Btrfs: add a io_ctl struct and helpers for dealing with the space cache

In writing and reading the space cache we have one big loop that keeps track of
which page we are on and then a bunch of sizeable loops underneath this big loop
to try and read/write out properly.  Especially in the write case this makes
things hugely complicated and hard to follow, and makes our error checking and
recovery equally as complex.  So add a io_ctl struct with a bunch of helpers to
keep track of the pages we have, where we are, if we have enough space etc.
This unifies how we deal with the pages we're writing and keeps all the messy
tracking internal.  This allows us to kill the big loops in both the read and
write case and makes reviewing and chaning the write and read paths much
simpler.  I've run xfstests and stress.sh on this code and it survives.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/free-space-cache.c

@@ -241,27 +241,275 @@ static int readahead_cache(struct inode *inode)
 	return 0;
 }
 
+struct io_ctl {
+	void *cur, *orig;
+	struct page *page;
+	struct page **pages;
+	struct btrfs_root *root;
+	unsigned long size;
+	int index;
+	int num_pages;
+};
+
+static int io_ctl_init(struct io_ctl *io_ctl, struct inode *inode,
+		       struct btrfs_root *root)
+{
+	memset(io_ctl, 0, sizeof(struct io_ctl));
+	io_ctl->num_pages = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
+		PAGE_CACHE_SHIFT;
+	io_ctl->pages = kzalloc(sizeof(struct page *) * io_ctl->num_pages,
+				GFP_NOFS);
+	if (!io_ctl->pages)
+		return -ENOMEM;
+	io_ctl->root = root;
+	return 0;
+}
+
+static void io_ctl_free(struct io_ctl *io_ctl)
+{
+	kfree(io_ctl->pages);
+}
+
+static void io_ctl_unmap_page(struct io_ctl *io_ctl)
+{
+	if (io_ctl->cur) {
+		kunmap(io_ctl->page);
+		io_ctl->cur = NULL;
+		io_ctl->orig = NULL;
+	}
+}
+
+static void io_ctl_map_page(struct io_ctl *io_ctl, int clear)
+{
+	WARN_ON(io_ctl->cur);
+	BUG_ON(io_ctl->index >= io_ctl->num_pages);
+	io_ctl->page = io_ctl->pages[io_ctl->index++];
+	io_ctl->cur = kmap(io_ctl->page);
+	io_ctl->orig = io_ctl->cur;
+	io_ctl->size = PAGE_CACHE_SIZE;
+	if (clear)
+		memset(io_ctl->cur, 0, PAGE_CACHE_SIZE);
+}
+
+static void io_ctl_drop_pages(struct io_ctl *io_ctl)
+{
+	int i;
+
+	io_ctl_unmap_page(io_ctl);
+
+	for (i = 0; i < io_ctl->num_pages; i++) {
+		ClearPageChecked(io_ctl->pages[i]);
+		unlock_page(io_ctl->pages[i]);
+		page_cache_release(io_ctl->pages[i]);
+	}
+}
+
+static int io_ctl_prepare_pages(struct io_ctl *io_ctl, struct inode *inode,
+				int uptodate)
+{
+	struct page *page;
+	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
+	int i;
+
+	for (i = 0; i < io_ctl->num_pages; i++) {
+		page = find_or_create_page(inode->i_mapping, i, mask);
+		if (!page) {
+			io_ctl_drop_pages(io_ctl);
+			return -ENOMEM;
+		}
+		io_ctl->pages[i] = page;
+		if (uptodate && !PageUptodate(page)) {
+			btrfs_readpage(NULL, page);
+			lock_page(page);
+			if (!PageUptodate(page)) {
+				printk(KERN_ERR "btrfs: error reading free "
+				       "space cache\n");
+				io_ctl_drop_pages(io_ctl);
+				return -EIO;
+			}
+		}
+	}
+
+	return 0;
+}
+
+static void io_ctl_set_generation(struct io_ctl *io_ctl, u64 generation)
+{
+	u64 *val;
+
+	io_ctl_map_page(io_ctl, 1);
+
+	/*
+	 * Skip the first 64bits to make sure theres a bogus crc for old
+	 * kernels
+	 */
+	io_ctl->cur += sizeof(u64);
+
+	val = io_ctl->cur;
+	*val = cpu_to_le64(generation);
+	io_ctl->cur += sizeof(u64);
+	io_ctl->size -= sizeof(u64) * 2;
+}
+
+static int io_ctl_check_generation(struct io_ctl *io_ctl, u64 generation)
+{
+	u64 *gen;
+
+	io_ctl_map_page(io_ctl, 0);
+
+	/* Skip the bogus crc area */
+	io_ctl->cur += sizeof(u64);
+	gen = io_ctl->cur;
+	if (le64_to_cpu(*gen) != generation) {
