xfs: optimize bio handling in the buffer writeback path

This patch implements two closely related changes:  First it embeds
a bio the ioend structure so that we don't have to allocate one
separately.  Second it uses the block layer bio chaining mechanism
to chain additional bios off this first one if needed instead of
manually accounting for multiple bio completions in the ioend
structure.  Together this removes a memory allocation per ioend and
greatly simplifies the ioend setup and I/O completion path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_aops.c

@@ -124,18 +124,25 @@ next_bh:
  */
 STATIC void
 xfs_destroy_ioend(
-	struct xfs_ioend	*ioend)
+	struct xfs_ioend	*ioend,
+	int			error)
 {
 	struct inode		*inode = ioend->io_inode;
-	int			error = ioend->io_error;
+	struct bio		*last = ioend->io_bio;
 	struct bio		*bio, *next;
 
-	for (bio = ioend->io_bio_done; bio; bio = next) {
+	for (bio = &ioend->io_inline_bio; bio; bio = next) {
 		struct bio_vec	*bvec;
 		int		i;
 
-		next = bio->bi_private;
-		bio->bi_private = NULL;
+		/*
+		 * For the last bio, bi_private points to the ioend, so we
+		 * need to explicitly end the iteration here.
+		 */
+		if (bio == last)
+			next = NULL;
+		else
+			next = bio->bi_private;
 
 		/* walk each page on bio, ending page IO on them */
 		bio_for_each_segment_all(bvec, bio, i)
@@ -143,8 +150,6 @@ xfs_destroy_ioend(
 
 		bio_put(bio);
 	}
-
-	mempool_free(ioend, xfs_ioend_pool);
 }
 
 /*
@@ -218,7 +223,8 @@ xfs_setfilesize(
 
 STATIC int
 xfs_setfilesize_ioend(
-	struct xfs_ioend	*ioend)
+	struct xfs_ioend	*ioend,
+	int			error)
 {
 	struct xfs_inode	*ip = XFS_I(ioend->io_inode);
 	struct xfs_trans	*tp = ioend->io_append_trans;
@@ -232,36 +238,14 @@ xfs_setfilesize_ioend(
 	__sb_writers_acquired(VFS_I(ip)->i_sb, SB_FREEZE_FS);
 
 	/* we abort the update if there was an IO error */
-	if (ioend->io_error) {
+	if (error) {
 		xfs_trans_cancel(tp);
-		return ioend->io_error;
+		return error;
 	}
 
 	return xfs_setfilesize(ip, tp, ioend->io_offset, ioend->io_size);
 }
 
-/*
- * Schedule IO completion handling on the final put of an ioend.
- *
- * If there is no work to do we might as well call it a day and free the
- * ioend right now.
- */
-STATIC void
-xfs_finish_ioend(
-	struct xfs_ioend	*ioend)
-{
-	if (atomic_dec_and_test(&ioend->io_remaining)) {
-		struct xfs_mount	*mp = XFS_I(ioend->io_inode)->i_mount;
-
-		if (ioend->io_type == XFS_IO_UNWRITTEN)
-			queue_work(mp->m_unwritten_workqueue, &ioend->io_work);
-		else if (ioend->io_append_trans)
-			queue_work(mp->m_data_workqueue, &ioend->io_work);
-		else
-			xfs_destroy_ioend(ioend);
-	}
-}
-
 /*
  * IO write completion.
  */
@@ -269,16 +253,17 @@ STATIC void
 xfs_end_io(
 	struct work_struct *work)
 {
-	xfs_ioend_t	*ioend = container_of(work, xfs_ioend_t, io_work);
-	struct xfs_inode *ip = XFS_I(ioend->io_inode);
-	int		error = 0;
+	struct xfs_ioend	*ioend =
+		container_of(work, struct xfs_ioend, io_work);
+	struct xfs_inode	*ip = XFS_I(ioend->io_inode);
+	int			error = ioend->io_bio->bi_error;
 
 	/*
 	 * Set an error if the mount has shut down and proceed with end I/O
 	 * processing so it can perform whatever cleanups are necessary.
 	 */
 	if (XFS_FORCED_SHUTDOWN(ip->i_mount))
-		ioend->io_error = -EIO;
+		error = -EIO;
 
