/*
   * mm/balloon_compaction.c
   *
   * Common interface for making balloon pages movable by compaction.
   *
   * Copyright (C) 2012, Red Hat, Inc.  Rafael Aquini <aquini@redhat.com>
   */
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/export.h>
  #include <linux/balloon_compaction.h>
  
  /*
   * balloon_devinfo_alloc - allocates a balloon device information descriptor.
   * @balloon_dev_descriptor: pointer to reference the balloon device which
   *                          this struct balloon_dev_info will be servicing.
   *
   * Driver must call it to properly allocate and initialize an instance of
   * struct balloon_dev_info which will be used to reference a balloon device
   * as well as to keep track of the balloon device page list.
   */
  struct balloon_dev_info *balloon_devinfo_alloc(void *balloon_dev_descriptor)
  {
  	struct balloon_dev_info *b_dev_info;
  	b_dev_info = kmalloc(sizeof(*b_dev_info), GFP_KERNEL);
  	if (!b_dev_info)
  		return ERR_PTR(-ENOMEM);
  
  	b_dev_info->balloon_device = balloon_dev_descriptor;
  	b_dev_info->mapping = NULL;
  	b_dev_info->isolated_pages = 0;
  	spin_lock_init(&b_dev_info->pages_lock);
  	INIT_LIST_HEAD(&b_dev_info->pages);
  
  	return b_dev_info;
  }
  EXPORT_SYMBOL_GPL(balloon_devinfo_alloc);
  
  /*
   * balloon_page_enqueue - allocates a new page and inserts it into the balloon
   *			  page list.
   * @b_dev_info: balloon device decriptor where we will insert a new page to
   *
   * Driver must call it to properly allocate a new enlisted balloon page
   * before definetively removing it from the guest system.
   * This function returns the page address for the recently enqueued page or
   * NULL in the case we fail to allocate a new page this turn.
   */
  struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info)
  {
  	unsigned long flags;
  	struct page *page = alloc_page(balloon_mapping_gfp_mask() |
  					__GFP_NOMEMALLOC | __GFP_NORETRY);
  	if (!page)
  		return NULL;
  
  	/*
  	 * Block others from accessing the 'page' when we get around to
  	 * establishing additional references. We should be the only one
  	 * holding a reference to the 'page' at this point.
  	 */
  	BUG_ON(!trylock_page(page));
  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
  	balloon_page_insert(page, b_dev_info->mapping, &b_dev_info->pages);
  	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  	unlock_page(page);
  	return page;
  }
  EXPORT_SYMBOL_GPL(balloon_page_enqueue);
  
  /*
   * balloon_page_dequeue - removes a page from balloon's page list and returns
   *			  the its address to allow the driver release the page.
   * @b_dev_info: balloon device decriptor where we will grab a page from.
   *
   * Driver must call it to properly de-allocate a previous enlisted balloon page
   * before definetively releasing it back to the guest system.
   * This function returns the page address for the recently dequeued page or
   * NULL in the case we find balloon's page list temporarily empty due to
   * compaction isolated pages.
   */
  struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
  {
  	struct page *page, *tmp;
  	unsigned long flags;
  	bool dequeued_page;
  
  	dequeued_page = false;
  	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
  		/*
  		 * Block others from accessing the 'page' while we get around
  		 * establishing additional references and preparing the 'page'
  		 * to be released by the balloon driver.
  		 */
  		if (trylock_page(page)) {
  			spin_lock_irqsave(&b_dev_info->pages_lock, flags);
  			/*
  			 * Raise the page refcount here to prevent any wrong
  			 * attempt to isolate this page, in case of coliding
  			 * with balloon_page_isolate() just after we release
  			 * the page lock.
  			 *
  			 * balloon_page_free() will take care of dropping
  			 * this extra refcount later.
  			 */
  			get_page(page);
  			balloon_page_delete(page);
  			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  			unlock_page(page);
  			dequeued_page = true;
  			break;
  		}
  	}
  
  	if (!dequeued_page) {
  		/*
  		 * If we are unable to dequeue a balloon page because the page
  		 * list is empty and there is no isolated pages, then something
  		 * went out of track and some balloon pages are lost.
  		 * BUG() here, otherwise the balloon driver may get stuck into
  		 * an infinite loop while attempting to release all its pages.
  		 */
  		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
  		if (unlikely(list_empty(&b_dev_info->pages) &&
  			     !b_dev_info->isolated_pages))
  			BUG();
  		spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  		page = NULL;
  	}
  	return page;
  }
  EXPORT_SYMBOL_GPL(balloon_page_dequeue);
  
  #ifdef CONFIG_BALLOON_COMPACTION
  /*
   * balloon_mapping_alloc - allocates a special ->mapping for ballooned pages.
   * @b_dev_info: holds the balloon device information descriptor.
   * @a_ops: balloon_mapping address_space_operations descriptor.
   *
   * Driver must call it to properly allocate and initialize an instance of
   * struct address_space which will be used as the special page->mapping for
   * balloon device enlisted page instances.
   */
  struct address_space *balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
  				const struct address_space_operations *a_ops)
  {
  	struct address_space *mapping;
  
  	mapping = kmalloc(sizeof(*mapping), GFP_KERNEL);
  	if (!mapping)
  		return ERR_PTR(-ENOMEM);
  