+		printk_ratelimited(KERN_ERR "btrfs: space cache generation "
+				   "(%Lu) does not match inode (%Lu)\n", *gen,
+				   generation);
+		io_ctl_unmap_page(io_ctl);
+		return -EIO;
+	}
+	io_ctl->cur += sizeof(u64);
+	io_ctl->size -= sizeof(u64) * 2;
+	return 0;
+}
+
+static int io_ctl_add_entry(struct io_ctl *io_ctl, u64 offset, u64 bytes,
+			    void *bitmap)
+{
+	struct btrfs_free_space_entry *entry;
+
+	if (!io_ctl->cur)
+		return -ENOSPC;
+
+	entry = io_ctl->cur;
+	entry->offset = cpu_to_le64(offset);
+	entry->bytes = cpu_to_le64(bytes);
+	entry->type = (bitmap) ? BTRFS_FREE_SPACE_BITMAP :
+		BTRFS_FREE_SPACE_EXTENT;
+	io_ctl->cur += sizeof(struct btrfs_free_space_entry);
+	io_ctl->size -= sizeof(struct btrfs_free_space_entry);
+
+	if (io_ctl->size >= sizeof(struct btrfs_free_space_entry))
+		return 0;
+
+	/*
+	 * index == 1 means the current page is 0, we need to generate a bogus
+	 * crc for older kernels.
+	 */
+	if (io_ctl->index == 1) {
+		u32 *tmp;
+		u32 crc = ~(u32)0;
+
+		crc = btrfs_csum_data(io_ctl->root, io_ctl->orig + sizeof(u64),
+				      crc, PAGE_CACHE_SIZE - sizeof(u64));
+		btrfs_csum_final(crc, (char *)&crc);
+		crc++;
+		tmp = io_ctl->orig;
+		*tmp = crc;
+	}
+	io_ctl_unmap_page(io_ctl);
+
+	/* No more pages to map */
+	if (io_ctl->index >= io_ctl->num_pages)
+		return 0;
+
+	/* map the next page */
+	io_ctl_map_page(io_ctl, 1);
+	return 0;
+}
+
+static int io_ctl_add_bitmap(struct io_ctl *io_ctl, void *bitmap)
+{
+	if (!io_ctl->cur)
+		return -ENOSPC;
+
+	/*
+	 * If we aren't at the start of the current page, unmap this one and
+	 * map the next one if there is any left.
+	 */
+	if (io_ctl->cur != io_ctl->orig) {
+		io_ctl_unmap_page(io_ctl);
+		if (io_ctl->index >= io_ctl->num_pages)
+			return -ENOSPC;
+		io_ctl_map_page(io_ctl, 0);
+	}
+
+	memcpy(io_ctl->cur, bitmap, PAGE_CACHE_SIZE);
+	io_ctl_unmap_page(io_ctl);
+	if (io_ctl->index < io_ctl->num_pages)
+		io_ctl_map_page(io_ctl, 0);
+	return 0;
+}
+
+static void io_ctl_zero_remaining_pages(struct io_ctl *io_ctl)
+{
+	io_ctl_unmap_page(io_ctl);
+
+	while (io_ctl->index < io_ctl->num_pages) {
+		io_ctl_map_page(io_ctl, 1);
+		io_ctl_unmap_page(io_ctl);
+	}
+}
+
+static u8 io_ctl_read_entry(struct io_ctl *io_ctl,
+			    struct btrfs_free_space *entry)
+{
+	struct btrfs_free_space_entry *e;
+	u8 type;
+
+	e = io_ctl->cur;
+	entry->offset = le64_to_cpu(e->offset);
+	entry->bytes = le64_to_cpu(e->bytes);
+	type = e->type;
+	io_ctl->cur += sizeof(struct btrfs_free_space_entry);
+	io_ctl->size -= sizeof(struct btrfs_free_space_entry);
+
+	if (io_ctl->size >= sizeof(struct btrfs_free_space_entry))
+		return type;
+
+	io_ctl_unmap_page(io_ctl);
+
+	if (io_ctl->index >= io_ctl->num_pages)
+		return type;
+
+	io_ctl_map_page(io_ctl, 0);
+	return type;
+}
+
+static void io_ctl_read_bitmap(struct io_ctl *io_ctl,
+			       struct btrfs_free_space *entry)
+{
+	BUG_ON(!io_ctl->cur);
+	if (io_ctl->cur != io_ctl->orig) {
+		io_ctl_unmap_page(io_ctl);
+		io_ctl_map_page(io_ctl, 0);
+	}
+	memcpy(entry->bitmap, io_ctl->cur, PAGE_CACHE_SIZE);
+	io_ctl_unmap_page(io_ctl);
+	if (io_ctl->index < io_ctl->num_pages)
+		io_ctl_map_page(io_ctl, 0);
+}
+
 int __load_free_space_cache(struct btrfs_root *root, struct inode *inode,
 			    struct btrfs_free_space_ctl *ctl,
 			    struct btrfs_path *path, u64 offset)
 {
 	struct btrfs_free_space_header *header;
 	struct extent_buffer *leaf;
-	struct page *page;
+	struct io_ctl io_ctl;
 	struct btrfs_key key;
+	struct btrfs_free_space *e, *n;
 	struct list_head bitmaps;
 	u64 num_entries;
 	u64 num_bitmaps;
 	u64 generation;
-	pgoff_t index = 0;
-	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
+	u8 type;
 	int ret = 0;
 
