lazytime bug fixes for 4.0

Jan Kara pointed out that if there is an inode which is constantly getting dirtied with I_DIRTY_PAGES, an inode with an updated timestamp will never be written since inode->dirtied_when is constantly getting updated. We fix this by adding an extra field to the inode, dirtied_time_when, so inodes with a stale dirtytime can get detected and handled.

In addition, if we have a dirtytime inode caused by an atime update, and there is no write activity on the file system, we need to have a secondary system to make sure these inodes get written out. We do this by setting up a second delayed work structure which wakes up the CPU much more rarely compared to writeback_expire_centisecs.

Signed-off-by Theodore Ts'o <[email protected]>

Cc [email protected]

---
fs/fs-writeback.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++------- include/linux/fs.h | 1 +

2 files changed, 73 insertions(+), 10 deletions(-)

fs/fs-writeback.c +72 -10

index e907052..ae13fba 100644
@@ -53,6 +53,18 @@ struct wb_writeback_work {
 	struct completion *done;	/* set if the caller waits */
 };
 
+/*
+ * If an inode is constantly having its pages dirtied, but then the
+ * updates stop dirtytime_expire_interval seconds in the past, it's
+ * possible for the worst case time between when an inode has its
+ * timestamps updated and when they finally get written out to be two
+ * dirtytime_expire_intervals.  We set the default to 12 hours (in
+ * seconds), which means most of the time inodes will have their
+ * timestamps written to disk after 12 hours, but in the worst case a
+ * few inodes might not their timestamps updated for 24 hours.
+ */
+unsigned int dirtytime_expire_interval = 12 * 60 * 60;
+
 /**
 * writeback_in_progress - determine whether there is writeback in progress
 * @bdi: the device's backing_dev_info structure.

@@ -275,8 +287,8 @@ static int move_expired_inodes(struct list_head *delaying_queue,
 
 	if ((flags & EXPIRE_DIRTY_ATIME) == 0)
 		older_than_this = work->older_than_this;
-	else if ((work->reason == WB_REASON_SYNC) == 0) {
-		expire_time = jiffies - (HZ * 86400);
+	else if (!work->for_sync) {
+		expire_time = jiffies - (dirtytime_expire_interval * HZ);
 		older_than_this = &expire_time;
 	}
 	while (!list_empty(delaying_queue)) {

@@ -458,6 +470,7 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
 		 */
 		redirty_tail(inode, wb);
 	} else if (inode->i_state & I_DIRTY_TIME) {
+		inode->dirtied_when = jiffies;
 		list_move(&inode->i_wb_list, &wb->b_dirty_time);
 	} else {
 		/* The inode is clean. Remove from writeback lists. */

@@ -505,12 +518,17 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 	spin_lock(&inode->i_lock);
 
 	dirty = inode->i_state & I_DIRTY;
-	if (((dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) &&
-	     (inode->i_state & I_DIRTY_TIME)) ||
-	    (inode->i_state & I_DIRTY_TIME_EXPIRED)) {
-		dirty |= I_DIRTY_TIME | I_DIRTY_TIME_EXPIRED;
-		trace_writeback_lazytime(inode);
-	}
+	if (inode->i_state & I_DIRTY_TIME) {
+		if ((dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) ||
+		    unlikely(inode->i_state & I_DIRTY_TIME_EXPIRED) ||
+		    unlikely(time_after((inode->dirtied_time_when +
+					 dirtytime_expire_interval * HZ),
+					jiffies))) {
+			dirty |= I_DIRTY_TIME | I_DIRTY_TIME_EXPIRED;
+			trace_writeback_lazytime(inode);
+		}
+	} else
+		inode->i_state &= ~I_DIRTY_TIME_EXPIRED;
 	inode->i_state &= ~dirty;
 
 	/*

@@ -1131,6 +1149,45 @@ void wakeup_flusher_threads(long nr_pages, enum wb_reason reason)
 	rcu_read_unlock();
 }
 
