ext4: use bio layer instead of buffer layer in mpage_da_submit_io

Call the block I/O layer directly instad of going through the buffer
layer.  This should give us much better performance and scalability,
as well as lowering our CPU utilization when doing buffered writeback.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/Makefile

@@ -4,7 +4,7 @@
 
 obj-$(CONFIG_EXT4_FS) += ext4.o
 
-ext4-y	:= balloc.o bitmap.o dir.o file.o fsync.o ialloc.o inode.o \
+ext4-y	:= balloc.o bitmap.o dir.o file.o fsync.o ialloc.o inode.o page-io.o \
 		ioctl.o namei.o super.o symlink.o hash.o resize.o extents.o \
 		ext4_jbd2.o migrate.o mballoc.o block_validity.o move_extent.o
 

fs/ext4/ext4.h

@@ -168,7 +168,20 @@ struct mpage_da_data {
 	int pages_written;
 	int retval;
 };
-#define	EXT4_IO_UNWRITTEN	0x1
+
+/*
+ * Flags for ext4_io_end->flags
+ */
+#define	EXT4_IO_END_UNWRITTEN	0x0001
+#define EXT4_IO_END_ERROR	0x0002
+
+struct ext4_io_page {
+	struct page	*p_page;
+	int		p_count;
+};
+
+#define MAX_IO_PAGES 128
+
 typedef struct ext4_io_end {
 	struct list_head	list;		/* per-file finished IO list */
 	struct inode		*inode;		/* file being written to */
@@ -179,8 +192,18 @@ typedef struct ext4_io_end {
 	struct work_struct	work;		/* data work queue */
 	struct kiocb		*iocb;		/* iocb struct for AIO */
 	int			result;		/* error value for AIO */
+	int			num_io_pages;
+	struct ext4_io_page	*pages[MAX_IO_PAGES];
 } ext4_io_end_t;
 
+struct ext4_io_submit {
+	int			io_op;
+	struct bio		*io_bio;
+	ext4_io_end_t		*io_end;
+	struct ext4_io_page	*io_page;
+	sector_t		io_next_block;
+};
+
 /*
  * Special inodes numbers
  */
@@ -2044,6 +2067,17 @@ extern int ext4_move_extents(struct file *o_filp, struct file *d_filp,
 			     __u64 start_orig, __u64 start_donor,
 			     __u64 len, __u64 *moved_len);
 
+/* page-io.c */
+extern int __init init_ext4_pageio(void);
+extern void exit_ext4_pageio(void);
+extern void ext4_free_io_end(ext4_io_end_t *io);
+extern ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags);
+extern int ext4_end_io_nolock(ext4_io_end_t *io);
+extern void ext4_io_submit(struct ext4_io_submit *io);
+extern int ext4_bio_write_page(struct ext4_io_submit *io,
+			       struct page *page,
+			       int len,
+			       struct writeback_control *wbc);
 
 /* BH_Uninit flag: blocks are allocated but uninitialized on disk */
 enum ext4_state_bits {

fs/ext4/extents.c

@@ -3202,7 +3202,7 @@ ext4_ext_handle_uninitialized_extents(handle_t *handle, struct inode *inode,
 		 * completed
 		 */
 		if (io)
-			io->flag = EXT4_IO_UNWRITTEN;
+			io->flag = EXT4_IO_END_UNWRITTEN;
 		else
 			ext4_set_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN);
 		if (ext4_should_dioread_nolock(inode))
@@ -3494,7 +3494,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		 */
 		if ((flags & EXT4_GET_BLOCKS_PRE_IO)) {
 			if (io)
-				io->flag = EXT4_IO_UNWRITTEN;
+				io->flag = EXT4_IO_END_UNWRITTEN;
 			else
 				ext4_set_inode_state(inode,
 						     EXT4_STATE_DIO_UNWRITTEN);

fs/ext4/inode.c

@@ -2016,8 +2016,10 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
 	struct buffer_head *bh, *page_bufs = NULL;
 	int journal_data = ext4_should_journal_data(inode);
 	sector_t pblock = 0, cur_logical = 0;
+	struct ext4_io_submit io_submit;
 