 	INIT_LIST_HEAD(&bitmaps);
 
 	/* Nothing in the space cache, goodbye */
 	if (!i_size_read(inode))
-		goto out;
+		return 0;
 
 	key.objectid = BTRFS_FREE_SPACE_OBJECTID;
 	key.offset = offset;
@@ -269,11 +517,10 @@ int __load_free_space_cache(struct btrfs_root *root, struct inode *inode,
 
 	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
 	if (ret < 0)
-		goto out;
+		return 0;
 	else if (ret > 0) {
 		btrfs_release_path(path);
-		ret = 0;
-		goto out;
+		return 0;
 	}
 
 	ret = -1;
@@ -291,170 +538,89 @@ int __load_free_space_cache(struct btrfs_root *root, struct inode *inode,
 		       " not match free space cache generation (%llu)\n",
 		       (unsigned long long)BTRFS_I(inode)->generation,
 		       (unsigned long long)generation);
-		goto out;
+		return 0;
 	}
 
 	if (!num_entries)
-		goto out;
+		return 0;
 
+	io_ctl_init(&io_ctl, inode, root);
 	ret = readahead_cache(inode);
 	if (ret)
 		goto out;
 
-	while (1) {
-		struct btrfs_free_space_entry *entry;
-		struct btrfs_free_space *e;
-		void *addr;
-		unsigned long offset = 0;
-		int need_loop = 0;
+	ret = io_ctl_prepare_pages(&io_ctl, inode, 1);
+	if (ret)
+		goto out;
 
-		if (!num_entries && !num_bitmaps)
-			break;
+	ret = io_ctl_check_generation(&io_ctl, generation);
+	if (ret)
+		goto free_cache;
 
-		page = find_or_create_page(inode->i_mapping, index, mask);
-		if (!page)
+	while (num_entries) {
+		e = kmem_cache_zalloc(btrfs_free_space_cachep,
+				      GFP_NOFS);
+		if (!e)
 			goto free_cache;
 
-		if (!PageUptodate(page)) {
-			btrfs_readpage(NULL, page);
-			lock_page(page);
-			if (!PageUptodate(page)) {
-				unlock_page(page);
-				page_cache_release(page);
-				printk(KERN_ERR "btrfs: error reading free "
-				       "space cache\n");
-				goto free_cache;
-			}
+		type = io_ctl_read_entry(&io_ctl, e);
+		if (!e->bytes) {
+			kmem_cache_free(btrfs_free_space_cachep, e);
+			goto free_cache;
 		}
-		addr = kmap(page);
 
