[PATCHv2 4/4] z3fold: support page migration

[Vitaly Wool]

Now that we are not using page address in handles directly, we
can make z3fold pages movable to decrease the memory fragmentation
z3fold may create over time.

This patch starts advertising non-headless z3fold pages as movable
and uses the existing kernel infrastructure to implement moving of
such pages per memory management subsystem's request. It thus
implements 3 required callbacks for page migration:

* isolation callback: z3fold_page_isolate(): try to isolate the
page by removing it from all lists. Pages scheduled for some activity
and mapped pages will not be isolated. Return true if isolation was
successful or false otherwise
* migration callback: z3fold_page_migrate(): re-check critical
conditions and migrate page contents to the new page provided by the
memory subsystem. Returns 0 on success or negative error code
otherwise
* putback callback: z3fold_page_putback(): put back the page if
z3fold_page_migrate() for it failed permanently (i. e. not with
-EAGAIN code).

Signed-off-by: Vitaly Wool <vitaly.vul@xxxxxxxx>
---
 mm/z3fold.c | 241 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 231 insertions(+), 10 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index bebc10083f1c..d9eabfdad0fe 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -24,10 +24,18 @@
 
 #include <linux/atomic.h>
 #include <linux/sched.h>
+#include <linux/cpumask.h>
+#include <linux/dcache.h>
 #include <linux/list.h>
 #include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/page-flags.h>
+#include <linux/migrate.h>
+#include <linux/node.h>
+#include <linux/compaction.h>
 #include <linux/percpu.h>
+#include <linux/mount.h>
+#include <linux/fs.h>
 #include <linux/preempt.h>
 #include <linux/workqueue.h>
 #include <linux/slab.h>
@@ -97,6 +105,7 @@ struct z3fold_buddy_slots {
  * @middle_chunks:	the size of the middle buddy in chunks, 0 if free
  * @last_chunks:	the size of the last buddy in chunks, 0 if free
  * @first_num:		the starting number (for the first handle)
+ * @mapped_count:	the number of objects currently mapped
  */
 struct z3fold_header {
 	struct list_head buddy;
@@ -110,6 +119,7 @@ struct z3fold_header {
 	unsigned short last_chunks;
 	unsigned short start_middle;
 	unsigned short first_num:2;
+	unsigned short mapped_count:2;
 };
 
 /**
@@ -130,6 +140,7 @@ struct z3fold_header {
  * @compact_wq:	workqueue for page layout background optimization
  * @release_wq:	workqueue for safe page release
  * @work:	work_struct for safe page release
+ * @inode:	inode for z3fold pseudo filesystem
  *
  * This structure is allocated at pool creation time and maintains metadata
  * pertaining to a particular z3fold pool.
@@ -149,6 +160,7 @@ struct z3fold_pool {
 	struct workqueue_struct *compact_wq;
 	struct workqueue_struct *release_wq;
 	struct work_struct work;
+	struct inode *inode;
 };
 
 /*
@@ -227,6 +239,59 @@ static inline void free_handle(unsigned long handle)
 	}
 }
 
