writeback: switch to per-bdi threads for flushing data

This gets rid of pdflush for bdi writeout and kupdated style cleaning.
pdflush writeout suffers from lack of locality and also requires more
threads to handle the same workload, since it has to work in a
non-blocking fashion against each queue. This also introduces lumpy
behaviour and potential request starvation, since pdflush can be starved
for queue access if others are accessing it. A sample ffsb workload that
does random writes to files is about 8% faster here on a simple SATA drive
during the benchmark phase. File layout also seems a LOT more smooth in
vmstat:

 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 0  1      0 608848   2652 375372    0    0     0 71024  604    24  1 10 48 42
 0  1      0 549644   2712 433736    0    0     0 60692  505    27  1  8 48 44
 1  0      0 476928   2784 505192    0    0     4 29540  553    24  0  9 53 37
 0  1      0 457972   2808 524008    0    0     0 54876  331    16  0  4 38 58
 0  1      0 366128   2928 614284    0    0     4 92168  710    58  0 13 53 34
 0  1      0 295092   3000 684140    0    0     0 62924  572    23  0  9 53 37
 0  1      0 236592   3064 741704    0    0     4 58256  523    17  0  8 48 44
 0  1      0 165608   3132 811464    0    0     0 57460  560    21  0  8 54 38
 0  1      0 102952   3200 873164    0    0     4 74748  540    29  1 10 48 41
 0  1      0  48604   3252 926472    0    0     0 53248  469    29  0  7 47 45

where vanilla tends to fluctuate a lot in the creation phase:

 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 1  1      0 678716   5792 303380    0    0     0 74064  565    50  1 11 52 36
 1  0      0 662488   5864 319396    0    0     4   352  302   329  0  2 47 51
 0  1      0 599312   5924 381468    0    0     0 78164  516    55  0  9 51 40
 0  1      0 519952   6008 459516    0    0     4 78156  622    56  1 11 52 37
 1  1      0 436640   6092 541632    0    0     0 82244  622    54  0 11 48 41
 0  1      0 436640   6092 541660    0    0     0     8  152    39  0  0 51 49
 0  1      0 332224   6200 644252    0    0     4 102800  728    46  1 13 49 36
 1  0      0 274492   6260 701056    0    0     4 12328  459    49  0  7 50 43
 0  1      0 211220   6324 763356    0    0     0 106940  515    37  1 10 51 39
 1  0      0 160412   6376 813468    0    0     0  8224  415    43  0  6 49 45
 1  1      0  85980   6452 886556    0    0     4 113516  575    39  1 11 54 34
 0  2      0  85968   6452 886620    0    0     0  1640  158   211  0  0 46 54

A 10 disk test with btrfs performs 26% faster with per-bdi flushing. A
SSD based writeback test on XFS performs over 20% better as well, with
the throughput being very stable around 1GB/sec, where pdflush only
manages 750MB/sec and fluctuates wildly while doing so. Random buffered
writes to many files behave a lot better as well, as does random mmap'ed
writes.

A separate thread is added to sync the super blocks. In the long term,
adding sync_supers_bdi() functionality could get rid of this thread again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 710 insertions, 289 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 45ad4bb700e6..7f6dae8aa47f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -19,6 +19,8 @@
 #include <linux/sched.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
+#include <linux/kthread.h>
+#include <linux/freezer.h>
 #include <linux/writeback.h>
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
@@ -27,165 +29,208 @@
 
 #define inode_to_bdi(inode)	((inode)->i_mapping->backing_dev_info)
 
-/**
- * writeback_acquire - attempt to get exclusive writeback access to a device
- * @bdi: the device's backing_dev_info structure
- *
- * It is a waste of resources to have more than one pdflush thread blocked on
- * a single request queue.  Exclusion at the request_queue level is obtained
- * via a flag in the request_queue's backing_dev_info.state.
- *
- * Non-request_queue-backed address_spaces will share default_backing_dev_info,
- * unless they implement their own.  Which is somewhat inefficient, as this
- * may prevent concurrent writeback against multiple devices.
+/*
+ * Work items for the bdi_writeback threads
  */
-static int writeback_acquire(struct backing_dev_info *bdi)
+struct bdi_work {
+	struct list_head list;
+	struct list_head wait_list;
+	struct rcu_head rcu_head;
+
+	unsigned long seen;
+	atomic_t pending;
+
+	struct super_block *sb;
+	unsigned long nr_pages;
+	enum writeback_sync_modes sync_mode;
+
+	unsigned long state;
+};
+
+enum {
+	WS_USED_B = 0,
+	WS_ONSTACK_B,
+};
+
+#define WS_USED (1 << WS_USED_B)
+#define WS_ONSTACK (1 << WS_ONSTACK_B)
+
+static inline bool bdi_work_on_stack(struct bdi_work *work)
+{
+	return test_bit(WS_ONSTACK_B, &work->state);
+}
+
+static inline void bdi_work_init(struct bdi_work *work,
+				 struct writeback_control *wbc)
+{
+	INIT_RCU_HEAD(&work->rcu_head);
+	work->sb = wbc->sb;
+	work->nr_pages = wbc->nr_to_write;
+	work->sync_mode = wbc->sync_mode;
+	work->state = WS_USED;
+}
+
+static inline void bdi_work_init_on_stack(struct bdi_work *work,
+					  struct writeback_control *wbc)
 {
-	return !test_and_set_bit(BDI_pdflush, &bdi->state);
+	bdi_work_init(work, wbc);
+	work->state |= WS_ONSTACK;
 }
 