-		if (index == 0) {
-			u64 *gen;
-
-			/*
-			 * We put a bogus crc in the front of the first page in
-			 * case old kernels try to mount a fs with the new
-			 * format to make sure they discard the cache.
-			 */
-			addr += sizeof(u64);
-			offset += sizeof(u64);
-
-			gen = addr;
-			if (*gen != BTRFS_I(inode)->generation) {
-				printk_ratelimited(KERN_ERR "btrfs: space cache"
-					" generation (%llu) does not match "
-					"inode (%llu)\n",
-					(unsigned long long)*gen,
-					(unsigned long long)
-					BTRFS_I(inode)->generation);
-				kunmap(page);
-				unlock_page(page);
-				page_cache_release(page);
-				goto free_cache;
-			}
-			addr += sizeof(u64);
-			offset += sizeof(u64);
-		}
-		entry = addr;
-
-		while (1) {
-			if (!num_entries)
-				break;
-
-			need_loop = 1;
-			e = kmem_cache_zalloc(btrfs_free_space_cachep,
-					      GFP_NOFS);
-			if (!e) {
-				kunmap(page);
-				unlock_page(page);
-				page_cache_release(page);
-				goto free_cache;
-			}
-
-			e->offset = le64_to_cpu(entry->offset);
-			e->bytes = le64_to_cpu(entry->bytes);
-			if (!e->bytes) {
-				kunmap(page);
+		if (type == BTRFS_FREE_SPACE_EXTENT) {
+			spin_lock(&ctl->tree_lock);
+			ret = link_free_space(ctl, e);
+			spin_unlock(&ctl->tree_lock);
+			if (ret) {
+				printk(KERN_ERR "Duplicate entries in "
+				       "free space cache, dumping\n");
 				kmem_cache_free(btrfs_free_space_cachep, e);
-				unlock_page(page);
-				page_cache_release(page);
 				goto free_cache;
 			}
-
-			if (entry->type == BTRFS_FREE_SPACE_EXTENT) {
-				spin_lock(&ctl->tree_lock);
-				ret = link_free_space(ctl, e);
-				spin_unlock(&ctl->tree_lock);
-				if (ret) {
-					printk(KERN_ERR "Duplicate entries in "
-					       "free space cache, dumping\n");
-					kunmap(page);
-					unlock_page(page);
-					page_cache_release(page);
-					goto free_cache;
-				}
-			} else {
-				e->bitmap = kzalloc(PAGE_CACHE_SIZE, GFP_NOFS);
-				if (!e->bitmap) {
-					kunmap(page);
-					kmem_cache_free(
-						btrfs_free_space_cachep, e);
-					unlock_page(page);
-					page_cache_release(page);
-					goto free_cache;
-				}
-				spin_lock(&ctl->tree_lock);
-				ret = link_free_space(ctl, e);
-				ctl->total_bitmaps++;
-				ctl->op->recalc_thresholds(ctl);
-				spin_unlock(&ctl->tree_lock);
-				if (ret) {
-					printk(KERN_ERR "Duplicate entries in "
-					       "free space cache, dumping\n");
-					kunmap(page);
-					unlock_page(page);
-					page_cache_release(page);
-					goto free_cache;
-				}
-				list_add_tail(&e->list, &bitmaps);
+		} else {
+			BUG_ON(!num_bitmaps);
+			num_bitmaps--;
+			e->bitmap = kzalloc(PAGE_CACHE_SIZE, GFP_NOFS);
+			if (!e->bitmap) {
+				kmem_cache_free(
+					btrfs_free_space_cachep, e);
+				goto free_cache;
 			}
-
-			num_entries--;
-			offset += sizeof(struct btrfs_free_space_entry);
-			if (offset + sizeof(struct btrfs_free_space_entry) >=
-			    PAGE_CACHE_SIZE)
-				break;
-			entry++;
+			spin_lock(&ctl->tree_lock);
+			ret = link_free_space(ctl, e);
+			ctl->total_bitmaps++;
+			ctl->op->recalc_thresholds(ctl);
+			spin_unlock(&ctl->tree_lock);
+			if (ret) {
+				printk(KERN_ERR "Duplicate entries in "
+				       "free space cache, dumping\n");
+				kmem_cache_free(btrfs_free_space_cachep, e);
+				goto free_cache;
+			}
+			list_add_tail(&e->list, &bitmaps);
 		}
 