 	BUG_ON(mpd->next_page <= mpd->first_page);
+	memset(&io_submit, 0, sizeof(io_submit));
 	/*
 	 * We need to start from the first_page to the next_page - 1
 	 * to make sure we also write the mapped dirty buffer_heads.
@@ -2109,16 +2111,16 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
 				/* mark the buffer_heads as dirty & uptodate */
 				block_commit_write(page, 0, len);
 
-			if (journal_data && PageChecked(page))
+			/*
+			 * Delalloc doesn't support data journalling,
+			 * but eventually maybe we'll lift this
+			 * restriction.
+			 */
+			if (unlikely(journal_data && PageChecked(page)))
 				err = __ext4_journalled_writepage(page, len);
-			else if (buffer_uninit(page_bufs)) {
+			else
-				ext4_set_bh_endio(page_bufs, inode);
+				err = ext4_bio_write_page(&io_submit, page,
-				err = block_write_full_page_endio(page,
+							  len, mpd->wbc);
-					noalloc_get_block_write,
-					mpd->wbc, ext4_end_io_buffer_write);
-			} else
-				err = block_write_full_page(page,
-					    noalloc_get_block_write, mpd->wbc);
 
 			if (!err)
 				mpd->pages_written++;
@@ -2131,6 +2133,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
 		}
 		pagevec_release(&pvec);
 	}
+	ext4_io_submit(&io_submit);
 	return ret;
 }
 
@@ -3426,15 +3429,6 @@ ext4_readpages(struct file *file, struct address_space *mapping,
 	return mpage_readpages(mapping, pages, nr_pages, ext4_get_block);
 }
 
-static void ext4_free_io_end(ext4_io_end_t *io)
-{
-	BUG_ON(!io);
-	if (io->page)
-		put_page(io->page);
-	iput(io->inode);
-	kfree(io);
-}
-
 static void ext4_invalidatepage_free_endio(struct page *page, unsigned long offset)
 {
 	struct buffer_head *head, *bh;
@@ -3639,68 +3633,6 @@ static void dump_completed_IO(struct inode * inode)
 #endif
 }
 
-/*
- * check a range of space and convert unwritten extents to written.
- */
-static int ext4_end_io_nolock(ext4_io_end_t *io)
-{
-	struct inode *inode = io->inode;
-	loff_t offset = io->offset;
-	ssize_t size = io->size;
-	int ret = 0;
-
-	ext4_debug("ext4_end_io_nolock: io 0x%p from inode %lu,list->next 0x%p,"
-		   "list->prev 0x%p\n",
-	           io, inode->i_ino, io->list.next, io->list.prev);
-
-	if (list_empty(&io->list))
-		return ret;
-
-	if (io->flag != EXT4_IO_UNWRITTEN)
-		return ret;
-
-	ret = ext4_convert_unwritten_extents(inode, offset, size);
-	if (ret < 0) {
-		printk(KERN_EMERG "%s: failed to convert unwritten"
-			"extents to written extents, error is %d"
-			" io is still on inode %lu aio dio list\n",
-                       __func__, ret, inode->i_ino);
-		return ret;
-	}
-
-	if (io->iocb)
-		aio_complete(io->iocb, io->result, 0);
-	/* clear the DIO AIO unwritten flag */
-	io->flag = 0;
-	return ret;
-}
-
-/*
- * work on completed aio dio IO, to convert unwritten extents to extents
- */
-static void ext4_end_io_work(struct work_struct *work)
-{
-	ext4_io_end_t		*io = container_of(work, ext4_io_end_t, work);
-	struct inode		*inode = io->inode;
-	struct ext4_inode_info	*ei = EXT4_I(inode);
-	unsigned long		flags;
-	int			ret;
-
-	mutex_lock(&inode->i_mutex);
-	ret = ext4_end_io_nolock(io);
-	if (ret < 0) {
-		mutex_unlock(&inode->i_mutex);
-		return;
-	}
-
-	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
-	if (!list_empty(&io->list))
-		list_del_init(&io->list);
-	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
-	mutex_unlock(&inode->i_mutex);
-	ext4_free_io_end(io);
-}
-
 /*
  * This function is called from ext4_sync_file().
  *
@@ -3756,28 +3688,6 @@ int flush_completed_IO(struct inode *inode)
 	return (ret2 < 0) ? ret2 : 0;
 }
 
