/*
   * include/linux/balloon_compaction.h
   *
   * Common interface definitions for making balloon pages movable by compaction.
   *
   * Despite being perfectly possible to perform ballooned pages migration, they
   * make a special corner case to compaction scans because balloon pages are not
   * enlisted at any LRU list like the other pages we do compact / migrate.
   *
   * As the page isolation scanning step a compaction thread does is a lockless
   * procedure (from a page standpoint), it might bring some racy situations while
   * performing balloon page compaction. In order to sort out these racy scenarios
   * and safely perform balloon's page compaction and migration we must, always,
   * ensure following these three simple rules:
   *
   *   i. when updating a balloon's page ->mapping element, strictly do it under
   *      the following lock order, independently of the far superior
   *      locking scheme (lru_lock, balloon_lock):
   *	    +-page_lock(page);
   *	      +--spin_lock_irq(&b_dev_info->pages_lock);
   *	            ... page->mapping updates here ...
   *
   *  ii. before isolating or dequeueing a balloon page from the balloon device
   *      pages list, the page reference counter must be raised by one and the
   *      extra refcount must be dropped when the page is enqueued back into
   *      the balloon device page list, thus a balloon page keeps its reference
   *      counter raised only while it is under our special handling;
   *
   * iii. after the lockless scan step have selected a potential balloon page for
   *      isolation, re-test the page->mapping flags and the page ref counter
   *      under the proper page lock, to ensure isolating a valid balloon page
   *      (not yet isolated, nor under release procedure)
   *
   * The functions provided by this interface are placed to help on coping with
   * the aforementioned balloon page corner case, as well as to ensure the simple
   * set of exposed rules are satisfied while we are dealing with balloon pages
   * compaction / migration.
   *
   * Copyright (C) 2012, Red Hat, Inc.  Rafael Aquini <aquini@redhat.com>
   */
  #ifndef _LINUX_BALLOON_COMPACTION_H
  #define _LINUX_BALLOON_COMPACTION_H
  #include <linux/pagemap.h>
  #include <linux/page-flags.h>
  #include <linux/migrate.h>
  #include <linux/gfp.h>
  #include <linux/err.h>
  
  /*
   * Balloon device information descriptor.
   * This struct is used to allow the common balloon compaction interface
   * procedures to find the proper balloon device holding memory pages they'll
   * have to cope for page compaction / migration, as well as it serves the
   * balloon driver as a page book-keeper for its registered balloon devices.
   */
  struct balloon_dev_info {
  	void *balloon_device;		/* balloon device descriptor */
  	struct address_space *mapping;	/* balloon special page->mapping */
  	unsigned long isolated_pages;	/* # of isolated pages for migration */
  	spinlock_t pages_lock;		/* Protection to pages list */
  	struct list_head pages;		/* Pages enqueued & handled to Host */
  };
  
  extern struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info);
  extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
  extern struct balloon_dev_info *balloon_devinfo_alloc(
  						void *balloon_dev_descriptor);
  
  static inline void balloon_devinfo_free(struct balloon_dev_info *b_dev_info)
  {
  	kfree(b_dev_info);
  }
  
  /*
   * balloon_page_free - release a balloon page back to the page free lists
   * @page: ballooned page to be set free
   *
   * This function must be used to properly set free an isolated/dequeued balloon
   * page at the end of a sucessful page migration, or at the balloon driver's
   * page release procedure.
   */
  static inline void balloon_page_free(struct page *page)
  {
  	/*
  	 * Balloon pages always get an extra refcount before being isolated
  	 * and before being dequeued to help on sorting out fortuite colisions
  	 * between a thread attempting to isolate and another thread attempting
  	 * to release the very same balloon page.
  	 *
  	 * Before we handle the page back to Buddy, lets drop its extra refcnt.
  	 */
  	put_page(page);
  	__free_page(page);
  }
  
  #ifdef CONFIG_BALLOON_COMPACTION
  extern bool balloon_page_isolate(struct page *page);
  extern void balloon_page_putback(struct page *page);
  extern int balloon_page_migrate(struct page *newpage,
  				struct page *page, enum migrate_mode mode);
  extern struct address_space
  *balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
  			const struct address_space_operations *a_ops);
  
  static inline void balloon_mapping_free(struct address_space *balloon_mapping)
  {
  	kfree(balloon_mapping);
  }
  
  /*
   * page_flags_cleared - helper to perform balloon @page ->flags tests.
   *
   * As balloon pages are obtained from buddy and we do not play with page->flags
   * at driver level (exception made when we get the page lock for compaction),
   * we can safely identify a ballooned page by checking if the
   * PAGE_FLAGS_CHECK_AT_PREP page->flags are all cleared.  This approach also
   * helps us skip ballooned pages that are locked for compaction or release, thus
   * mitigating their racy check at balloon_page_movable()
   */
  static inline bool page_flags_cleared(struct page *page)
  {
  	return !(page->flags & PAGE_FLAGS_CHECK_AT_PREP);
  }
  
