FreeBSD/Linux Kernel Cross Reference
sys/mm/balloon_compaction.c

     /*
      * 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);
           }
           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);
                   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 */

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