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.
  *
- * Determine whether there is writeback in progress against a backing device.
+ * Determine whether there is writeback waiting to be handled against a
+ * backing device.
  */
 int writeback_in_progress(struct backing_dev_info *bdi)
 {
-	return test_bit(BDI_pdflush, &bdi->state);
+	return !list_empty(&bdi->work_list);
 }
 
-/**
- * writeback_release - relinquish exclusive writeback access against a device.
- * @bdi: the device's backing_dev_info structure
- */
-static void writeback_release(struct backing_dev_info *bdi)
+static void bdi_work_clear(struct bdi_work *work)
 {
-	BUG_ON(!writeback_in_progress(bdi));
-	clear_bit(BDI_pdflush, &bdi->state);
+	clear_bit(WS_USED_B, &work->state);
+	smp_mb__after_clear_bit();
+	wake_up_bit(&work->state, WS_USED_B);
 }
 
-static noinline void block_dump___mark_inode_dirty(struct inode *inode)
+static void bdi_work_free(struct rcu_head *head)
 {
-	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
-		struct dentry *dentry;
-		const char *name = "?";
+	struct bdi_work *work = container_of(head, struct bdi_work, rcu_head);
 
-		dentry = d_find_alias(inode);
-		if (dentry) {
-			spin_lock(&dentry->d_lock);
-			name = (const char *) dentry->d_name.name;
-		}
-		printk(KERN_DEBUG
-		       "%s(%d): dirtied inode %lu (%s) on %s\n",
-		       current->comm, task_pid_nr(current), inode->i_ino,
-		       name, inode->i_sb->s_id);
-		if (dentry) {
-			spin_unlock(&dentry->d_lock);
-			dput(dentry);
-		}
-	}
+	if (!bdi_work_on_stack(work))
+		kfree(work);
+	else
+		bdi_work_clear(work);
 }
 
-/**
- *	__mark_inode_dirty -	internal function
- *	@inode: inode to mark
- *	@flags: what kind of dirty (i.e. I_DIRTY_SYNC)
- *	Mark an inode as dirty. Callers should use mark_inode_dirty or
- *  	mark_inode_dirty_sync.
- *
- * Put the inode on the super block's dirty list.
- *
- * CAREFUL! We mark it dirty unconditionally, but move it onto the
- * dirty list only if it is hashed or if it refers to a blockdev.
- * If it was not hashed, it will never be added to the dirty list
- * even if it is later hashed, as it will have been marked dirty already.
- *
- * In short, make sure you hash any inodes _before_ you start marking
- * them dirty.
- *
- * This function *must* be atomic for the I_DIRTY_PAGES case -
- * set_page_dirty() is called under spinlock in several places.
- *
- * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
- * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
- * the kernel-internal blockdev inode represents the dirtying time of the
- * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
- * page->mapping->host, so the page-dirtying time is recorded in the internal
- * blockdev inode.
- */
-void __mark_inode_dirty(struct inode *inode, int flags)
+static void wb_work_complete(struct bdi_work *work)
 {
-	struct super_block *sb = inode->i_sb;
+	const enum writeback_sync_modes sync_mode = work->sync_mode;
 
 	/*
-	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
-	 * dirty the inode itself
+	 * For allocated work, we can clear the done/seen bit right here.
+	 * For on-stack work, we need to postpone both the clear and free
+	 * to after the RCU grace period, since the stack could be invalidated
+	 * as soon as bdi_work_clear() has done the wakeup.
 	 */
-	if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
-		if (sb->s_op->dirty_inode)
-			sb->s_op->dirty_inode(inode);
-	}
+	if (!bdi_work_on_stack(work))
+		bdi_work_clear(work);
+	if (sync_mode == WB_SYNC_NONE || bdi_work_on_stack(work))
+		call_rcu(&work->rcu_head, bdi_work_free);
+}
 
+static void wb_clear_pending(struct bdi_writeback *wb, struct bdi_work *work)
+{
 	/*
-	 * make sure that changes are seen by all cpus before we test i_state
-	 * -- mikulas
+	 * The caller has retrieved the work arguments from this work,
+	 * drop our reference. If this is the last ref, delete and free it
 	 */
-	smp_mb();
+	if (atomic_dec_and_test(&work->pending)) {
+		struct backing_dev_info *bdi = wb->bdi;
 
-	/* avoid the locking if we can */
-	if ((inode->i_state & flags) == flags)
-		return;
-
-	if (unlikely(block_dump))
-		block_dump___mark_inode_dirty(inode);
+		spin_lock(&bdi->wb_lock);
+		list_del_rcu(&work->list);
+		spin_unlock(&bdi->wb_lock);
 
-	spin_lock(&inode_lock);
-	if ((inode->i_state & flags) != flags) {
-		const int was_dirty = inode->i_state & I_DIRTY;
+		wb_work_complete(work);
+	}
+}
 
-		inode->i_state |= flags;
+static void bdi_queue_work(struct backing_dev_info *bdi, struct bdi_work *work)
+{
+	if (work) {
+		work->seen = bdi->wb_mask;
+		BUG_ON(!work->seen);
+		atomic_set(&work->pending, bdi->wb_cnt);
+		BUG_ON(!bdi->wb_cnt);
 
 		/*
-		 * If the inode is being synced, just update its dirty state.
-		 * The unlocker will place the inode on the appropriate
-		 * superblock list, based upon its state.
+		 * Make sure stores are seen before it appears on the list
 		 */
-		if (inode->i_state & I_SYNC)
-			goto out;
+		smp_mb();
 
