/*
   * Lockless hierarchical page accounting & limiting
   *
   * Copyright (C) 2014 Red Hat, Inc., Johannes Weiner
   */
  
  #include <linux/page_counter.h>
  #include <linux/atomic.h>
  #include <linux/kernel.h>
  #include <linux/string.h>
  #include <linux/sched.h>
  #include <linux/bug.h>
  #include <asm/page.h>
  
  /**
   * page_counter_cancel - take pages out of the local counter
   * @counter: counter
   * @nr_pages: number of pages to cancel

   */

  void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)

  {
  	long new;
  
  	new = atomic_long_sub_return(nr_pages, &counter->count);

  	/* More uncharges than charges? */
  	WARN_ON_ONCE(new < 0);

  }
  
  /**
   * page_counter_charge - hierarchically charge pages
   * @counter: counter
   * @nr_pages: number of pages to charge
   *
   * NOTE: This does not consider any configured counter limits.
   */
  void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
  {
  	struct page_counter *c;
  
  	for (c = counter; c; c = c->parent) {
  		long new;
  
  		new = atomic_long_add_return(nr_pages, &c->count);
  		/*
  		 * This is indeed racy, but we can live with some
  		 * inaccuracy in the watermark.
  		 */
  		if (new > c->watermark)
  			c->watermark = new;
  	}
  }
  
  /**
   * page_counter_try_charge - try to hierarchically charge pages
   * @counter: counter
   * @nr_pages: number of pages to charge
   * @fail: points first counter to hit its limit, if any
   *

   * Returns %true on success, or %false and @fail if the counter or one
   * of its ancestors has hit its configured limit.

   */

  bool page_counter_try_charge(struct page_counter *counter,
  			     unsigned long nr_pages,
  			     struct page_counter **fail)

  {
  	struct page_counter *c;
  
  	for (c = counter; c; c = c->parent) {
  		long new;
  		/*
  		 * Charge speculatively to avoid an expensive CAS.  If
  		 * a bigger charge fails, it might falsely lock out a
  		 * racing smaller charge and send it into reclaim
  		 * early, but the error is limited to the difference
  		 * between the two sizes, which is less than 2M/4M in
  		 * case of a THP locking out a regular page charge.
  		 *
  		 * The atomic_long_add_return() implies a full memory
  		 * barrier between incrementing the count and reading
  		 * the limit.  When racing with page_counter_limit(),
  		 * we either see the new limit or the setter sees the
  		 * counter has changed and retries.
  		 */
  		new = atomic_long_add_return(nr_pages, &c->count);
  		if (new > c->limit) {
  			atomic_long_sub(nr_pages, &c->count);
  			/*
  			 * This is racy, but we can live with some
  			 * inaccuracy in the failcnt.
  			 */
  			c->failcnt++;
  			*fail = c;
  			goto failed;
  		}
  		/*
  		 * Just like with failcnt, we can live with some
  		 * inaccuracy in the watermark.
  		 */
  		if (new > c->watermark)
  			c->watermark = new;
  	}

  	return true;

  
  failed:
  	for (c = counter; c != *fail; c = c->parent)
  		page_counter_cancel(c, nr_pages);

  	return false;

  }
  
  /**
   * page_counter_uncharge - hierarchically uncharge pages
   * @counter: counter
   * @nr_pages: number of pages to uncharge

   */

  void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)

  {
  	struct page_counter *c;

  	for (c = counter; c; c = c->parent)
  		page_counter_cancel(c, nr_pages);

  }
  
  /**
   * page_counter_limit - limit the number of pages allowed
   * @counter: counter
   * @limit: limit to set
   *
   * Returns 0 on success, -EBUSY if the current number of pages on the
   * counter already exceeds the specified limit.
   *
   * The caller must serialize invocations on the same counter.
   */
  int page_counter_limit(struct page_counter *counter, unsigned long limit)
  {
  	for (;;) {
  		unsigned long old;
  		long count;
  
  		/*
  		 * Update the limit while making sure that it's not
  		 * below the concurrently-changing counter value.
  		 *
  		 * The xchg implies two full memory barriers before
  		 * and after, so the read-swap-read is ordered and
  		 * ensures coherency with page_counter_try_charge():
  		 * that function modifies the count before checking
  		 * the limit, so if it sees the old limit, we see the
  		 * modified counter and retry.
  		 */
  		count = atomic_long_read(&counter->count);
  
  		if (count > limit)
  			return -EBUSY;
  
  		old = xchg(&counter->limit, limit);
  
  		if (atomic_long_read(&counter->count) <= count)
  			return 0;
  
  		counter->limit = old;
  		cond_resched();
  	}
  }
  
  /**
   * page_counter_memparse - memparse() for page counter limits
   * @buf: string to parse

   * @max: string meaning maximum possible value

   * @nr_pages: returns the result in number of pages
   *
   * Returns -EINVAL, or 0 and @nr_pages on success.  @nr_pages will be
   * limited to %PAGE_COUNTER_MAX.
   */

  int page_counter_memparse(const char *buf, const char *max,
  			  unsigned long *nr_pages)

  {

  	char *end;
  	u64 bytes;

  	if (!strcmp(buf, max)) {

  		*nr_pages = PAGE_COUNTER_MAX;
  		return 0;
  	}
  
  	bytes = memparse(buf, &end);
  	if (*end != '\0')
  		return -EINVAL;
  
  	*nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
  
  	return 0;
  }