+static struct dentry *z3fold_do_mount(struct file_system_type *fs_type,
+				int flags, const char *dev_name, void *data)
+{
+	static const struct dentry_operations ops = {
+		.d_dname = simple_dname,
+	};
+
+	return mount_pseudo(fs_type, "z3fold:", NULL, &ops, 0x33);
+}
+
+static struct file_system_type z3fold_fs = {
+	.name		= "z3fold",
+	.mount		= z3fold_do_mount,
+	.kill_sb	= kill_anon_super,
+};
+
+static struct vfsmount *z3fold_mnt;
+static int z3fold_mount(void)
+{
+	int ret = 0;
+
+	z3fold_mnt = kern_mount(&z3fold_fs);
+	if (IS_ERR(z3fold_mnt))
+		ret = PTR_ERR(z3fold_mnt);
+
+	return ret;
+}
+
+static void z3fold_unmount(void)
+{
+	kern_unmount(z3fold_mnt);
+}
+
+static const struct address_space_operations z3fold_aops;
+static int z3fold_register_migration(struct z3fold_pool *pool)
+{
+	pool->inode = alloc_anon_inode(z3fold_mnt->mnt_sb);
+	if (IS_ERR(pool->inode)) {
+		pool->inode = NULL;
+		return 1;
+	}
+
+	pool->inode->i_mapping->private_data = pool;
+	pool->inode->i_mapping->a_ops = &z3fold_aops;
+	return 0;
+}
+
+static void z3fold_unregister_migration(struct z3fold_pool *pool)
+{
+	if (pool->inode)
+		iput(pool->inode);
+ }
+
 /* Initializes the z3fold header of a newly allocated z3fold page */
 static struct z3fold_header *init_z3fold_page(struct page *page,
 					struct z3fold_pool *pool)
@@ -259,8 +324,14 @@ static struct z3fold_header *init_z3fold_page(struct page *page,
 }
 
 /* Resets the struct page fields and frees the page */
-static void free_z3fold_page(struct page *page)
+static void free_z3fold_page(struct page *page, bool headless)
 {
+	if (!headless) {
+		lock_page(page);
+		__ClearPageMovable(page);
+		unlock_page(page);
+	}
+	ClearPagePrivate(page);
 	__free_page(page);
 }
 
@@ -317,12 +388,12 @@ static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
 }
 
 /* Returns the z3fold page where a given handle is stored */
-static inline struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
+static inline struct z3fold_header *handle_to_z3fold_header(unsigned long h)
 {
-	unsigned long addr = handle;
+	unsigned long addr = h;
 
 	if (!(addr & (1 << PAGE_HEADLESS)))
-		addr = *(unsigned long *)handle;
+		addr = *(unsigned long *)h;
 
 	return (struct z3fold_header *)(addr & PAGE_MASK);
 }
@@ -366,7 +437,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 	clear_bit(NEEDS_COMPACTING, &page->private);
 	spin_lock(&pool->lock);
 	if (!list_empty(&page->lru))
-		list_del(&page->lru);
+		list_del_init(&page->lru);
 	spin_unlock(&pool->lock);
 	if (locked)
 		z3fold_page_unlock(zhdr);
@@ -420,7 +491,7 @@ static void free_pages_work(struct work_struct *w)
 			continue;
 		spin_unlock(&pool->stale_lock);
 		cancel_work_sync(&zhdr->work);
-		free_z3fold_page(page);
+		free_z3fold_page(page, false);
 		cond_resched();
 		spin_lock(&pool->stale_lock);
 	}
@@ -486,6 +557,9 @@ static int z3fold_compact_page(struct z3fold_header *zhdr)
 	if (test_bit(MIDDLE_CHUNK_MAPPED, &page->private))
 		return 0; /* can't move middle chunk, it's used */
 
+	if (unlikely(PageIsolated(page)))
+		return 0;
+
 	if (zhdr->middle_chunks == 0)
 		return 0; /* nothing to compact */
 
@@ -546,6 +620,12 @@ static void do_compact_page(struct z3fold_header *zhdr, bool locked)
 		return;
 	}
 
+	if (unlikely(PageIsolated(page) ||
+		     test_bit(PAGE_STALE, &page->private))) {
+		z3fold_page_unlock(zhdr);
+		return;
+	}
+
 	z3fold_compact_page(zhdr);
 	add_to_unbuddied(pool, zhdr);
 	z3fold_page_unlock(zhdr);
@@ -705,10 +785,14 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 	pool->release_wq = create_singlethread_workqueue(pool->name);
 	if (!pool->release_wq)
 		goto out_wq;
+	if (z3fold_register_migration(pool))
+		goto out_rwq;
 	INIT_WORK(&pool->work, free_pages_work);
 	pool->ops = ops;
 	return pool;
 