-		/*
-		 * Only add valid (hashed) inodes to the superblock's
-		 * dirty list.  Add blockdev inodes as well.
-		 */
-		if (!S_ISBLK(inode->i_mode)) {
-			if (hlist_unhashed(&inode->i_hash))
-				goto out;
-		}
-		if (inode->i_state & (I_FREEING|I_CLEAR))
-			goto out;
+		spin_lock(&bdi->wb_lock);
+		list_add_tail_rcu(&work->list, &bdi->work_list);
+		spin_unlock(&bdi->wb_lock);
+	}
+
+	/*
+	 * If the default thread isn't there, make sure we add it. When
+	 * it gets created and wakes up, we'll run this work.
+	 */
+	if (unlikely(list_empty_careful(&bdi->wb_list)))
+		wake_up_process(default_backing_dev_info.wb.task);
+	else {
+		struct bdi_writeback *wb = &bdi->wb;
 
 		/*
-		 * If the inode was already on b_dirty/b_io/b_more_io, don't
-		 * reposition it (that would break b_dirty time-ordering).
+		 * If we failed allocating the bdi work item, wake up the wb
+		 * thread always. As a safety precaution, it'll flush out
+		 * everything
 		 */
-		if (!was_dirty) {
-			inode->dirtied_when = jiffies;
-			list_move(&inode->i_list,
-					&inode_to_bdi(inode)->b_dirty);
-		}
+		if (!wb_has_dirty_io(wb)) {
+			if (work)
+				wb_clear_pending(wb, work);
+		} else if (wb->task)
+			wake_up_process(wb->task);
 	}
-out:
-	spin_unlock(&inode_lock);
 }
 
-EXPORT_SYMBOL(__mark_inode_dirty);
+/*
+ * Used for on-stack allocated work items. The caller needs to wait until
+ * the wb threads have acked the work before it's safe to continue.
+ */
+static void bdi_wait_on_work_clear(struct bdi_work *work)
+{
+	wait_on_bit(&work->state, WS_USED_B, bdi_sched_wait,
+		    TASK_UNINTERRUPTIBLE);
+}
 
-static int write_inode(struct inode *inode, int sync)
+static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
 {
-	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
-		return inode->i_sb->s_op->write_inode(inode, sync);
-	return 0;
+	struct bdi_work *work;
+
+	work = kmalloc(sizeof(*work), GFP_ATOMIC);
+	if (work)
+		bdi_work_init(work, wbc);
+
+	return work;
+}
+
+void bdi_start_writeback(struct writeback_control *wbc)
+{
+	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
+	struct bdi_work work_stack, *work = NULL;
+
+	if (!must_wait)
+		work = bdi_alloc_work(wbc);
+
+	if (!work) {
+		work = &work_stack;
+		bdi_work_init_on_stack(work, wbc);
+	}
+
+	bdi_queue_work(wbc->bdi, work);
+
+	/*
+	 * If the sync mode is WB_SYNC_ALL, block waiting for the work to
+	 * complete. If not, we only need to wait for the work to be started,
+	 * if we allocated it on-stack. We use the same mechanism, if the
+	 * wait bit is set in the bdi_work struct, then threads will not
+	 * clear pending until after they are done.
+	 *
+	 * Note that work == &work_stack if must_wait is true, so we don't
+	 * need to do call_rcu() here ever, since the completion path will
+	 * have done that for us.
+	 */
+	if (must_wait || work == &work_stack) {
+		bdi_wait_on_work_clear(work);
+		if (work != &work_stack)
+			call_rcu(&work->rcu_head, bdi_work_free);
+	}
 }
 
 /*
@@ -199,16 +244,16 @@ static int write_inode(struct inode *inode, int sync)
  */
 static void redirty_tail(struct inode *inode)
 {
-	struct backing_dev_info *bdi = inode_to_bdi(inode);
+	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
 
-	if (!list_empty(&bdi->b_dirty)) {
+	if (!list_empty(&wb->b_dirty)) {
 		struct inode *tail;
 
-		tail = list_entry(bdi->b_dirty.next, struct inode, i_list);
+		tail = list_entry(wb->b_dirty.next, struct inode, i_list);
 		if (time_before(inode->dirtied_when, tail->dirtied_when))
 			inode->dirtied_when = jiffies;
 	}
-	list_move(&inode->i_list, &bdi->b_dirty);
+	list_move(&inode->i_list, &wb->b_dirty);
 }
 
 /*
@@ -216,7 +261,9 @@ static void redirty_tail(struct inode *inode)
  */
 static void requeue_io(struct inode *inode)
 {
-	list_move(&inode->i_list, &inode_to_bdi(inode)->b_more_io);
+	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
+
+	list_move(&inode->i_list, &wb->b_more_io);
 }
 
 static void inode_sync_complete(struct inode *inode)
@@ -263,52 +310,18 @@ static void move_expired_inodes(struct list_head *delaying_queue,
 /*
  * Queue all expired dirty inodes for io, eldest first.
  */
-static void queue_io(struct backing_dev_info *bdi,
-		     unsigned long *older_than_this)
+static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
 {
-	list_splice_init(&bdi->b_more_io, bdi->b_io.prev);
-	move_expired_inodes(&bdi->b_dirty, &bdi->b_io, older_than_this);
+	list_splice_init(&wb->b_more_io, wb->b_io.prev);
+	move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
 }
 
