[PATCH 30/49] writeback: implement WB_has_dirty_io wb_state flag

Tejun Heo
2015-04-06T20:15:10Z

Currently, wb_has_dirty_io() determines whether a wb (bdi_writeback)
has any dirty inode by testing all three IO lists on each invocation
without actively keeping track. For cgroup writeback support, a
single bdi will host multiple wb's each of which will host dirty
inodes separately and we'll need to make bdi_has_dirty_io(), which
currently only represents the root wb, aggregate has_dirty_io from all
member wb's, which requires tracking transitions in has_dirty_io state
on each wb.

This patch introduces inode_wb_list_{move|del}_locked() to consolidate
IO list operations leaving queue_io() the only other function which
directly manipulates IO lists (via move_expired_inodes()). All three
functions are updated to call wb_io_lists_[de]populated() which keep
track of whether the wb has dirty inodes or not and record it using
the new WB_has_dirty_io flag. inode_wb_list_moved_locked()'s return
value indicates whether the wb had no dirty inodes before.

mark_inode_dirty() is restructured so that the return value of
inode_wb_list_move_locked() can be used for deciding whether to wake
up the wb.

While at it, change {bdi|wb}_has_dirty_io()'s return values to bool.
These functions were returning 0 and 1 before. Also, add a comment
explaining the synchronization of wb_state flags.

v2: Updated to accommodate b_dirty_time.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
---
fs/fs-writeback.c | 104 ++++++++++++++++++++++++++++++---------
include/linux/backing-dev-defs.h | 1 +
include/linux/backing-dev.h | 8 ++-
mm/backing-dev.c | 2 +-
4 files changed, 86 insertions(+), 29 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1d30249..29b4f8f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -81,6 +81,66 @@ static inline struct inode *wb_inode(struct list_head *head)

EXPORT_TRACEPOINT_SYMBOL_GPL(wbc_writepage);

+static bool wb_io_lists_populated(struct bdi_writeback *wb)
+{
+ if (wb_has_dirty_io(wb)) {
+ return false;
+ } else {
+ set_bit(WB_has_dirty_io, &wb->state);
+ return true;
+ }
+}
+
+static void wb_io_lists_depopulated(struct bdi_writeback *wb)
+{
+ if (wb_has_dirty_io(wb) && list_empty(&wb->b_dirty) &&
+ list_empty(&wb->b_io) && list_empty(&wb->b_more_io))
+ clear_bit(WB_has_dirty_io, &wb->state);
+}
+
+/**
+ * inode_wb_list_move_locked - move an inode onto a bdi_writeback IO list
+ * @inode: inode to be moved
+ * @wb: target bdi_writeback
+ * @head: one of @wb->b_{dirty|io|more_io}
+ *
+ * Move @inode->i_wb_list to @list of @wb and set %WB_has_dirty_io.
+ * Returns %true if @inode is the first occupant of the !dirty_time IO
+ * lists; otherwise, %false.
+ */
+static bool inode_wb_list_move_locked(struct inode *inode,
+ struct bdi_writeback *wb,
+ struct list_head *head)
+{
+ assert_spin_locked(&wb->list_lock);
+
+ list_move(&inode->i_wb_list, head);
+
+ /* dirty_time doesn't count as dirty_io until expiration */
+ if (head != &wb->b_dirty_time)
+ return wb_io_lists_populated(wb);
+
+ wb_io_lists_depopulated(wb);
+ return false;
+}
+
+/**
+ * inode_wb_list_del_locked - remove an inode from its bdi_writeback IO list
+ * @inode: inode to be removed
+ * @wb: bdi_writeback @inode is being removed from
+ *
+ * Remove @inode which may be on one of @wb->b_{dirty|io|more_io} lists and
+ * clear %WB_has_dirty_io if all are empty afterwards.
+ */
+static void inode_wb_list_del_locked(struct inode *inode,
+ struct bdi_writeback *wb)
+{
+ assert_spin_locked(&wb->list_lock);
+
+ list_del_init(&inode->i_wb_list);
+ wb_io_lists_depopulated(wb);
+}
+
static void wb_wakeup(struct bdi_writeback *wb)
{
spin_lock_bh(&wb->work_lock);
@@ -205,7 +265,7 @@ void inode_wb_list_del(struct inode *inode)
struct bdi_writeback *wb = inode_to_wb(inode);

spin_lock(&wb->list_lock);
- list_del_init(&inode->i_wb_list);
+ inode_wb_list_del_locked(inode, wb);
spin_unlock(&wb->list_lock);
}

@@ -220,7 +280,6 @@ void inode_wb_list_del(struct inode *inode)
*/
static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
{
- assert_spin_locked(&wb->list_lock);
if (!list_empty(&wb->b_dirty)) {
struct inode *tail;

@@ -228,7 +287,7 @@ static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
if (time_before(inode->dirtied_when, tail->dirtied_when))
inode->dirtied_when = jiffies;
}
- list_move(&inode->i_wb_list, &wb->b_dirty);
+ inode_wb_list_move_locked(inode, wb, &wb->b_dirty);
}