-		/*
-		 * We read an entry out of this page, we need to move on to the
-		 * next page.
-		 */
-		if (need_loop) {
-			kunmap(page);
-			goto next;
-		}
-
-		/*
-		 * We add the bitmaps at the end of the entries in order that
-		 * the bitmap entries are added to the cache.
-		 */
-		e = list_entry(bitmaps.next, struct btrfs_free_space, list);
-		list_del_init(&e->list);
-		memcpy(e->bitmap, addr, PAGE_CACHE_SIZE);
-		kunmap(page);
-		num_bitmaps--;
-next:
-		unlock_page(page);
-		page_cache_release(page);
-		index++;
+		num_entries--;
 	}
 
+	/*
+	 * We add the bitmaps at the end of the entries in order that
+	 * the bitmap entries are added to the cache.
+	 */
+	list_for_each_entry_safe(e, n, &bitmaps, list) {
+		list_del_init(&e->list);
+		io_ctl_read_bitmap(&io_ctl, e);
+	}
+
+	io_ctl_drop_pages(&io_ctl);
 	ret = 1;
 out:
+	io_ctl_free(&io_ctl);
 	return ret;
 free_cache:
+	io_ctl_drop_pages(&io_ctl);
 	__btrfs_remove_free_space_cache(ctl);
 	goto out;
 }
@@ -554,40 +720,28 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 	struct extent_buffer *leaf;
 	struct rb_node *node;
 	struct list_head *pos, *n;
-	struct page **pages;
-	struct page *page;
 	struct extent_state *cached_state = NULL;
 	struct btrfs_free_cluster *cluster = NULL;
 	struct extent_io_tree *unpin = NULL;
+	struct io_ctl io_ctl;
 	struct list_head bitmap_list;
 	struct btrfs_key key;
 	u64 start, end, len;
-	u64 bytes = 0;
-	u32 crc = ~(u32)0;
-	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
-	int index = 0, num_pages = 0;
 	int entries = 0;
 	int bitmaps = 0;
 	int ret;
 	int err = -1;
-	bool next_page = false;
-	bool out_of_space = false;
 
 	INIT_LIST_HEAD(&bitmap_list);
 
 	if (!i_size_read(inode))
 		return -1;
 
-	num_pages = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
-		PAGE_CACHE_SHIFT;
-
 	filemap_write_and_wait(inode->i_mapping);
 	btrfs_wait_ordered_range(inode, inode->i_size &
 				 ~(root->sectorsize - 1), (u64)-1);
 
-	pages = kzalloc(sizeof(struct page *) * num_pages, GFP_NOFS);
-	if (!pages)
-		return -1;
+	io_ctl_init(&io_ctl, inode, root);
 
 	/* Get the cluster for this block_group if it exists */
 	if (block_group && !list_empty(&block_group->cluster_list))
@@ -601,30 +755,9 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 	 */
 	unpin = root->fs_info->pinned_extents;
 
-	/*
-	 * Lock all pages first so we can lock the extent safely.
-	 *
-	 * NOTE: Because we hold the ref the entire time we're going to write to
-	 * the page find_get_page should never fail, so we don't do a check
-	 * after find_get_page at this point.  Just putting this here so people
-	 * know and don't freak out.
-	 */
-	while (index < num_pages) {
-		page = find_or_create_page(inode->i_mapping, index, mask);
-		if (!page) {
-			int i;
+	/* Lock all pages first so we can lock the extent safely. */
+	io_ctl_prepare_pages(&io_ctl, inode, 0);
 
-			for (i = 0; i < num_pages; i++) {
-				unlock_page(pages[i]);
-				page_cache_release(pages[i]);
-			}
-			goto out;
-		}
-		pages[index] = page;
-		index++;
-	}
-
-	index = 0;
 	lock_extent_bits(&BTRFS_I(inode)->io_tree, 0, i_size_read(inode) - 1,
 			 0, &cached_state, GFP_NOFS);
 
@@ -641,166 +774,78 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 		cluster = NULL;
 	}
 
+	io_ctl_set_generation(&io_ctl, trans->transid);
+
 	/* Write out the extent entries */
-	do {
-		struct btrfs_free_space_entry *entry;
-		void *addr, *orig;
-		unsigned long offset = 0;
+	while (node) {
+		struct btrfs_free_space *e;
 
-		next_page = false;
+		e = rb_entry(node, struct btrfs_free_space, offset_index);
+		entries++;
 