-static int sb_on_inode_list(struct super_block *sb, struct list_head *list)
-{
-	struct inode *inode;
-	int ret = 0;
-
-	spin_lock(&inode_lock);
-	list_for_each_entry(inode, list, i_list) {
-		if (inode->i_sb == sb) {
-			ret = 1;
-			break;
-		}
-	}
-	spin_unlock(&inode_lock);
-	return ret;
-}
-
-int sb_has_dirty_inodes(struct super_block *sb)
+static int write_inode(struct inode *inode, int sync)
 {
-	struct backing_dev_info *bdi;
-	int ret = 0;
-
-	/*
-	 * This is REALLY expensive right now, but it'll go away
-	 * when the bdi writeback is introduced
-	 */
-	mutex_lock(&bdi_lock);
-	list_for_each_entry(bdi, &bdi_list, bdi_list) {
-		if (sb_on_inode_list(sb, &bdi->b_dirty) ||
-		    sb_on_inode_list(sb, &bdi->b_io) ||
-		    sb_on_inode_list(sb, &bdi->b_more_io)) {
-			ret = 1;
-			break;
-		}
-	}
-	mutex_unlock(&bdi_lock);
-
-	return ret;
+	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
+		return inode->i_sb->s_op->write_inode(inode, sync);
+	return 0;
 }
-EXPORT_SYMBOL(sb_has_dirty_inodes);
 
 /*
  * Wait for writeback on an inode to complete.
@@ -466,20 +479,71 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 	return ret;
 }
 
-static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
-				    struct writeback_control *wbc,
-				    struct super_block *sb)
+/*
+ * For WB_SYNC_NONE writeback, the caller does not have the sb pinned
+ * before calling writeback. So make sure that we do pin it, so it doesn't
+ * go away while we are writing inodes from it.
+ *
+ * Returns 0 if the super was successfully pinned (or pinning wasn't needed),
+ * 1 if we failed.
+ */
+static int pin_sb_for_writeback(struct writeback_control *wbc,
+				   struct inode *inode)
+{
+	struct super_block *sb = inode->i_sb;
+
+	/*
+	 * Caller must already hold the ref for this
+	 */
+	if (wbc->sync_mode == WB_SYNC_ALL) {
+		WARN_ON(!rwsem_is_locked(&sb->s_umount));
+		return 0;
+	}
+
+	spin_lock(&sb_lock);
+	sb->s_count++;
+	if (down_read_trylock(&sb->s_umount)) {
+		if (sb->s_root) {
+			spin_unlock(&sb_lock);
+			return 0;
+		}
+		/*
+		 * umounted, drop rwsem again and fall through to failure
+		 */
+		up_read(&sb->s_umount);
+	}
+
+	sb->s_count--;
+	spin_unlock(&sb_lock);
+	return 1;
+}
+
+static void unpin_sb_for_writeback(struct writeback_control *wbc,
+				   struct inode *inode)
+{
+	struct super_block *sb = inode->i_sb;
+
+	if (wbc->sync_mode == WB_SYNC_ALL)
+		return;
+
+	up_read(&sb->s_umount);
+	put_super(sb);
+}
+
+static void writeback_inodes_wb(struct bdi_writeback *wb,
+				struct writeback_control *wbc)
 {
+	struct super_block *sb = wbc->sb;
 	const int is_blkdev_sb = sb_is_blkdev_sb(sb);
 	const unsigned long start = jiffies;	/* livelock avoidance */
 
 	spin_lock(&inode_lock);
 
-	if (!wbc->for_kupdate || list_empty(&bdi->b_io))
-		queue_io(bdi, wbc->older_than_this);
+	if (!wbc->for_kupdate || list_empty(&wb->b_io))
+		queue_io(wb, wbc->older_than_this);
 
-	while (!list_empty(&bdi->b_io)) {
-		struct inode *inode = list_entry(bdi->b_io.prev,
+	while (!list_empty(&wb->b_io)) {
+		struct inode *inode = list_entry(wb->b_io.prev,
 						struct inode, i_list);
 		long pages_skipped;
 
@@ -491,7 +555,7 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 			continue;
 		}
 
-		if (!bdi_cap_writeback_dirty(bdi)) {
+		if (!bdi_cap_writeback_dirty(wb->bdi)) {
 			redirty_tail(inode);
 			if (is_blkdev_sb) {
 				/*
@@ -513,7 +577,7 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 			continue;
 		}
 
-		if (wbc->nonblocking && bdi_write_congested(bdi)) {
+		if (wbc->nonblocking && bdi_write_congested(wb->bdi)) {
 			wbc->encountered_congestion = 1;
 			if (!is_blkdev_sb)
 				break;		/* Skip a congested fs */
@@ -521,13 +585,6 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 			continue;		/* Skip a congested blockdev */
 		}
 
-		if (wbc->bdi && bdi != wbc->bdi) {
-			if (!is_blkdev_sb)
-				break;		/* fs has the wrong queue */
-			requeue_io(inode);
-			continue;		/* blockdev has wrong queue */
-		}
-
 		/*
 		 * Was this inode dirtied after sync_sb_inodes was called?
 		 * This keeps sync from extra jobs and livelock.
@@ -535,16 +592,16 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 		if (inode_dirtied_after(inode, start))
 			break;
 
-		/* Is another pdflush already flushing this queue? */
-		if (current_is_pdflush() && !writeback_acquire(bdi))
-			break;
+		if (pin_sb_for_writeback(wbc, inode)) {
+			requeue_io(inode);
+			continue;
+		}
 
 		BUG_ON(inode->i_state & (I_FREEING | I_CLEAR));
 		__iget(inode);
 		pages_skipped = wbc->pages_skipped;
 		writeback_single_inode(inode, wbc);
-		if (current_is_pdflush())
-			writeback_release(bdi);
+		unpin_sb_for_writeback(wbc, inode);
 		if (wbc->pages_skipped != pages_skipped) {
 			/*
 			 * writeback is not making progress due to locked
@@ -560,7 +617,7 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 			wbc->more_io = 1;
 			break;
 		}
-		if (!list_empty(&bdi->b_more_io))
+		if (!list_empty(&wb->b_more_io))
 			wbc->more_io = 1;
 	}
 
