/*
 * 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 */