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authorRafael Aquini <aquini@redhat.com>2012-12-11 16:02:38 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-12-11 17:22:26 -0800
commit18468d93e53b037e1a04ec58398eab763d054064 (patch)
tree74392b876400391567f55bb5292107b65e3791a2
parent252aa6f5be64c90c67b9f066ccff880f6b487d32 (diff)
downloadlinux-18468d93e53b037e1a04ec58398eab763d054064.tar.gz
mm: introduce a common interface for balloon pages mobility
Memory fragmentation introduced by ballooning might reduce significantly the number of 2MB contiguous memory blocks that can be used within a guest, thus imposing performance penalties associated with the reduced number of transparent huge pages that could be used by the guest workload. This patch introduces a common interface to help a balloon driver on making its page set movable to compaction, and thus allowing the system to better leverage the compation efforts on memory defragmentation. [akpm@linux-foundation.org: use PAGE_FLAGS_CHECK_AT_PREP, s/__balloon_page_flags/page_flags_cleared/, small cleanups] [rientjes@google.com: allow balloon compaction for any system with memory compaction enabled, which is the defconfig] Signed-off-by: Rafael Aquini <aquini@redhat.com> Acked-by: Mel Gorman <mel@csn.ul.ie> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: "Michael S. Tsirkin" <mst@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Minchan Kim <minchan@kernel.org> Signed-off-by: David Rientjes <rientjes@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/balloon_compaction.h272
-rw-r--r--include/linux/migrate.h10
-rw-r--r--include/linux/pagemap.h16
-rw-r--r--mm/Kconfig15
-rw-r--r--mm/Makefile3
-rw-r--r--mm/balloon_compaction.c302
6 files changed, 617 insertions, 1 deletions
diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
new file mode 100644
index 00000000000..f7f1d7169b1
--- /dev/null
+++ b/include/linux/balloon_compaction.h
@@ -0,0 +1,272 @@
+/*
+ * 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;
+}
+
+/*
+ * 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 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 */
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index a4e886d17f8..ce42847ed35 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -11,8 +11,18 @@ typedef struct page *new_page_t(struct page *, unsigned long private, int **);
* Return values from addresss_space_operations.migratepage():
* - negative errno on page migration failure;
* - zero on page migration success;
+ *
+ * The balloon page migration introduces this special case where a 'distinct'
+ * return code is used to flag a successful page migration to unmap_and_move().
+ * This approach is necessary because page migration can race against balloon
+ * deflation procedure, and for such case we could introduce a nasty page leak
+ * if a successfully migrated balloon page gets released concurrently with
+ * migration's unmap_and_move() wrap-up steps.
*/
#define MIGRATEPAGE_SUCCESS 0
+#define MIGRATEPAGE_BALLOON_SUCCESS 1 /* special ret code for balloon page
+ * sucessful migration case.
+ */
#ifdef CONFIG_MIGRATION
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e42c762f0dc..6da609d14c1 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -24,6 +24,7 @@ enum mapping_flags {
AS_ENOSPC = __GFP_BITS_SHIFT + 1, /* ENOSPC on async write */
AS_MM_ALL_LOCKS = __GFP_BITS_SHIFT + 2, /* under mm_take_all_locks() */
AS_UNEVICTABLE = __GFP_BITS_SHIFT + 3, /* e.g., ramdisk, SHM_LOCK */
+ AS_BALLOON_MAP = __GFP_BITS_SHIFT + 4, /* balloon page special map */
};
static inline void mapping_set_error(struct address_space *mapping, int error)
@@ -53,6 +54,21 @@ static inline int mapping_unevictable(struct address_space *mapping)
return !!mapping;
}
+static inline void mapping_set_balloon(struct address_space *mapping)
+{
+ set_bit(AS_BALLOON_MAP, &mapping->flags);
+}
+
+static inline void mapping_clear_balloon(struct address_space *mapping)
+{
+ clear_bit(AS_BALLOON_MAP, &mapping->flags);
+}
+
+static inline int mapping_balloon(struct address_space *mapping)
+{
+ return mapping && test_bit(AS_BALLOON_MAP, &mapping->flags);
+}
+
static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
{
return (__force gfp_t)mapping->flags & __GFP_BITS_MASK;
diff --git a/mm/Kconfig b/mm/Kconfig
index a3f8dddaaab..e6651c5de14 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -188,6 +188,21 @@ config SPLIT_PTLOCK_CPUS
default "4"
#
+# support for memory balloon compaction
+config BALLOON_COMPACTION
+ bool "Allow for balloon memory compaction/migration"
+ def_bool y
+ depends on COMPACTION && VIRTIO_BALLOON
+ help
+ Memory fragmentation introduced by ballooning might reduce
+ significantly the number of 2MB contiguous memory blocks that can be
+ used within a guest, thus imposing performance penalties associated
+ with the reduced number of transparent huge pages that could be used
+ by the guest workload. Allowing the compaction & migration for memory
+ pages enlisted as being part of memory balloon devices avoids the
+ scenario aforementioned and helps improving memory defragmentation.
+
+#
# support for memory compaction
config COMPACTION
bool "Allow for memory compaction"
diff --git a/mm/Makefile b/mm/Makefile
index 6b025f80af3..3a4628751f8 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -16,7 +16,8 @@ obj-y := filemap.o mempool.o oom_kill.o fadvise.o \
readahead.o swap.o truncate.o vmscan.o shmem.o \
util.o mmzone.o vmstat.o backing-dev.o \
mm_init.o mmu_context.o percpu.o slab_common.o \
- compaction.o interval_tree.o $(mmu-y)
+ compaction.o balloon_compaction.o \
+ interval_tree.o $(mmu-y)
obj-y += init-mm.o
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
new file mode 100644
index 00000000000..07dbc8ec46c
--- /dev/null
+++ b/mm/balloon_compaction.c
@@ -0,0 +1,302 @@
+/*
+ * 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 */