@@ -568,139 +625,500 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
 	/* Leave any unwritten inodes on b_io */
 }
 
+void writeback_inodes_wbc(struct writeback_control *wbc)
+{
+	struct backing_dev_info *bdi = wbc->bdi;
+
+	writeback_inodes_wb(&bdi->wb, wbc);
+}
+
 /*
- * Write out a superblock's list of dirty inodes.  A wait will be performed
- * upon no inodes, all inodes or the final one, depending upon sync_mode.
- *
- * If older_than_this is non-NULL, then only write out inodes which
- * had their first dirtying at a time earlier than *older_than_this.
- *
- * If we're a pdlfush thread, then implement pdflush collision avoidance
- * against the entire list.
+ * The maximum number of pages to writeout in a single bdi flush/kupdate
+ * operation.  We do this so we don't hold I_SYNC against an inode for
+ * enormous amounts of time, which would block a userspace task which has
+ * been forced to throttle against that inode.  Also, the code reevaluates
+ * the dirty each time it has written this many pages.
+ */
+#define MAX_WRITEBACK_PAGES     1024
+
+static inline bool over_bground_thresh(void)
+{
+	unsigned long background_thresh, dirty_thresh;
+
+	get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
+
+	return (global_page_state(NR_FILE_DIRTY) +
+		global_page_state(NR_UNSTABLE_NFS) >= background_thresh);
+}
+
+/*
+ * Explicit flushing or periodic writeback of "old" data.
  *
- * If `bdi' is non-zero then we're being asked to writeback a specific queue.
- * This function assumes that the blockdev superblock's inodes are backed by
- * a variety of queues, so all inodes are searched.  For other superblocks,
- * assume that all inodes are backed by the same queue.
+ * Define "old": the first time one of an inode's pages is dirtied, we mark the
+ * dirtying-time in the inode's address_space.  So this periodic writeback code
+ * just walks the superblock inode list, writing back any inodes which are
+ * older than a specific point in time.
  *
- * FIXME: this linear search could get expensive with many fileystems.  But
- * how to fix?  We need to go from an address_space to all inodes which share
- * a queue with that address_space.  (Easy: have a global "dirty superblocks"
- * list).
+ * Try to run once per dirty_writeback_interval.  But if a writeback event
+ * takes longer than a dirty_writeback_interval interval, then leave a
+ * one-second gap.
  *
- * The inodes to be written are parked on bdi->b_io.  They are moved back onto
- * bdi->b_dirty as they are selected for writing.  This way, none can be missed
- * on the writer throttling path, and we get decent balancing between many
- * throttled threads: we don't want them all piling up on inode_sync_wait.
+ * older_than_this takes precedence over nr_to_write.  So we'll only write back
+ * all dirty pages if they are all attached to "old" mappings.
  */
-static void generic_sync_sb_inodes(struct super_block *sb,
-				   struct writeback_control *wbc)
+static long wb_writeback(struct bdi_writeback *wb, long nr_pages,
+			 struct super_block *sb,
+			 enum writeback_sync_modes sync_mode, int for_kupdate)
 {
-	struct backing_dev_info *bdi;
-
-	if (!wbc->bdi) {
-		mutex_lock(&bdi_lock);
-		list_for_each_entry(bdi, &bdi_list, bdi_list)
-			generic_sync_bdi_inodes(bdi, wbc, sb);
-		mutex_unlock(&bdi_lock);
-	} else
-		generic_sync_bdi_inodes(wbc->bdi, wbc, sb);
+	struct writeback_control wbc = {
+		.bdi			= wb->bdi,
+		.sb			= sb,
+		.sync_mode		= sync_mode,
+		.older_than_this	= NULL,
+		.for_kupdate		= for_kupdate,
+		.range_cyclic		= 1,
+	};
+	unsigned long oldest_jif;
+	long wrote = 0;
 
-	if (wbc->sync_mode == WB_SYNC_ALL) {
-		struct inode *inode, *old_inode = NULL;
+	if (wbc.for_kupdate) {
+		wbc.older_than_this = &oldest_jif;
+		oldest_jif = jiffies -
+				msecs_to_jiffies(dirty_expire_interval * 10);
+	}
 
-		spin_lock(&inode_lock);
+	for (;;) {
+		/*
+		 * Don't flush anything for non-integrity writeback where
+		 * no nr_pages was given
+		 */
+		if (!for_kupdate && nr_pages <= 0 && sync_mode == WB_SYNC_NONE)
+			break;
 
 		/*
-		 * Data integrity sync. Must wait for all pages under writeback,
-		 * because there may have been pages dirtied before our sync
-		 * call, but which had writeout started before we write it out.
-		 * In which case, the inode may not be on the dirty list, but
-		 * we still have to wait for that writeout.
+		 * If no specific pages were given and this is just a
+		 * periodic background writeout and we are below the
+		 * background dirty threshold, don't do anything
 		 */
-		list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
-			struct address_space *mapping;
+		if (for_kupdate && nr_pages <= 0 && !over_bground_thresh())
+			break;
 