-static ext4_io_end_t *ext4_init_io_end (struct inode *inode, gfp_t flags)
-{
-	ext4_io_end_t *io = NULL;
-
-	io = kmalloc(sizeof(*io), flags);
-
-	if (io) {
-		igrab(inode);
-		io->inode = inode;
-		io->flag = 0;
-		io->offset = 0;
-		io->size = 0;
-		io->page = NULL;
-		io->iocb = NULL;
-		io->result = 0;
-		INIT_WORK(&io->work, ext4_end_io_work);
-		INIT_LIST_HEAD(&io->list);
-	}
-
-	return io;
-}
-
 static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
 			    ssize_t size, void *private, int ret,
 			    bool is_async)
@@ -3797,7 +3707,7 @@ static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
 		  size);
 
 	/* if not aio dio with unwritten extents, just free io and return */
-	if (io_end->flag != EXT4_IO_UNWRITTEN){
+	if (!(io_end->flag & EXT4_IO_END_UNWRITTEN)) {
 		ext4_free_io_end(io_end);
 		iocb->private = NULL;
 out:
@@ -3842,7 +3752,7 @@ static void ext4_end_io_buffer_write(struct buffer_head *bh, int uptodate)
 		goto out;
 	}
 
-	io_end->flag = EXT4_IO_UNWRITTEN;
+	io_end->flag = EXT4_IO_END_UNWRITTEN;
 	inode = io_end->inode;
 