 	/*
 	 * For unwritten extents we need to issue transactions to convert a
@@ -288,50 +273,33 @@ xfs_end_io(
 	 * on error.
 	 */
 	if (ioend->io_type == XFS_IO_UNWRITTEN) {
-		if (ioend->io_error)
+		if (error)
 			goto done;
 		error = xfs_iomap_write_unwritten(ip, ioend->io_offset,
 						  ioend->io_size);
 	} else if (ioend->io_append_trans) {
-		error = xfs_setfilesize_ioend(ioend);
+		error = xfs_setfilesize_ioend(ioend, error);
 	} else {
 		ASSERT(!xfs_ioend_is_append(ioend));
 	}
 
 done:
-	if (error)
-		ioend->io_error = error;
-	xfs_destroy_ioend(ioend);
+	xfs_destroy_ioend(ioend, error);
 }
 
-/*
- * Allocate and initialise an IO completion structure.
- * We need to track unwritten extent write completion here initially.
- * We'll need to extend this for updating the ondisk inode size later
- * (vs. incore size).
- */
-STATIC xfs_ioend_t *
-xfs_alloc_ioend(
-	struct inode		*inode,
-	unsigned int		type)
+STATIC void
+xfs_end_bio(
+	struct bio		*bio)
 {
-	xfs_ioend_t		*ioend;
-
-	ioend = mempool_alloc(xfs_ioend_pool, GFP_NOFS);
-	memset(ioend, 0, sizeof(*ioend));
+	struct xfs_ioend	*ioend = bio->bi_private;
+	struct xfs_mount	*mp = XFS_I(ioend->io_inode)->i_mount;
 
-	/*
-	 * Set the count to 1 initially, which will prevent an I/O
-	 * completion callback from happening before we have started
-	 * all the I/O from calling the completion routine too early.
-	 */
-	atomic_set(&ioend->io_remaining, 1);
-	INIT_LIST_HEAD(&ioend->io_list);
-	ioend->io_type = type;
-	ioend->io_inode = inode;
-	INIT_WORK(&ioend->io_work, xfs_end_io);
-	spin_lock_init(&ioend->io_lock);
-	return ioend;
+	if (ioend->io_type == XFS_IO_UNWRITTEN)
+		queue_work(mp->m_unwritten_workqueue, &ioend->io_work);
+	else if (ioend->io_append_trans)
+		queue_work(mp->m_data_workqueue, &ioend->io_work);
+	else
+		xfs_destroy_ioend(ioend, bio->bi_error);
 }
 
 STATIC int
@@ -403,56 +371,6 @@ xfs_imap_valid(
 		offset < imap->br_startoff + imap->br_blockcount;
 }
 
-/*
- * BIO completion handler for buffered IO.
- */
-STATIC void
-xfs_end_bio(
-	struct bio		*bio)
-{
-	struct xfs_ioend	*ioend = bio->bi_private;
-	unsigned long		flags;
-
-	bio->bi_private = NULL;
-	bio->bi_end_io = NULL;
-
-	spin_lock_irqsave(&ioend->io_lock, flags);
-	if (!ioend->io_error)
-	       ioend->io_error = bio->bi_error;
-	if (!ioend->io_bio_done)
-		ioend->io_bio_done = bio;
-	else
-		ioend->io_bio_done_tail->bi_private = bio;
-	ioend->io_bio_done_tail = bio;
-	spin_unlock_irqrestore(&ioend->io_lock, flags);
-
-	xfs_finish_ioend(ioend);
-}
-
-STATIC void
-xfs_submit_ioend_bio(
-	struct writeback_control *wbc,
-	xfs_ioend_t		*ioend,
-	struct bio		*bio)
-{
-	atomic_inc(&ioend->io_remaining);
-	bio->bi_private = ioend;
-	bio->bi_end_io = xfs_end_bio;
-	submit_bio(wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE, bio);
-}
-
-STATIC struct bio *
-xfs_alloc_ioend_bio(
-	struct buffer_head	*bh)
-{
-	struct bio		*bio = bio_alloc(GFP_NOIO, BIO_MAX_PAGES);
-
-	ASSERT(bio->bi_private == NULL);
-	bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
-	bio->bi_bdev = bh->b_bdev;
-	return bio;
-}
-
 STATIC void
 xfs_start_buffer_writeback(
 	struct buffer_head	*bh)
@@ -513,16 +431,19 @@ static inline int xfs_bio_add_buffer(struct bio *bio, struct buffer_head *bh)
 STATIC int
 xfs_submit_ioend(
 	struct writeback_control *wbc,
-	xfs_ioend_t		*ioend,
+	struct xfs_ioend	*ioend,
 	int			status)
 {
 	/* Reserve log space if we might write beyond the on-disk inode size. */
 	if (!status &&
-	    ioend->io_bio && ioend->io_type != XFS_IO_UNWRITTEN &&
+	    ioend->io_type != XFS_IO_UNWRITTEN &&
 	    xfs_ioend_is_append(ioend) &&
 	    !ioend->io_append_trans)
 		status = xfs_setfilesize_trans_alloc(ioend);
 