-			if (inode->i_state &
-					(I_FREEING|I_CLEAR|I_WILL_FREE|I_NEW))
-				continue;
-			mapping = inode->i_mapping;
-			if (mapping->nrpages == 0)
+		wbc.more_io = 0;
+		wbc.encountered_congestion = 0;
+		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+		wbc.pages_skipped = 0;
+		writeback_inodes_wb(wb, &wbc);
+		nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+
+		/*
+		 * If we ran out of stuff to write, bail unless more_io got set
+		 */
+		if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
+			if (wbc.more_io && !wbc.for_kupdate)
 				continue;
-			__iget(inode);
-			spin_unlock(&inode_lock);
+			break;
+		}
+	}
+
+	return wrote;
+}
+
+/*
+ * Return the next bdi_work struct that hasn't been processed by this
+ * wb thread yet
+ */
+static struct bdi_work *get_next_work_item(struct backing_dev_info *bdi,
+					   struct bdi_writeback *wb)
+{
+	struct bdi_work *work, *ret = NULL;
+
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(work, &bdi->work_list, list) {
+		if (!test_and_clear_bit(wb->nr, &work->seen))
+			continue;
+
+		ret = work;
+		break;
+	}
+
+	rcu_read_unlock();
+	return ret;
+}
+
+static long wb_check_old_data_flush(struct bdi_writeback *wb)
+{
+	unsigned long expired;
+	long nr_pages;
+
+	expired = wb->last_old_flush +
+			msecs_to_jiffies(dirty_writeback_interval * 10);
+	if (time_before(jiffies, expired))
+		return 0;
+
+	wb->last_old_flush = jiffies;
+	nr_pages = global_page_state(NR_FILE_DIRTY) +
+			global_page_state(NR_UNSTABLE_NFS) +
+			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
+
+	if (nr_pages)
+		return wb_writeback(wb, nr_pages, NULL, WB_SYNC_NONE, 1);
+
+	return 0;
+}
+
+/*
+ * Retrieve work items and do the writeback they describe
+ */
+long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
+{
+	struct backing_dev_info *bdi = wb->bdi;
+	struct bdi_work *work;
+	long nr_pages, wrote = 0;
+
+	while ((work = get_next_work_item(bdi, wb)) != NULL) {
+		enum writeback_sync_modes sync_mode;
+
+		nr_pages = work->nr_pages;
+
+		/*
+		 * Override sync mode, in case we must wait for completion
+		 */
+		if (force_wait)
+			work->sync_mode = sync_mode = WB_SYNC_ALL;
+		else
+			sync_mode = work->sync_mode;
+
+		/*
+		 * If this isn't a data integrity operation, just notify
+		 * that we have seen this work and we are now starting it.
+		 */
+		if (sync_mode == WB_SYNC_NONE)
+			wb_clear_pending(wb, work);
+
+		wrote += wb_writeback(wb, nr_pages, work->sb, sync_mode, 0);
+
+		/*
+		 * This is a data integrity writeback, so only do the
+		 * notification when we have completed the work.
+		 */
+		if (sync_mode == WB_SYNC_ALL)
+			wb_clear_pending(wb, work);
+	}
+
+	/*
+	 * Check for periodic writeback, kupdated() style
+	 */
+	wrote += wb_check_old_data_flush(wb);
+
+	return wrote;
+}
+
+/*
+ * Handle writeback of dirty data for the device backed by this bdi. Also
+ * wakes up periodically and does kupdated style flushing.
+ */
+int bdi_writeback_task(struct bdi_writeback *wb)
+{
+	unsigned long last_active = jiffies;
+	unsigned long wait_jiffies = -1UL;
+	long pages_written;
+
+	while (!kthread_should_stop()) {
+		pages_written = wb_do_writeback(wb, 0);
+
+		if (pages_written)
+			last_active = jiffies;
+		else if (wait_jiffies != -1UL) {
+			unsigned long max_idle;
+
 			/*
-			 * We hold a reference to 'inode' so it couldn't have
-			 * been removed from s_inodes list while we dropped the
-			 * inode_lock.  We cannot iput the inode now as we can
-			 * be holding the last reference and we cannot iput it
-			 * under inode_lock. So we keep the reference and iput
-			 * it later.
+			 * Longest period of inactivity that we tolerate. If we
+			 * see dirty data again later, the task will get
+			 * recreated automatically.
 			 */
-			iput(old_inode);
-			old_inode = inode;
+			max_idle = max(5UL * 60 * HZ, wait_jiffies);
+			if (time_after(jiffies, max_idle + last_active))
+				break;
+		}
+
+		wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(wait_jiffies);
+		try_to_freeze();
+	}
+
+	return 0;
+}
+
+/*
+ * Schedule writeback for all backing devices. Expensive! If this is a data
+ * integrity operation, writeback will be complete when this returns. If
+ * we are simply called for WB_SYNC_NONE, then writeback will merely be
+ * scheduled to run.
+ */
+static void bdi_writeback_all(struct writeback_control *wbc)
+{
+	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
+	struct backing_dev_info *bdi;
+	struct bdi_work *work;
+	LIST_HEAD(list);
+
+restart:
+	spin_lock(&bdi_lock);
+
+	list_for_each_entry(bdi, &bdi_list, bdi_list) {
+		struct bdi_work *work;
+
+		if (!bdi_has_dirty_io(bdi))
+			continue;
 
-			filemap_fdatawait(mapping);
+		/*
+		 * If work allocation fails, do the writes inline. We drop
+		 * the lock and restart the list writeout. This should be OK,
+		 * since this happens rarely and because the writeout should
+		 * eventually make more free memory available.
+		 */
+		work = bdi_alloc_work(wbc);
+		if (!work) {
+			struct writeback_control __wbc;
 
-			cond_resched();
+			/*
+			 * Not a data integrity writeout, just continue
+			 */
+			if (!must_wait)
+				continue;
 