  /*
   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
   */
  static inline bool __is_movable_balloon_page(struct page *page)
  {
  	struct address_space *mapping = page->mapping;
  	return mapping_balloon(mapping);
  }
  
  /*
   * balloon_page_movable - test page->mapping->flags to identify balloon pages
   *			  that can be moved by compaction/migration.
   *
   * This function is used at core compaction's page isolation scheme, therefore
   * most pages exposed to it are not enlisted as balloon pages and so, to avoid
   * undesired side effects like racing against __free_pages(), we cannot afford
   * holding the page locked while testing page->mapping->flags here.
   *
   * As we might return false positives in the case of a balloon page being just
   * released under us, the page->mapping->flags need to be re-tested later,
   * under the proper page lock, at the functions that will be coping with the
   * balloon page case.
   */
  static inline bool balloon_page_movable(struct page *page)
  {
  	/*
  	 * Before dereferencing and testing mapping->flags, let's make sure
  	 * this is not a page that uses ->mapping in a different way
  	 */
  	if (page_flags_cleared(page) && !page_mapped(page) &&
  	    page_count(page) == 1)
  		return __is_movable_balloon_page(page);
  
  	return false;
  }
  
  /*
   * isolated_balloon_page - identify an isolated balloon page on private
   *			   compaction/migration page lists.
   *
   * After a compaction thread isolates a balloon page for migration, it raises
   * the page refcount to prevent concurrent compaction threads from re-isolating
   * the same page. For that reason putback_movable_pages(), or other routines
   * that need to identify isolated balloon pages on private pagelists, cannot
   * rely on balloon_page_movable() to accomplish the task.
   */
  static inline bool isolated_balloon_page(struct page *page)
  {
  	/* Already isolated balloon pages, by default, have a raised refcount */
  	if (page_flags_cleared(page) && !page_mapped(page) &&
  	    page_count(page) >= 2)
  		return __is_movable_balloon_page(page);
  
  	return false;
  }
  
  /*
   * balloon_page_insert - insert a page into the balloon's page list and make
   *		         the page->mapping assignment accordingly.
   * @page    : page to be assigned as a 'balloon page'
   * @mapping : allocated special 'balloon_mapping'
   * @head    : balloon's device page list head
   *
   * Caller must ensure the page is locked and the spin_lock protecting balloon
   * pages list is held before inserting a page into the balloon device.
   */
  static inline void balloon_page_insert(struct page *page,
  				       struct address_space *mapping,
  				       struct list_head *head)
  {
  	page->mapping = mapping;
  	list_add(&page->lru, head);
  }
  
  /*
   * balloon_page_delete - delete a page from balloon's page list and clear
   *			 the page->mapping assignement accordingly.
   * @page    : page to be released from balloon's page list
   *
   * Caller must ensure the page is locked and the spin_lock protecting balloon
   * pages list is held before deleting a page from the balloon device.
   */
  static inline void balloon_page_delete(struct page *page)
  {
  	page->mapping = NULL;
  	list_del(&page->lru);
  }
  
  /*
   * balloon_page_device - get the b_dev_info descriptor for the balloon device
   *			 that enqueues the given page.
   */
  static inline struct balloon_dev_info *balloon_page_device(struct page *page)
  {
  	struct address_space *mapping = page->mapping;
  	if (likely(mapping))
  		return mapping->private_data;
  
  	return NULL;
  }
  
  static inline gfp_t balloon_mapping_gfp_mask(void)
  {
  	return GFP_HIGHUSER_MOVABLE;
  }
  
  static inline bool balloon_compaction_check(void)
  {
  	return true;
  }
  
  #else /* !CONFIG_BALLOON_COMPACTION */
  
  static inline void *balloon_mapping_alloc(void *balloon_device,
  				const struct address_space_operations *a_ops)
  {
  	return ERR_PTR(-EOPNOTSUPP);
  }
  
  static inline void balloon_mapping_free(struct address_space *balloon_mapping)
  {
  	return;
  }
  
  static inline void balloon_page_insert(struct page *page,
  				       struct address_space *mapping,
  				       struct list_head *head)
  {
  	list_add(&page->lru, head);
  }
  
  static inline void balloon_page_delete(struct page *page)
  {
  	list_del(&page->lru);
  }
  
  static inline bool balloon_page_movable(struct page *page)
  {
  	return false;
  }
  
  static inline bool isolated_balloon_page(struct page *page)
  {
  	return false;
  }
  
  static inline bool balloon_page_isolate(struct page *page)
  {
  	return false;
  }
  
  static inline void balloon_page_putback(struct page *page)
  {
  	return;
  }
  
  static inline int balloon_page_migrate(struct page *newpage,
  				struct page *page, enum migrate_mode mode)
  {
  	return 0;
  }
  
  static inline gfp_t balloon_mapping_gfp_mask(void)
  {
  	return GFP_HIGHUSER;
  }
  
  static inline bool balloon_compaction_check(void)
  {
  	return false;
  }
  #endif /* CONFIG_BALLOON_COMPACTION */
  #endif /* _LINUX_BALLOON_COMPACTION_H */