-		if (index >= num_pages) {
-			out_of_space = true;
+		ret = io_ctl_add_entry(&io_ctl, e->offset, e->bytes,
+				       e->bitmap);
+		if (ret)
+			goto out_nospc;
+
+		if (e->bitmap) {
+			list_add_tail(&e->list, &bitmap_list);
+			bitmaps++;
+		}
+		node = rb_next(node);
+		if (!node && cluster) {
+			node = rb_first(&cluster->root);
+			cluster = NULL;
+		}
+	}
+
+	/*
+	 * We want to add any pinned extents to our free space cache
+	 * so we don't leak the space
+	 */
+	while (block_group && (start < block_group->key.objectid +
+			       block_group->key.offset)) {
+		ret = find_first_extent_bit(unpin, start, &start, &end,
+					    EXTENT_DIRTY);
+		if (ret) {
+			ret = 0;
 			break;
 		}
 
-		page = pages[index];
+		/* This pinned extent is out of our range */
+		if (start >= block_group->key.objectid +
+		    block_group->key.offset)
+			break;
 
-		orig = addr = kmap(page);
-		if (index == 0) {
-			u64 *gen;
+		len = block_group->key.objectid +
+			block_group->key.offset - start;
+		len = min(len, end + 1 - start);
 
-			/*
-			 * We're going to put in a bogus crc for this page to
-			 * make sure that old kernels who aren't aware of this
-			 * format will be sure to discard the cache.
-			 */
-			addr += sizeof(u64);
-			offset += sizeof(u64);
+		entries++;
+		ret = io_ctl_add_entry(&io_ctl, start, len, NULL);
+		if (ret)
+			goto out_nospc;
 
-			gen = addr;
-			*gen = trans->transid;
-			addr += sizeof(u64);
-			offset += sizeof(u64);
-		}
-		entry = addr;
-
-		memset(addr, 0, PAGE_CACHE_SIZE - offset);
-		while (node && !next_page) {
-			struct btrfs_free_space *e;
-
-			e = rb_entry(node, struct btrfs_free_space, offset_index);
-			entries++;
-
-			entry->offset = cpu_to_le64(e->offset);
-			entry->bytes = cpu_to_le64(e->bytes);
-			if (e->bitmap) {
-				entry->type = BTRFS_FREE_SPACE_BITMAP;
-				list_add_tail(&e->list, &bitmap_list);
-				bitmaps++;
-			} else {
-				entry->type = BTRFS_FREE_SPACE_EXTENT;
-			}
-			node = rb_next(node);
-			if (!node && cluster) {
-				node = rb_first(&cluster->root);
-				cluster = NULL;
-			}
-			offset += sizeof(struct btrfs_free_space_entry);
-			if (offset + sizeof(struct btrfs_free_space_entry) >=
-			    PAGE_CACHE_SIZE)
-				next_page = true;
-			entry++;
-		}
-
-		/*
-		 * We want to add any pinned extents to our free space cache
-		 * so we don't leak the space
-		 */
-		while (block_group && !next_page &&
-		       (start < block_group->key.objectid +
-			block_group->key.offset)) {
-			ret = find_first_extent_bit(unpin, start, &start, &end,
-						    EXTENT_DIRTY);
-			if (ret) {
-				ret = 0;
-				break;
-			}
-
-			/* This pinned extent is out of our range */
-			if (start >= block_group->key.objectid +
-			    block_group->key.offset)
-				break;
-
-			len = block_group->key.objectid +
-				block_group->key.offset - start;
-			len = min(len, end + 1 - start);
-
-			entries++;
-			entry->offset = cpu_to_le64(start);
-			entry->bytes = cpu_to_le64(len);
-			entry->type = BTRFS_FREE_SPACE_EXTENT;
-
-			start = end + 1;
-			offset += sizeof(struct btrfs_free_space_entry);
-			if (offset + sizeof(struct btrfs_free_space_entry) >=
-			    PAGE_CACHE_SIZE)
-				next_page = true;
-			entry++;
-		}
-
-		/* Generate bogus crc value */
-		if (index == 0) {
-			u32 *tmp;
-			crc = btrfs_csum_data(root, orig + sizeof(u64), crc,
-					      PAGE_CACHE_SIZE - sizeof(u64));
-			btrfs_csum_final(crc, (char *)&crc);
-			crc++;
-			tmp = orig;
-			*tmp = crc;
-		}
-
-		kunmap(page);
-
-		bytes += PAGE_CACHE_SIZE;
-
-		index++;
-	} while (node || next_page);
+		start = end + 1;
+	}
 