 	/* Add the io_end to per-inode completed io list*/

fs/ext4/page-io.c

@@ -0,0 +1,430 @@
+/*
+ * linux/fs/ext4/page-io.c
+ *
+ * This contains the new page_io functions for ext4
+ *
+ * Written by Theodore Ts'o, 2010.
+ */
+
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/time.h>
+#include <linux/jbd2.h>
+#include <linux/highuid.h>
+#include <linux/pagemap.h>
+#include <linux/quotaops.h>
+#include <linux/string.h>
+#include <linux/buffer_head.h>
+#include <linux/writeback.h>
+#include <linux/pagevec.h>
+#include <linux/mpage.h>
+#include <linux/namei.h>
+#include <linux/uio.h>
+#include <linux/bio.h>
+#include <linux/workqueue.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+
+#include "ext4_jbd2.h"
+#include "xattr.h"
+#include "acl.h"
+#include "ext4_extents.h"
+
+static struct kmem_cache *io_page_cachep, *io_end_cachep;
+
+int __init init_ext4_pageio(void)
+{
+	io_page_cachep = KMEM_CACHE(ext4_io_page, SLAB_RECLAIM_ACCOUNT);
+	if (io_page_cachep == NULL)
+		return -ENOMEM;
+	io_end_cachep = KMEM_CACHE(ext4_io_end, SLAB_RECLAIM_ACCOUNT);
+	if (io_page_cachep == NULL) {
+		kmem_cache_destroy(io_page_cachep);
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+void exit_ext4_pageio(void)
+{
+	kmem_cache_destroy(io_end_cachep);
+	kmem_cache_destroy(io_page_cachep);
+}
+
+void ext4_free_io_end(ext4_io_end_t *io)
+{
+	int i;
+
+	BUG_ON(!io);
+	if (io->page)
+		put_page(io->page);
+	for (i = 0; i < io->num_io_pages; i++) {
+		if (--io->pages[i]->p_count == 0) {
+			struct page *page = io->pages[i]->p_page;
+
+			end_page_writeback(page);
+			put_page(page);
+			kmem_cache_free(io_page_cachep, io->pages[i]);
+		}
+	}
+	io->num_io_pages = 0;
+	iput(io->inode);
+	kmem_cache_free(io_end_cachep, io);
+}
+
+/*
+ * check a range of space and convert unwritten extents to written.
+ */
+int ext4_end_io_nolock(ext4_io_end_t *io)
+{
+	struct inode *inode = io->inode;
+	loff_t offset = io->offset;
+	ssize_t size = io->size;
+	int ret = 0;
+
+	ext4_debug("ext4_end_io_nolock: io 0x%p from inode %lu,list->next 0x%p,"
+		   "list->prev 0x%p\n",
+		   io, inode->i_ino, io->list.next, io->list.prev);
+
+	if (list_empty(&io->list))
+		return ret;
+
+	if (!(io->flag & EXT4_IO_END_UNWRITTEN))
+		return ret;
+
+	ret = ext4_convert_unwritten_extents(inode, offset, size);
+	if (ret < 0) {
+		printk(KERN_EMERG "%s: failed to convert unwritten "
+			"extents to written extents, error is %d "
+			"io is still on inode %lu aio dio list\n",
+		       __func__, ret, inode->i_ino);
+		return ret;
+	}
+
+	if (io->iocb)
+		aio_complete(io->iocb, io->result, 0);
+	/* clear the DIO AIO unwritten flag */
+	io->flag &= ~EXT4_IO_END_UNWRITTEN;
+	return ret;
+}
+
+/*
+ * work on completed aio dio IO, to convert unwritten extents to extents
+ */
+static void ext4_end_io_work(struct work_struct *work)
+{
+	ext4_io_end_t		*io = container_of(work, ext4_io_end_t, work);
+	struct inode		*inode = io->inode;
+	struct ext4_inode_info	*ei = EXT4_I(inode);
+	unsigned long		flags;
+	int			ret;
+
+	mutex_lock(&inode->i_mutex);
+	ret = ext4_end_io_nolock(io);
+	if (ret < 0) {
+		mutex_unlock(&inode->i_mutex);
+		return;
+	}
+
+	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
+	if (!list_empty(&io->list))
+		list_del_init(&io->list);
+	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
+	mutex_unlock(&inode->i_mutex);
+	ext4_free_io_end(io);
+}
+
+ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)
+{
+	ext4_io_end_t *io = NULL;
+
+	io = kmem_cache_alloc(io_end_cachep, flags);
+	if (io) {
+		memset(io, 0, sizeof(*io));
+		io->inode = igrab(inode);
+		BUG_ON(!io->inode);
+		INIT_WORK(&io->work, ext4_end_io_work);
+		INIT_LIST_HEAD(&io->list);
+	}
+	return io;
+}
+
+/*
+ * Print an buffer I/O error compatible with the fs/buffer.c.  This
+ * provides compatibility with dmesg scrapers that look for a specific
+ * buffer I/O error message.  We really need a unified error reporting
+ * structure to userspace ala Digital Unix's uerf system, but it's
+ * probably not going to happen in my lifetime, due to LKML politics...
