/*
   * Implement the manual drop-all-pagecache function
   */
  
  #include <linux/kernel.h>
  #include <linux/mm.h>
  #include <linux/fs.h>
  #include <linux/writeback.h>
  #include <linux/sysctl.h>
  #include <linux/gfp.h>
  
  /* A global variable is a bit ugly, but it keeps the code simple */
  int sysctl_drop_caches;
  
  static void drop_pagecache_sb(struct super_block *sb)
  {

  	struct inode *inode, *toput_inode = NULL;

  
  	spin_lock(&inode_lock);
  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
  		if (inode->i_state & (I_FREEING|I_WILL_FREE))
  			continue;

  		if (inode->i_mapping->nrpages == 0)
  			continue;

  		__iget(inode);
  		spin_unlock(&inode_lock);

  		__invalidate_mapping_pages(inode->i_mapping, 0, -1, true);

  		iput(toput_inode);
  		toput_inode = inode;
  		spin_lock(&inode_lock);

  	}
  	spin_unlock(&inode_lock);

  	iput(toput_inode);

  }

  static void drop_pagecache(void)

  {
  	struct super_block *sb;
  
  	spin_lock(&sb_lock);
  restart:
  	list_for_each_entry(sb, &super_blocks, s_list) {
  		sb->s_count++;
  		spin_unlock(&sb_lock);
  		down_read(&sb->s_umount);
  		if (sb->s_root)
  			drop_pagecache_sb(sb);
  		up_read(&sb->s_umount);
  		spin_lock(&sb_lock);
  		if (__put_super_and_need_restart(sb))
  			goto restart;
  	}
  	spin_unlock(&sb_lock);
  }

  static void drop_slab(void)

  {
  	int nr_objects;
  
  	do {
  		nr_objects = shrink_slab(1000, GFP_KERNEL, 1000);
  	} while (nr_objects > 10);
  }
  
  int drop_caches_sysctl_handler(ctl_table *table, int write,
  	struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
  {
  	proc_dointvec_minmax(table, write, file, buffer, length, ppos);
  	if (write) {
  		if (sysctl_drop_caches & 1)
  			drop_pagecache();
  		if (sysctl_drop_caches & 2)
  			drop_slab();
  	}
  	return 0;
  }