 	/* Write out the bitmaps */
 	list_for_each_safe(pos, n, &bitmap_list) {
-		void *addr;
 		struct btrfs_free_space *entry =
 			list_entry(pos, struct btrfs_free_space, list);
 
-		if (index >= num_pages) {
-			out_of_space = true;
-			break;
-		}
-		page = pages[index];
-
-		addr = kmap(page);
-		memcpy(addr, entry->bitmap, PAGE_CACHE_SIZE);
-		kunmap(page);
-		bytes += PAGE_CACHE_SIZE;
-
+		ret = io_ctl_add_bitmap(&io_ctl, entry->bitmap);
+		if (ret)
+			goto out_nospc;
 		list_del_init(&entry->list);
-		index++;
-	}
-
-	if (out_of_space) {
-		btrfs_drop_pages(pages, num_pages);
-		unlock_extent_cached(&BTRFS_I(inode)->io_tree, 0,
-				     i_size_read(inode) - 1, &cached_state,
-				     GFP_NOFS);
-		goto out;
 	}
 
 	/* Zero out the rest of the pages just to make sure */
-	while (index < num_pages) {
-		void *addr;
+	io_ctl_zero_remaining_pages(&io_ctl);
 
-		page = pages[index];
-		addr = kmap(page);
-		memset(addr, 0, PAGE_CACHE_SIZE);
-		kunmap(page);
-		bytes += PAGE_CACHE_SIZE;
-		index++;
-	}
-
-	ret = btrfs_dirty_pages(root, inode, pages, num_pages, 0,
-					    bytes, &cached_state);
-	btrfs_drop_pages(pages, num_pages);
+	ret = btrfs_dirty_pages(root, inode, io_ctl.pages, io_ctl.num_pages,
+				0, i_size_read(inode), &cached_state);
+	io_ctl_drop_pages(&io_ctl);
 	unlock_extent_cached(&BTRFS_I(inode)->io_tree, 0,
 			     i_size_read(inode) - 1, &cached_state, GFP_NOFS);
 
@@ -817,7 +862,7 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 
 	ret = btrfs_search_slot(trans, root, &key, path, 0, 1);
 	if (ret < 0) {
-		clear_extent_bit(&BTRFS_I(inode)->io_tree, 0, bytes - 1,
+		clear_extent_bit(&BTRFS_I(inode)->io_tree, 0, inode->i_size - 1,
 				 EXTENT_DIRTY | EXTENT_DELALLOC |
 				 EXTENT_DO_ACCOUNTING, 0, 0, NULL, GFP_NOFS);
 		goto out;
@@ -830,7 +875,8 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 		btrfs_item_key_to_cpu(leaf, &found_key, path->slots[0]);
 		if (found_key.objectid != BTRFS_FREE_SPACE_OBJECTID ||
 		    found_key.offset != offset) {
-			clear_extent_bit(&BTRFS_I(inode)->io_tree, 0, bytes - 1,
+			clear_extent_bit(&BTRFS_I(inode)->io_tree, 0,
+					 inode->i_size - 1,
 					 EXTENT_DIRTY | EXTENT_DELALLOC |
 					 EXTENT_DO_ACCOUNTING, 0, 0, NULL,
 					 GFP_NOFS);
@@ -848,13 +894,24 @@ int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 
 	err = 0;
 out:
-	kfree(pages);
+	io_ctl_free(&io_ctl);
 	if (err) {
-		invalidate_inode_pages2_range(inode->i_mapping, 0, index);
+		invalidate_inode_pages2(inode->i_mapping);
 		BTRFS_I(inode)->generation = 0;
 	}
 	btrfs_update_inode(trans, root, inode);
 	return err;
+
+out_nospc:
+	list_for_each_safe(pos, n, &bitmap_list) {
+		struct btrfs_free_space *entry =
+			list_entry(pos, struct btrfs_free_space, list);
+		list_del_init(&entry->list);
+	}
+	io_ctl_drop_pages(&io_ctl);
+	unlock_extent_cached(&BTRFS_I(inode)->io_tree, 0,
+			     i_size_read(inode) - 1, &cached_state, GFP_NOFS);
+	goto out;
 }
 
 int btrfs_write_out_cache(struct btrfs_root *root,