-			spin_lock(&inode_lock);
+			spin_unlock(&bdi_lock);
+			__wbc = *wbc;
+			__wbc.bdi = bdi;
+			writeback_inodes_wbc(&__wbc);
+			goto restart;
 		}
-		spin_unlock(&inode_lock);
-		iput(old_inode);
+		if (must_wait)
+			list_add_tail(&work->wait_list, &list);
+
+		bdi_queue_work(bdi, work);
+	}
+
+	spin_unlock(&bdi_lock);
+
+	/*
+	 * If this is for WB_SYNC_ALL, wait for pending work to complete
+	 * before returning.
+	 */
+	while (!list_empty(&list)) {
+		work = list_entry(list.next, struct bdi_work, wait_list);
+		list_del(&work->wait_list);
+		bdi_wait_on_work_clear(work);
+		call_rcu(&work->rcu_head, bdi_work_free);
 	}
 }
 
 /*
- * Start writeback of dirty pagecache data against all unlocked inodes.
+ * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
+ * the whole world.
+ */
+void wakeup_flusher_threads(long nr_pages)
+{
+	struct writeback_control wbc = {
+		.sync_mode	= WB_SYNC_NONE,
+		.older_than_this = NULL,
+		.range_cyclic	= 1,
+	};
+
+	if (nr_pages == 0)
+		nr_pages = global_page_state(NR_FILE_DIRTY) +
+				global_page_state(NR_UNSTABLE_NFS);
+	wbc.nr_to_write = nr_pages;
+	bdi_writeback_all(&wbc);
+}
+
+static noinline void block_dump___mark_inode_dirty(struct inode *inode)
+{
+	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
+		struct dentry *dentry;
+		const char *name = "?";
+
+		dentry = d_find_alias(inode);
+		if (dentry) {
+			spin_lock(&dentry->d_lock);
+			name = (const char *) dentry->d_name.name;
+		}
+		printk(KERN_DEBUG
+		       "%s(%d): dirtied inode %lu (%s) on %s\n",
+		       current->comm, task_pid_nr(current), inode->i_ino,
+		       name, inode->i_sb->s_id);
+		if (dentry) {
+			spin_unlock(&dentry->d_lock);
+			dput(dentry);
+		}
+	}
+}
+
+/**
+ *	__mark_inode_dirty -	internal function
+ *	@inode: inode to mark
+ *	@flags: what kind of dirty (i.e. I_DIRTY_SYNC)
+ *	Mark an inode as dirty. Callers should use mark_inode_dirty or
+ *  	mark_inode_dirty_sync.
  *
- * Note:
- * We don't need to grab a reference to superblock here. If it has non-empty
- * ->b_dirty it's hadn't been killed yet and kill_super() won't proceed
- * past sync_inodes_sb() until the ->b_dirty/b_io/b_more_io lists are all
- * empty. Since __sync_single_inode() regains inode_lock before it finally moves
- * inode from superblock lists we are OK.
+ * Put the inode on the super block's dirty list.
+ *
+ * CAREFUL! We mark it dirty unconditionally, but move it onto the
+ * dirty list only if it is hashed or if it refers to a blockdev.
+ * If it was not hashed, it will never be added to the dirty list
+ * even if it is later hashed, as it will have been marked dirty already.
+ *
+ * In short, make sure you hash any inodes _before_ you start marking
+ * them dirty.
  *
- * If `older_than_this' is non-zero then only flush inodes which have a
- * flushtime older than *older_than_this.
+ * This function *must* be atomic for the I_DIRTY_PAGES case -
+ * set_page_dirty() is called under spinlock in several places.
  *
- * If `bdi' is non-zero then we will scan the first inode against each
- * superblock until we find the matching ones.  One group will be the dirty
- * inodes against a filesystem.  Then when we hit the dummy blockdev superblock,
- * sync_sb_inodes will seekout the blockdev which matches `bdi'.  Maybe not
- * super-efficient but we're about to do a ton of I/O...
+ * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
+ * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
+ * the kernel-internal blockdev inode represents the dirtying time of the
+ * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
+ * page->mapping->host, so the page-dirtying time is recorded in the internal
+ * blockdev inode.
  */
-void
-writeback_inodes(struct writeback_control *wbc)
+void __mark_inode_dirty(struct inode *inode, int flags)
 {
-	struct super_block *sb;
+	struct super_block *sb = inode->i_sb;
 