+	ioend->io_bio->bi_private = ioend;
+	ioend->io_bio->bi_end_io = xfs_end_bio;
+
 	/*
 	 * If we are failing the IO now, just mark the ioend with an
 	 * error and finish it. This will run IO completion immediately
@@ -530,19 +451,75 @@ xfs_submit_ioend(
 	 * time.
 	 */
 	if (status) {
-		if (ioend->io_bio)
-			bio_put(ioend->io_bio);
-		ioend->io_error = status;
-		xfs_finish_ioend(ioend);
+		ioend->io_bio->bi_error = status;
+		bio_endio(ioend->io_bio);
 		return status;
 	}
 
-	xfs_submit_ioend_bio(wbc, ioend, ioend->io_bio);
-	ioend->io_bio = NULL;
-	xfs_finish_ioend(ioend);
+	submit_bio(wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE,
+		   ioend->io_bio);
 	return 0;
 }
 
+static void
+xfs_init_bio_from_bh(
+	struct bio		*bio,
+	struct buffer_head	*bh)
+{
+	bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
+	bio->bi_bdev = bh->b_bdev;
+}
+
+static struct xfs_ioend *
+xfs_alloc_ioend(
+	struct inode		*inode,
+	unsigned int		type,
+	xfs_off_t		offset,
+	struct buffer_head	*bh)
+{
+	struct xfs_ioend	*ioend;
+	struct bio		*bio;
+
+	bio = bio_alloc_bioset(GFP_NOFS, BIO_MAX_PAGES, xfs_ioend_bioset);
+	xfs_init_bio_from_bh(bio, bh);
+
+	ioend = container_of(bio, struct xfs_ioend, io_inline_bio);
+	INIT_LIST_HEAD(&ioend->io_list);
+	ioend->io_type = type;
+	ioend->io_inode = inode;
+	ioend->io_size = 0;
+	ioend->io_offset = offset;
+	INIT_WORK(&ioend->io_work, xfs_end_io);
+	ioend->io_append_trans = NULL;
+	ioend->io_bio = bio;
+	return ioend;
+}
+
+/*
+ * Allocate a new bio, and chain the old bio to the new one.
+ *
+ * Note that we have to do perform the chaining in this unintuitive order
+ * so that the bi_private linkage is set up in the right direction for the
+ * traversal in xfs_destroy_ioend().
+ */
+static void
+xfs_chain_bio(
+	struct xfs_ioend	*ioend,
+	struct writeback_control *wbc,
+	struct buffer_head	*bh)
+{
+	struct bio *new;
+
+	new = bio_alloc(GFP_NOFS, BIO_MAX_PAGES);
+	xfs_init_bio_from_bh(new, bh);
+
+	bio_chain(ioend->io_bio, new);
+	bio_get(ioend->io_bio);		/* for xfs_destroy_ioend */
+	submit_bio(wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE,
+		   ioend->io_bio);
+	ioend->io_bio = new;
+}
+
 /*
  * Test to see if we've been building up a completion structure for
  * earlier buffers -- if so, we try to append to this ioend if we
@@ -564,19 +541,15 @@ xfs_add_to_ioend(
 	    offset != wpc->ioend->io_offset + wpc->ioend->io_size) {
 		if (wpc->ioend)
 			list_add(&wpc->ioend->io_list, iolist);
-		wpc->ioend = xfs_alloc_ioend(inode, wpc->io_type);
-		wpc->ioend->io_offset = offset;
+		wpc->ioend = xfs_alloc_ioend(inode, wpc->io_type, offset, bh);
 	}
 
-retry:
-	if (!wpc->ioend->io_bio)
-		wpc->ioend->io_bio = xfs_alloc_ioend_bio(bh);
-
-	if (xfs_bio_add_buffer(wpc->ioend->io_bio, bh) != bh->b_size) {
-		xfs_submit_ioend_bio(wbc, wpc->ioend, wpc->ioend->io_bio);
-		wpc->ioend->io_bio = NULL;
-		goto retry;
-	}
+	/*
+	 * If the buffer doesn't fit into the bio we need to allocate a new
+	 * one.  This shouldn't happen more than once for a given buffer.
+	 */
+	while (xfs_bio_add_buffer(wpc->ioend->io_bio, bh) != bh->b_size)
+		xfs_chain_bio(wpc->ioend, wbc, bh);
 
 	wpc->ioend->io_size += bh->b_size;
 	wpc->last_block = bh->b_blocknr;

fs/xfs/xfs_aops.h

@@ -18,7 +18,7 @@
 #ifndef __XFS_AOPS_H__
 #define __XFS_AOPS_H__
 
-extern mempool_t *xfs_ioend_pool;
+extern struct bio_set *xfs_ioend_bioset;
 