  	/*
  	 * Give a clean 'zeroed' status to all elements of this special
  	 * balloon page->mapping struct address_space instance.
  	 */
  	address_space_init_once(mapping);
  
  	/*
  	 * Set mapping->flags appropriately, to allow balloon pages
  	 * ->mapping identification.
  	 */
  	mapping_set_balloon(mapping);
  	mapping_set_gfp_mask(mapping, balloon_mapping_gfp_mask());
  
  	/* balloon's page->mapping->a_ops callback descriptor */
  	mapping->a_ops = a_ops;
  
  	/*
  	 * Establish a pointer reference back to the balloon device descriptor
  	 * this particular page->mapping will be servicing.
  	 * This is used by compaction / migration procedures to identify and
  	 * access the balloon device pageset while isolating / migrating pages.
  	 *
  	 * As some balloon drivers can register multiple balloon devices
  	 * for a single guest, this also helps compaction / migration to
  	 * properly deal with multiple balloon pagesets, when required.
  	 */
  	mapping->private_data = b_dev_info;
  	b_dev_info->mapping = mapping;
  
  	return mapping;
  }
  EXPORT_SYMBOL_GPL(balloon_mapping_alloc);
  
  static inline void __isolate_balloon_page(struct page *page)
  {
  	struct balloon_dev_info *b_dev_info = page->mapping->private_data;
  	unsigned long flags;
  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
  	list_del(&page->lru);
  	b_dev_info->isolated_pages++;
  	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  }
  
  static inline void __putback_balloon_page(struct page *page)
  {
  	struct balloon_dev_info *b_dev_info = page->mapping->private_data;
  	unsigned long flags;
  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
  	list_add(&page->lru, &b_dev_info->pages);
  	b_dev_info->isolated_pages--;
  	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  }
  
  static inline int __migrate_balloon_page(struct address_space *mapping,
  		struct page *newpage, struct page *page, enum migrate_mode mode)
  {
  	return page->mapping->a_ops->migratepage(mapping, newpage, page, mode);
  }
  
  /* __isolate_lru_page() counterpart for a ballooned page */
  bool balloon_page_isolate(struct page *page)
  {
  	/*
  	 * Avoid burning cycles with pages that are yet under __free_pages(),
  	 * or just got freed under us.
  	 *
  	 * In case we 'win' a race for a balloon page being freed under us and
  	 * raise its refcount preventing __free_pages() from doing its job
  	 * the put_page() at the end of this block will take care of
  	 * release this page, thus avoiding a nasty leakage.
  	 */
  	if (likely(get_page_unless_zero(page))) {
  		/*
  		 * As balloon pages are not isolated from LRU lists, concurrent
  		 * compaction threads can race against page migration functions
  		 * as well as race against the balloon driver releasing a page.
  		 *
  		 * In order to avoid having an already isolated balloon page
  		 * being (wrongly) re-isolated while it is under migration,
  		 * or to avoid attempting to isolate pages being released by
  		 * the balloon driver, lets be sure we have the page lock
  		 * before proceeding with the balloon page isolation steps.
  		 */
  		if (likely(trylock_page(page))) {
  			/*
  			 * A ballooned page, by default, has just one refcount.
  			 * Prevent concurrent compaction threads from isolating
  			 * an already isolated balloon page by refcount check.
  			 */
  			if (__is_movable_balloon_page(page) &&
  			    page_count(page) == 2) {
  				__isolate_balloon_page(page);
  				unlock_page(page);
  				return true;
  			}
  			unlock_page(page);
  		}
  		put_page(page);
  	}
  	return false;
  }
  
  /* putback_lru_page() counterpart for a ballooned page */
  void balloon_page_putback(struct page *page)
  {
  	/*
  	 * 'lock_page()' stabilizes the page and prevents races against
  	 * concurrent isolation threads attempting to re-isolate it.
  	 */
  	lock_page(page);
  
  	if (__is_movable_balloon_page(page)) {
  		__putback_balloon_page(page);
  		/* drop the extra ref count taken for page isolation */
  		put_page(page);
  	} else {
  		WARN_ON(1);
  		dump_page(page, "not movable balloon page");
  	}
  	unlock_page(page);
  }
  
  /* move_to_new_page() counterpart for a ballooned page */
  int balloon_page_migrate(struct page *newpage,
  			 struct page *page, enum migrate_mode mode)
  {
  	struct address_space *mapping;
  	int rc = -EAGAIN;
  
  	/*
  	 * Block others from accessing the 'newpage' when we get around to
  	 * establishing additional references. We should be the only one
  	 * holding a reference to the 'newpage' at this point.
  	 */
  	BUG_ON(!trylock_page(newpage));
  
  	if (WARN_ON(!__is_movable_balloon_page(page))) {
  		dump_page(page, "not movable balloon page");
  		unlock_page(newpage);
  		return rc;
  	}
  
  	mapping = page->mapping;
  	if (mapping)
  		rc = __migrate_balloon_page(mapping, newpage, page, mode);
  
  	unlock_page(newpage);
  	return rc;
  }
  #endif /* CONFIG_BALLOON_COMPACTION */