-	might_sleep();
-	spin_lock(&sb_lock);
-restart:
-	list_for_each_entry_reverse(sb, &super_blocks, s_list) {
-		if (sb_has_dirty_inodes(sb)) {
-			/* we're making our own get_super here */
-			sb->s_count++;
-			spin_unlock(&sb_lock);
-			/*
-			 * If we can't get the readlock, there's no sense in
-			 * waiting around, most of the time the FS is going to
-			 * be unmounted by the time it is released.
-			 */
-			if (down_read_trylock(&sb->s_umount)) {
-				if (sb->s_root)
-					generic_sync_sb_inodes(sb, wbc);
-				up_read(&sb->s_umount);
-			}
-			spin_lock(&sb_lock);
-			if (__put_super_and_need_restart(sb))
-				goto restart;
+	/*
+	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
+	 * dirty the inode itself
+	 */
+	if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
+		if (sb->s_op->dirty_inode)
+			sb->s_op->dirty_inode(inode);
+	}
+
+	/*
+	 * make sure that changes are seen by all cpus before we test i_state
+	 * -- mikulas
+	 */
+	smp_mb();
+
+	/* avoid the locking if we can */
+	if ((inode->i_state & flags) == flags)
+		return;
+
+	if (unlikely(block_dump))
+		block_dump___mark_inode_dirty(inode);
+
+	spin_lock(&inode_lock);
+	if ((inode->i_state & flags) != flags) {
+		const int was_dirty = inode->i_state & I_DIRTY;
+
+		inode->i_state |= flags;
+
+		/*
+		 * If the inode is being synced, just update its dirty state.
+		 * The unlocker will place the inode on the appropriate
+		 * superblock list, based upon its state.
+		 */
+		if (inode->i_state & I_SYNC)
+			goto out;
+
+		/*
+		 * Only add valid (hashed) inodes to the superblock's
+		 * dirty list.  Add blockdev inodes as well.
+		 */
+		if (!S_ISBLK(inode->i_mode)) {
+			if (hlist_unhashed(&inode->i_hash))
+				goto out;
+		}
+		if (inode->i_state & (I_FREEING|I_CLEAR))
+			goto out;
+
+		/*
+		 * If the inode was already on b_dirty/b_io/b_more_io, don't
+		 * reposition it (that would break b_dirty time-ordering).
+		 */
+		if (!was_dirty) {
+			struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
+
+			inode->dirtied_when = jiffies;
+			list_move(&inode->i_list, &wb->b_dirty);
 		}
-		if (wbc->nr_to_write <= 0)
-			break;
 	}
-	spin_unlock(&sb_lock);
+out:
+	spin_unlock(&inode_lock);
+}
+EXPORT_SYMBOL(__mark_inode_dirty);
+
+/*
+ * Write out a superblock's list of dirty inodes.  A wait will be performed
+ * upon no inodes, all inodes or the final one, depending upon sync_mode.
+ *
+ * If older_than_this is non-NULL, then only write out inodes which
+ * had their first dirtying at a time earlier than *older_than_this.
+ *
+ * If we're a pdlfush thread, then implement pdflush collision avoidance
+ * against the entire list.
+ *
+ * If `bdi' is non-zero then we're being asked to writeback a specific queue.
+ * This function assumes that the blockdev superblock's inodes are backed by
+ * a variety of queues, so all inodes are searched.  For other superblocks,
+ * assume that all inodes are backed by the same queue.
+ *
+ * The inodes to be written are parked on bdi->b_io.  They are moved back onto
+ * bdi->b_dirty as they are selected for writing.  This way, none can be missed
+ * on the writer throttling path, and we get decent balancing between many
+ * throttled threads: we don't want them all piling up on inode_sync_wait.
+ */
+static void wait_sb_inodes(struct writeback_control *wbc)
+{
+	struct inode *inode, *old_inode = NULL;
+
+	/*
+	 * We need to be protected against the filesystem going from
+	 * r/o to r/w or vice versa.
+	 */
+	WARN_ON(!rwsem_is_locked(&wbc->sb->s_umount));
+
+	spin_lock(&inode_lock);
+
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback,
+	 * because there may have been pages dirtied before our sync
+	 * call, but which had writeout started before we write it out.
+	 * In which case, the inode may not be on the dirty list, but
+	 * we still have to wait for that writeout.
+	 */
+	list_for_each_entry(inode, &wbc->sb->s_inodes, i_sb_list) {
+		struct address_space *mapping;
+
+		if (inode->i_state & (I_FREEING|I_CLEAR|I_WILL_FREE|I_NEW))
+			continue;
+		mapping = inode->i_mapping;
+		if (mapping->nrpages == 0)
+			continue;
+		__iget(inode);
+		spin_unlock(&inode_lock);
+		/*
+		 * We hold a reference to 'inode' so it couldn't have
+		 * been removed from s_inodes list while we dropped the
+		 * inode_lock.  We cannot iput the inode now as we can
+		 * be holding the last reference and we cannot iput it
+		 * under inode_lock. So we keep the reference and iput
+		 * it later.
+		 */
+		iput(old_inode);
+		old_inode = inode;
+
+		filemap_fdatawait(mapping);
+
+		cond_resched();
+
+		spin_lock(&inode_lock);
+	}
+	spin_unlock(&inode_lock);
+	iput(old_inode);
 }
 
 /**
@@ -715,6 +1133,7 @@ restart:
 long writeback_inodes_sb(struct super_block *sb)
 {
 	struct writeback_control wbc = {
+		.sb		= sb,
 		.sync_mode	= WB_SYNC_NONE,
 		.range_start	= 0,
 		.range_end	= LLONG_MAX,
@@ -727,7 +1146,7 @@ long writeback_inodes_sb(struct super_block *sb)
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 
 	wbc.nr_to_write = nr_to_write;
-	generic_sync_sb_inodes(sb, &wbc);
+	bdi_writeback_all(&wbc);
 	return nr_to_write - wbc.nr_to_write;
 }
 EXPORT_SYMBOL(writeback_inodes_sb);
@@ -742,6 +1161,7 @@ EXPORT_SYMBOL(writeback_inodes_sb);
 long sync_inodes_sb(struct super_block *sb)
 {
 	struct writeback_control wbc = {
+		.sb		= sb,
 		.sync_mode	= WB_SYNC_ALL,
 		.range_start	= 0,
 		.range_end	= LLONG_MAX,
@@ -749,7 +1169,8 @@ long sync_inodes_sb(struct super_block *sb)
 	long nr_to_write = LONG_MAX; /* doesn't actually matter */
 
 	wbc.nr_to_write = nr_to_write;
-	generic_sync_sb_inodes(sb, &wbc);
+	bdi_writeback_all(&wbc);
+	wait_sb_inodes(&wbc);
 	return nr_to_write - wbc.nr_to_write;
 }
 EXPORT_SYMBOL(sync_inodes_sb);