+out_rwq:
+	destroy_workqueue(pool->release_wq);
 out_wq:
 	destroy_workqueue(pool->compact_wq);
 out_unbuddied:
@@ -730,6 +814,7 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 static void z3fold_destroy_pool(struct z3fold_pool *pool)
 {
 	kmem_cache_destroy(pool->c_handle);
+	z3fold_unregister_migration(pool);
 	destroy_workqueue(pool->release_wq);
 	destroy_workqueue(pool->compact_wq);
 	kfree(pool);
@@ -837,6 +922,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 		set_bit(PAGE_HEADLESS, &page->private);
 		goto headless;
 	}
+	__SetPageMovable(page, pool->inode->i_mapping);
 	z3fold_page_lock(zhdr);
 
 found:
@@ -895,7 +981,7 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 			spin_lock(&pool->lock);
 			list_del(&page->lru);
 			spin_unlock(&pool->lock);
-			free_z3fold_page(page);
+			free_z3fold_page(page, true);
 			atomic64_dec(&pool->pages_nr);
 		}
 		return;
@@ -931,7 +1017,8 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		z3fold_page_unlock(zhdr);
 		return;
 	}
-	if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
+	if (unlikely(PageIsolated(page)) ||
+	    test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
 		z3fold_page_unlock(zhdr);
 		return;
 	}
@@ -1012,10 +1099,12 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			if (test_and_set_bit(PAGE_CLAIMED, &page->private))
 				continue;
 
-			zhdr = page_address(page);
+			if (unlikely(PageIsolated(page)))
+				continue;
 			if (test_bit(PAGE_HEADLESS, &page->private))
 				break;
 
+			zhdr = page_address(page);
 			if (!z3fold_page_trylock(zhdr)) {
 				zhdr = NULL;
 				continue; /* can't evict at this point */
@@ -1076,7 +1165,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 next:
 		if (test_bit(PAGE_HEADLESS, &page->private)) {
 			if (ret == 0) {
-				free_z3fold_page(page);
+				free_z3fold_page(page, true);
 				atomic64_dec(&pool->pages_nr);
 				return 0;
 			}
@@ -1153,6 +1242,8 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
 		break;
 	}
 
+	if (addr)
+		zhdr->mapped_count++;
 	z3fold_page_unlock(zhdr);
 out:
 	return addr;
@@ -1179,6 +1270,7 @@ static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
 	buddy = handle_to_buddy(handle);
 	if (buddy == MIDDLE)
 		clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
+	zhdr->mapped_count--;
 	z3fold_page_unlock(zhdr);
 }
 
@@ -1193,6 +1285,128 @@ static u64 z3fold_get_pool_size(struct z3fold_pool *pool)
 	return atomic64_read(&pool->pages_nr);
 }
 
+bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
+{
+	struct z3fold_header *zhdr;
+	struct z3fold_pool *pool;
+
+	VM_BUG_ON_PAGE(!PageMovable(page), page);
+	VM_BUG_ON_PAGE(PageIsolated(page), page);
+
+	if (test_bit(PAGE_HEADLESS, &page->private))
+		return false;
+
+	zhdr = page_address(page);
+	z3fold_page_lock(zhdr);
+	if (test_bit(NEEDS_COMPACTING, &page->private) ||
+	    test_bit(PAGE_STALE, &page->private))
+		goto out;
+
+	pool = zhdr_to_pool(zhdr);
+
+	if (zhdr->mapped_count == 0) {
+		kref_get(&zhdr->refcount);
+		if (!list_empty(&zhdr->buddy))
+			list_del_init(&zhdr->buddy);
+		spin_lock(&pool->lock);
+		if (!list_empty(&page->lru))
+			list_del(&page->lru);
+		spin_unlock(&pool->lock);
+		z3fold_page_unlock(zhdr);
+		return true;
+	}
+out:
+	z3fold_page_unlock(zhdr);
+	return false;
+}
+
+int z3fold_page_migrate(struct address_space *mapping, struct page *newpage,
+			struct page *page, enum migrate_mode mode)
+{
+	struct z3fold_header *zhdr, *new_zhdr;
+	struct z3fold_pool *pool;
+	struct address_space *new_mapping;
+
+	VM_BUG_ON_PAGE(!PageMovable(page), page);
+	VM_BUG_ON_PAGE(!PageIsolated(page), page);
+
+	zhdr = page_address(page);
+	pool = zhdr_to_pool(zhdr);
+
+	if (!trylock_page(page))
+		return -EAGAIN;
+
+	if (!z3fold_page_trylock(zhdr)) {
+		unlock_page(page);
+		return -EAGAIN;
+	}
+	if (zhdr->mapped_count != 0) {
+		z3fold_page_unlock(zhdr);
+		unlock_page(page);
+		return -EBUSY;
+	}
+	new_zhdr = page_address(newpage);
+	memcpy(new_zhdr, zhdr, PAGE_SIZE);
+	newpage->private = page->private;
+	page->private = 0;
+	z3fold_page_unlock(zhdr);
+	spin_lock_init(&new_zhdr->page_lock);
+	new_mapping = page_mapping(page);
+	__ClearPageMovable(page);
+	ClearPagePrivate(page);
+
+	get_page(newpage);
+	z3fold_page_lock(new_zhdr);
+	if (new_zhdr->first_chunks)
+		encode_handle(new_zhdr, FIRST);
+	if (new_zhdr->last_chunks)
+		encode_handle(new_zhdr, LAST);
+	if (new_zhdr->middle_chunks)
+		encode_handle(new_zhdr, MIDDLE);
+	set_bit(NEEDS_COMPACTING, &newpage->private);
+	new_zhdr->cpu = smp_processor_id();
+	spin_lock(&pool->lock);
+	list_add(&newpage->lru, &pool->lru);
+	spin_unlock(&pool->lock);
+	__SetPageMovable(newpage, new_mapping);
+	z3fold_page_unlock(new_zhdr);
+
+	queue_work_on(new_zhdr->cpu, pool->compact_wq, &new_zhdr->work);
+
+	page_mapcount_reset(page);
+	unlock_page(page);
+	put_page(page);
+	return 0;
+}
+
+void z3fold_page_putback(struct page *page)
+{
+	struct z3fold_header *zhdr;
+	struct z3fold_pool *pool;
+
+	zhdr = page_address(page);
+	pool = zhdr_to_pool(zhdr);
+
+	z3fold_page_lock(zhdr);
+	if (!list_empty(&zhdr->buddy))
+		list_del_init(&zhdr->buddy);
+	INIT_LIST_HEAD(&page->lru);
+	if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
+		atomic64_dec(&pool->pages_nr);
+		return;
+	}
+	spin_lock(&pool->lock);
+	list_add(&page->lru, &pool->lru);
+	spin_unlock(&pool->lock);
+	z3fold_page_unlock(zhdr);
+}
+
+static const struct address_space_operations z3fold_aops = {
+	.isolate_page = z3fold_page_isolate,
+	.migratepage = z3fold_page_migrate,
+	.putback_page = z3fold_page_putback,
+};
+
 /*****************
  * zpool
  ****************/
@@ -1290,8 +1504,14 @@ MODULE_ALIAS("zpool-z3fold");
 
 static int __init init_z3fold(void)
 {
+	int ret;
+
 	/* Make sure the z3fold header is not larger than the page size */
 	BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE);
+	ret = z3fold_mount();
+	if (ret)
+		return ret;
+
 	zpool_register_driver(&z3fold_zpool_driver);
 
 	return 0;
@@ -1299,6 +1519,7 @@ static int __init init_z3fold(void)
 
 static void __exit exit_z3fold(void)
 {
+	z3fold_unmount();
 	zpool_unregister_driver(&z3fold_zpool_driver);
 }
 
-- 
2.17.1