAD)
  			continue;
  
  		p = find_lock_task_mm(tsk);
  		if (!p)
  			continue;
  
  		if (task_lmk_waiting(p) &&
  		    time_before_eq(jiffies, lowmem_deathpending_timeout)) {
  			task_unlock(p);
  			rcu_read_unlock();
  			return 0;
  		}
  		oom_score_adj = p->signal->oom_score_adj;
  		if (oom_score_adj < min_score_adj) {
  			task_unlock(p);
  			continue;
  		}
  		tasksize = get_mm_rss(p->mm);
  		task_unlock(p);
  		if (tasksize <= 0)
  			continue;
  		if (selected) {
  			if (oom_score_adj < selected_oom_score_adj)
  				continue;
  			if (oom_score_adj == selected_oom_score_adj &&
  			    tasksize <= selected_tasksize)
  				continue;
  		}
  		selected = p;
  		selected_tasksize = tasksize;
  		selected_oom_score_adj = oom_score_adj;
  		lowmem_print(2, "select '%s' (%d), adj %hd, size %d, to kill\n",
  			     p->comm, p->pid, oom_score_adj, tasksize);
  	}
  	if (selected) {
  		task_lock(selected);
  		send_sig(SIGKILL, selected, 0);
  		if (selected->mm)
  			task_set_lmk_waiting(selected);
  		task_unlock(selected);
  		lowmem_print(1, "Killing '%s' (%d), adj %hd,\n"
  				 "   to free %ldkB on behalf of '%s' (%d) because\n"
  				 "   cache %ldkB is below limit %ldkB for oom_score_adj %hd\n"
  				 "   Free memory is %ldkB above reserved\n",
  			     selected->comm, selected->pid,
  			     selected_oom_score_adj,
  			     selected_tasksize * (long)(PAGE_SIZE / 1024),
  			     current->comm, current->pid,
  			     other_file * (long)(PAGE_SIZE / 1024),
  			     minfree * (long)(PAGE_SIZE / 1024),
  			     min_score_adj,
  			     other_free * (long)(PAGE_SIZE / 1024));
  		lowmem_deathpending_timeout = jiffies + HZ;
  		rem += selected_tasksize;
  	}
  
  	lowmem_print(4, "lowmem_scan %lu, %x, return %lu\n",
  		     sc->nr_to_scan, sc->gfp_mask, rem);
  	rcu_read_unlock();
  	return rem;
  }
  
  static struct shrinker lowmem_shrinker = {
  	.scan_objects = lowmem_scan,
  	.count_objects = lowmem_count,
  	.seeks = DEFAULT_SEEKS * 16
  };
  
  static int __init lowmem_init(void)
  {
  	register_shrinker(&lowmem_shrinker);
  	return 0;
  }
  device_initcall(lowmem_init);
  
  /*
   * not really modular, but the easiest way to keep compat with existing
   * bootargs behaviour is to continue using module_param here.
   */
  module_param_named(cost, lowmem_shrinker.seeks, int, S_IRUGO | S_IWUSR);
  module_param_array_named(adj, lowmem_adj, short, &lowmem_adj_size,
  			 S_IRUGO | S_IWUSR);
  module_param_array_named(minfree, lowmem_minfree, uint, &lowmem_minfree_size,
  			 S_IRUGO | S_IWUSR);
  module_param_named(debug_level, lowmem_debug_level, uint, S_IRUGO | S_IWUSR);
  
  