+ */
+static void buffer_io_error(struct buffer_head *bh)
+{
+	char b[BDEVNAME_SIZE];
+	printk(KERN_ERR "Buffer I/O error on device %s, logical block %llu\n",
+			bdevname(bh->b_bdev, b),
+			(unsigned long long)bh->b_blocknr);
+}
+
+static void ext4_end_bio(struct bio *bio, int error)
+{
+	ext4_io_end_t *io_end = bio->bi_private;
+	struct workqueue_struct *wq;
+	struct inode *inode;
+	unsigned long flags;
+	ext4_fsblk_t err_block;
+	int i;
+
+	BUG_ON(!io_end);
+	inode = io_end->inode;
+	bio->bi_private = NULL;
+	bio->bi_end_io = NULL;
+	if (test_bit(BIO_UPTODATE, &bio->bi_flags))
+		error = 0;
+	err_block = bio->bi_sector >> (inode->i_blkbits - 9);
+	bio_put(bio);
+
+	if (!(inode->i_sb->s_flags & MS_ACTIVE)) {
+		pr_err("sb umounted, discard end_io request for inode %lu\n",
+			io_end->inode->i_ino);
+		ext4_free_io_end(io_end);
+		return;
+	}
+
+	if (error) {
+		io_end->flag |= EXT4_IO_END_ERROR;
+		ext4_warning(inode->i_sb, "I/O error writing to inode %lu "
+			     "(offset %llu size %ld starting block %llu)",
+			     inode->i_ino,
+			     (unsigned long long) io_end->offset,
+			     (long) io_end->size,
+			     (unsigned long long) err_block);
+	}
+
+	for (i = 0; i < io_end->num_io_pages; i++) {
+		struct page *page = io_end->pages[i]->p_page;
+		struct buffer_head *bh, *head;
+		int partial_write = 0;
+
+		head = page_buffers(page);
+		if (error)
+			SetPageError(page);
+		BUG_ON(!head);
+		if (head->b_size == PAGE_CACHE_SIZE)
+			clear_buffer_dirty(head);
+		else {
+			loff_t offset;
+			loff_t io_end_offset = io_end->offset + io_end->size;
+
+			offset = (sector_t) page->index << PAGE_CACHE_SHIFT;
+			bh = head;
+			do {
+				if ((offset >= io_end->offset) &&
+				    (offset+bh->b_size <= io_end_offset)) {
+					if (error)
+						buffer_io_error(bh);
+
+					clear_buffer_dirty(bh);
+				}
+				if (buffer_delay(bh))
+					partial_write = 1;
+				else if (!buffer_mapped(bh))
+					clear_buffer_dirty(bh);
+				else if (buffer_dirty(bh))
+					partial_write = 1;
+				offset += bh->b_size;
+				bh = bh->b_this_page;
+			} while (bh != head);
+		}
+
+		if (--io_end->pages[i]->p_count == 0) {
+			struct page *page = io_end->pages[i]->p_page;
+
+			end_page_writeback(page);
+			put_page(page);
+			kmem_cache_free(io_page_cachep, io_end->pages[i]);
+		}
+
+		/*
+		 * If this is a partial write which happened to make
+		 * all buffers uptodate then we can optimize away a
+		 * bogus readpage() for the next read(). Here we
+		 * 'discover' whether the page went uptodate as a
+		 * result of this (potentially partial) write.
+		 */
+		if (!partial_write)
+			SetPageUptodate(page);
+	}
+
+	io_end->num_io_pages = 0;
+
+	/* Add the io_end to per-inode completed io list*/
+	spin_lock_irqsave(&EXT4_I(inode)->i_completed_io_lock, flags);
+	list_add_tail(&io_end->list, &EXT4_I(inode)->i_completed_io_list);
+	spin_unlock_irqrestore(&EXT4_I(inode)->i_completed_io_lock, flags);
+
+	wq = EXT4_SB(inode->i_sb)->dio_unwritten_wq;
+	/* queue the work to convert unwritten extents to written */
+	queue_work(wq, &io_end->work);
+}
+
+void ext4_io_submit(struct ext4_io_submit *io)
+{
+	struct bio *bio = io->io_bio;
+
+	if (bio) {
+		bio_get(io->io_bio);
+		submit_bio(io->io_op, io->io_bio);
+		BUG_ON(bio_flagged(io->io_bio, BIO_EOPNOTSUPP));
+		bio_put(io->io_bio);
+	}
+	io->io_bio = 0;
+	io->io_op = 0;
+	io->io_end = 0;
+}
+
+static int io_submit_init(struct ext4_io_submit *io,
+			  struct inode *inode,
+			  struct writeback_control *wbc,
+			  struct buffer_head *bh)
+{
+	ext4_io_end_t *io_end;
+	struct page *page = bh->b_page;
+	int nvecs = bio_get_nr_vecs(bh->b_bdev);
+	struct bio *bio;
+
+	io_end = ext4_init_io_end(inode, GFP_NOFS);
+	if (!io_end)
+		return -ENOMEM;
+	do {
+		bio = bio_alloc(GFP_NOIO, nvecs);
+		nvecs >>= 1;
+	} while (bio == NULL);
+
+	bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
+	bio->bi_bdev = bh->b_bdev;
+	bio->bi_private = io->io_end = io_end;
+	bio->bi_end_io = ext4_end_bio;
+
+	io_end->inode = inode;