 /*
  * Types of I/O for bmap clustering and I/O completion tracking.
@@ -37,24 +37,19 @@ enum {
 	{ XFS_IO_OVERWRITE,		"overwrite" }
 
 /*
- * xfs_ioend struct manages large extent writes for XFS.
- * It can manage several multi-page bio's at once.
+ * Structure for buffered I/O completions.
  */
-typedef struct xfs_ioend {
+struct xfs_ioend {
 	struct list_head	io_list;	/* next ioend in chain */
 	unsigned int		io_type;	/* delalloc / unwritten */
-	int			io_error;	/* I/O error code */
-	atomic_t		io_remaining;	/* hold count */
 	struct inode		*io_inode;	/* file being written to */
 	size_t			io_size;	/* size of the extent */
 	xfs_off_t		io_offset;	/* offset in the file */
 	struct work_struct	io_work;	/* xfsdatad work queue */
 	struct xfs_trans	*io_append_trans;/* xact. for size update */
 	struct bio		*io_bio;	/* bio being built */
-	struct bio		*io_bio_done;	/* bios completed */
-	struct bio		*io_bio_done_tail; /* bios completed */
-	spinlock_t		io_lock;	/* for bio completion list */
-} xfs_ioend_t;
+	struct bio		io_inline_bio;	/* MUST BE LAST! */
+};
 
 extern const struct address_space_operations xfs_address_space_operations;
 

fs/xfs/xfs_super.c

@@ -58,8 +58,7 @@
 #include <linux/parser.h>
 
 static const struct super_operations xfs_super_operations;
-static kmem_zone_t *xfs_ioend_zone;
-mempool_t *xfs_ioend_pool;
+struct bio_set *xfs_ioend_bioset;
 
 static struct kset *xfs_kset;		/* top-level xfs sysfs dir */
 #ifdef DEBUG
@@ -1688,20 +1687,15 @@ MODULE_ALIAS_FS("xfs");
 STATIC int __init
 xfs_init_zones(void)
 {
-
-	xfs_ioend_zone = kmem_zone_init(sizeof(xfs_ioend_t), "xfs_ioend");
-	if (!xfs_ioend_zone)
+	xfs_ioend_bioset = bioset_create(4 * MAX_BUF_PER_PAGE,
+			offsetof(struct xfs_ioend, io_inline_bio));
+	if (!xfs_ioend_bioset)
 		goto out;
 
-	xfs_ioend_pool = mempool_create_slab_pool(4 * MAX_BUF_PER_PAGE,
-						  xfs_ioend_zone);
-	if (!xfs_ioend_pool)
-		goto out_destroy_ioend_zone;
-
 	xfs_log_ticket_zone = kmem_zone_init(sizeof(xlog_ticket_t),
 						"xfs_log_ticket");
 	if (!xfs_log_ticket_zone)
-		goto out_destroy_ioend_pool;
+		goto out_free_ioend_bioset;
 
 	xfs_bmap_free_item_zone = kmem_zone_init(sizeof(xfs_bmap_free_item_t),
 						"xfs_bmap_free_item");
@@ -1797,10 +1791,8 @@ xfs_init_zones(void)
 	kmem_zone_destroy(xfs_bmap_free_item_zone);
  out_destroy_log_ticket_zone:
 	kmem_zone_destroy(xfs_log_ticket_zone);
- out_destroy_ioend_pool:
-	mempool_destroy(xfs_ioend_pool);
- out_destroy_ioend_zone:
-	kmem_zone_destroy(xfs_ioend_zone);
+ out_free_ioend_bioset:
+	bioset_free(xfs_ioend_bioset);
  out:
 	return -ENOMEM;
 }
@@ -1826,9 +1818,7 @@ xfs_destroy_zones(void)
 	kmem_zone_destroy(xfs_btree_cur_zone);
 	kmem_zone_destroy(xfs_bmap_free_item_zone);
 	kmem_zone_destroy(xfs_log_ticket_zone);
-	mempool_destroy(xfs_ioend_pool);
-	kmem_zone_destroy(xfs_ioend_zone);
-
+	bioset_free(xfs_ioend_bioset);
 }
 
 STATIC int __init