/*
@@ -236,8 +295,7 @@ static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
*/
static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
{
- assert_spin_locked(&wb->list_lock);
- list_move(&inode->i_wb_list, &wb->b_more_io);
+ inode_wb_list_move_locked(inode, wb, &wb->b_more_io);
}

static void inode_sync_complete(struct inode *inode)
@@ -346,6 +404,8 @@ static void queue_io(struct bdi_writeback *wb, struct wb_writeback_work *work)
moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, 0, work);
moved += move_expired_inodes(&wb->b_dirty_time, &wb->b_io,
EXPIRE_DIRTY_ATIME, work);
+ if (moved)
+ wb_io_lists_populated(wb);
trace_writeback_queue_io(wb, work, moved);
}

@@ -470,10 +530,10 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
*/
redirty_tail(inode, wb);
} else if (inode->i_state & I_DIRTY_TIME) {
- list_move(&inode->i_wb_list, &wb->b_dirty_time);
+ inode_wb_list_move_locked(inode, wb, &wb->b_dirty_time);
} else {
/* The inode is clean. Remove from writeback lists. */
- list_del_init(&inode->i_wb_list);
+ inode_wb_list_del_locked(inode, wb);
}
}

@@ -610,7 +670,7 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
* touch it. See comment above for explanation.
*/
if (!(inode->i_state & I_DIRTY_ALL))
- list_del_init(&inode->i_wb_list);
+ inode_wb_list_del_locked(inode, wb);
spin_unlock(&wb->list_lock);
inode_sync_complete(inode);
out:
@@ -1264,27 +1324,25 @@ void __mark_inode_dirty(struct inode *inode, int flags)

spin_unlock(&inode->i_lock);
spin_lock(&bdi->wb.list_lock);
- if (bdi_cap_writeback_dirty(bdi)) {
- WARN(!test_bit(WB_registered, &bdi->wb.state),
- "bdi-%s not registered\n", bdi->name);

- /*
- * If this is the first dirty inode for this
- * bdi, we have to wake-up the corresponding
- * bdi thread to make sure background
- * write-back happens later.
- */
- if (!wb_has_dirty_io(&bdi->wb))
- wakeup_bdi = true;
- }
+ WARN(bdi_cap_writeback_dirty(bdi) &&
+ !test_bit(WB_registered, &bdi->wb.state),
+ "bdi-%s not registered\n", bdi->name);

inode->dirtied_when = jiffies;
- list_move(&inode->i_wb_list, dirtytime ?
- &bdi->wb.b_dirty_time : &bdi->wb.b_dirty);
+ wakeup_bdi = inode_wb_list_move_locked(inode, &bdi->wb,
+ dirtytime ? &bdi->wb.b_dirty_time :
+ &bdi->wb.b_dirty);
spin_unlock(&bdi->wb.list_lock);
trace_writeback_dirty_inode_enqueue(inode);

- if (wakeup_bdi)
+ /*
+ * If this is the first dirty inode for this bdi,
+ * we have to wake-up the corresponding bdi thread
+ * to make sure background write-back happens
+ * later.
+ */
+ if (bdi_cap_writeback_dirty(bdi) && wakeup_bdi)
wb_wakeup_delayed(&bdi->wb);
return;
}
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index eb38676..7a94b78 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -21,6 +21,7 @@ struct dentry;
enum wb_state {
WB_registered, /* bdi_register() was done */
WB_writeback_running, /* Writeback is in progress */
+ WB_has_dirty_io, /* Dirty inodes on ->b_{dirty|io|more_io} */
};

enum wb_congested_state {
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 6f08821..3c8403c 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -29,7 +29,7 @@ void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
enum wb_reason reason);
void bdi_start_background_writeback(struct backing_dev_info *bdi);
void wb_workfn(struct work_struct *work);
-int bdi_has_dirty_io(struct backing_dev_info *bdi);
+bool bdi_has_dirty_io(struct backing_dev_info *bdi);
void wb_wakeup_delayed(struct bdi_writeback *wb);

extern spinlock_t bdi_lock;
@@ -37,11 +37,9 @@ extern struct list_head bdi_list;

extern struct workqueue_struct *bdi_wq;

-static inline int wb_has_dirty_io(struct bdi_writeback *wb)
+static inline bool wb_has_dirty_io(struct bdi_writeback *wb)
{
- return !list_empty(&wb->b_dirty) ||
- !list_empty(&wb->b_io) ||
- !list_empty(&wb->b_more_io);
+ return test_bit(WB_has_dirty_io, &wb->state);
}

static inline void __add_wb_stat(struct bdi_writeback *wb,
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 5029c4a..161ddf1 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -256,7 +256,7 @@ static int __init default_bdi_init(void)
}
subsys_initcall(default_bdi_init);

-int bdi_has_dirty_io(struct backing_dev_info *bdi)
+bool bdi_has_dirty_io(struct backing_dev_info *bdi)
{
return wb_has_dirty_io(&bdi->wb);
}
--
2.1.0