+	io_end->offset = (page->index << PAGE_CACHE_SHIFT) + bh_offset(bh);
+
+	io->io_bio = bio;
+	io->io_op = (wbc->sync_mode == WB_SYNC_ALL ?
+			WRITE_SYNC_PLUG : WRITE);
+	io->io_next_block = bh->b_blocknr;
+	return 0;
+}
+
+static int io_submit_add_bh(struct ext4_io_submit *io,
+			    struct ext4_io_page *io_page,
+			    struct inode *inode,
+			    struct writeback_control *wbc,
+			    struct buffer_head *bh)
+{
+	ext4_io_end_t *io_end;
+	int ret;
+
+	if (buffer_new(bh)) {
+		clear_buffer_new(bh);
+		unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
+	}
+
+	if (!buffer_mapped(bh) || buffer_delay(bh)) {
+		if (!buffer_mapped(bh))
+			clear_buffer_dirty(bh);
+		if (io->io_bio)
+			ext4_io_submit(io);
+		return 0;
+	}
+
+	if (io->io_bio && bh->b_blocknr != io->io_next_block) {
+submit_and_retry:
+		ext4_io_submit(io);
+	}
+	if (io->io_bio == NULL) {
+		ret = io_submit_init(io, inode, wbc, bh);
+		if (ret)
+			return ret;
+	}
+	io_end = io->io_end;
+	if ((io_end->num_io_pages >= MAX_IO_PAGES) &&
+	    (io_end->pages[io_end->num_io_pages-1] != io_page))
+		goto submit_and_retry;
+	if (buffer_uninit(bh))
+		io->io_end->flag |= EXT4_IO_END_UNWRITTEN;
+	io->io_end->size += bh->b_size;
+	io->io_next_block++;
+	ret = bio_add_page(io->io_bio, bh->b_page, bh->b_size, bh_offset(bh));
+	if (ret != bh->b_size)
+		goto submit_and_retry;
+	if ((io_end->num_io_pages == 0) ||
+	    (io_end->pages[io_end->num_io_pages-1] != io_page)) {
+		io_end->pages[io_end->num_io_pages++] = io_page;
+		io_page->p_count++;
+	}
+	return 0;
+}
+
+int ext4_bio_write_page(struct ext4_io_submit *io,
+			struct page *page,
+			int len,
+			struct writeback_control *wbc)
+{
+	struct inode *inode = page->mapping->host;
+	unsigned block_start, block_end, blocksize;
+	struct ext4_io_page *io_page;
+	struct buffer_head *bh, *head;
+	int ret = 0;
+
+	blocksize = 1 << inode->i_blkbits;
+
+	BUG_ON(PageWriteback(page));
+	set_page_writeback(page);
+	ClearPageError(page);
+
+	io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
+	if (!io_page) {
+		set_page_dirty(page);
+		unlock_page(page);
+		return -ENOMEM;
+	}
+	io_page->p_page = page;
+	io_page->p_count = 0;
+	get_page(page);
+
+	for (bh = head = page_buffers(page), block_start = 0;
+	     bh != head || !block_start;
+	     block_start = block_end, bh = bh->b_this_page) {
+		block_end = block_start + blocksize;
+		if (block_start >= len) {
+			clear_buffer_dirty(bh);
+			set_buffer_uptodate(bh);
+			continue;
+		}
+		ret = io_submit_add_bh(io, io_page, inode, wbc, bh);
+		if (ret) {
+			/*
+			 * We only get here on ENOMEM.  Not much else
+			 * we can do but mark the page as dirty, and
+			 * better luck next time.
+			 */
+			set_page_dirty(page);
+			break;
+		}
+	}
+	unlock_page(page);
+	/*
+	 * If the page was truncated before we could do the writeback,
+	 * or we had a memory allocation error while trying to write
+	 * the first buffer head, we won't have submitted any pages for
+	 * I/O.  In that case we need to make sure we've cleared the
+	 * PageWriteback bit from the page to prevent the system from
+	 * wedging later on.
+	 */
+	if (io_page->p_count == 0) {
+		put_page(page);
+		end_page_writeback(page);
+		kmem_cache_free(io_page_cachep, io_page);
+	}
+	return ret;
+}

fs/ext4/super.c

@@ -4769,9 +4769,12 @@ static int __init init_ext4_fs(void)
 	int err;
 
 	ext4_check_flag_values();
-	err = init_ext4_system_zone();
+	err = init_ext4_pageio();
 	if (err)
 		return err;
+	err = init_ext4_system_zone();
+	if (err)
+		goto out5;
 	ext4_kset = kset_create_and_add("ext4", NULL, fs_kobj);
 	if (!ext4_kset)
 		goto out4;
@@ -4812,6 +4815,8 @@ out3:
 	kset_unregister(ext4_kset);
 out4:
 	exit_ext4_system_zone();
+out5:
+	exit_ext4_pageio();
 	return err;
 }
 
@@ -4827,6 +4832,7 @@ static void __exit exit_ext4_fs(void)
 	remove_proc_entry("fs/ext4", NULL);
 	kset_unregister(ext4_kset);
 	exit_ext4_system_zone();
+	exit_ext4_pageio();
 }
 
 MODULE_AUTHOR("Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others");