+/*
+ * Wake up bdi's periodically to make sure dirtytime inodes gets
+ * written back periodically.  We deliberately do *not* check the
+ * b_dirtytime list in wb_has_dirty_io(), since this would cause the
+ * kernel to be constantly waking up once there are any dirtytime
+ * inodes on the system.  So instead we define a separate delayed work
+ * function which gets called much more rarely.  (By default, only
+ * once every 12 hours.)
+ *
+ * If there is any other write activity going on in the file system,
+ * this function won't be necessary.  But if the only thing that has
+ * happened on the file system is a dirtytime inode caused by an atime
+ * update, we need this infrastructure below to make sure that inode
+ * eventually gets pushed out to disk.
+ */
+static void wakeup_dirtytime_writeback(struct work_struct *w);
+static DECLARE_DELAYED_WORK(dirtytime_work, wakeup_dirtytime_writeback);
+
+static void wakeup_dirtytime_writeback(struct work_struct *w)
+{
+	struct backing_dev_info *bdi;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
+		if (list_empty(&bdi->wb.b_dirty_time))
+			continue;
+		bdi_wakeup_thread(bdi);
+	}
+	rcu_read_unlock();
+	schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
+}
+
+static int __init start_dirtytime_writeback(void)
+{
+	schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
+	return 0;
+}
+__initcall(start_dirtytime_writeback);
+
 static noinline void block_dump___mark_inode_dirty(struct inode *inode)
 {
 	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {

@@ -1269,8 +1326,13 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 			}
 
 			inode->dirtied_when = jiffies;
-			list_move(&inode->i_wb_list, dirtytime ?
-				  &bdi->wb.b_dirty_time : &bdi->wb.b_dirty);
+			if (dirtytime)
+				inode->dirtied_time_when = jiffies;
+			if (inode->i_state & (I_DIRTY_INODE | I_DIRTY_PAGES))
+				list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
+			else
+				list_move(&inode->i_wb_list,
+					  &bdi->wb.b_dirty_time);
 			spin_unlock(&bdi->wb.list_lock);
 			trace_writeback_dirty_inode_enqueue(inode);
 

include/linux/fs.h +1 -0

index b4d71b5..f4131e8 100644
@@ -604,6 +604,7 @@ struct inode {
 	struct mutex		i_mutex;
 
 	unsigned long		dirtied_when;	/* jiffies of first dirtying */
+	unsigned long		dirtied_time_when;
 
 
 	struct hlist_node	i_hash;

struct list_head i_wb_list; /* backing dev IO list */

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2.3.0

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Quoted 7 lines · Theodore Ts'o

Jan Kara pointed out that if there is an inode which is constantly getting dirtied with I_DIRTY_PAGES, an inode with an updated timestamp will never be written since inode->dirtied_when is constantly getting updated. We fix this by adding an extra field to the inode, dirtied_time_when, so inodes with a stale dirtytime can get detected and handled.

The drawback here is that this adds another 8 bytes to every inode for a field of marginal value, since this is only important for the rare case of a file that is being dirtied continuously.

I wonder if something more lightweight could be added to avoid this problem? For example, we only care about this case if it has been going on for more than the lazytime interval (about a day), so the inode could store a 16-bit i_dirtied_time_when that is approximately (jiffies >> bits_in_a_half_a_day) and only check time_after() that. The __u16 could fit into some existing hole (e.g. after i_bytes on my kernel) and avoid expanding the size of the inode at all.

The remaining high bits of i_dirtied_time_when would be irrelevant, since a __u16 of half-days is about 80 years, so it would be enough to compare:

    time_after(i_dirtied_time_when, (__u16)(jiffies >> bits_in_half_a_day))

A day is 86400s, so 43200s is close to (1 << 22) jiffies for HZ=100, and (1 << 25) jiffies is about 3/8 of a day for HZ=1000. Since the exact times for inode writeout don't matter very much here, having only shifts to convert jiffies to i_dirtied_time_when in the kernel is better I think.

Minor issue, is there a good reason why dirtied_time_when doesn't have an "i_" prefix?

Yes, something like this should be possible. But I wanted that to happen
as a separate patch once we have everything working correctly. The code is subtle enough that I didn't want Ted to complicate it with further optimizations initially.

I wonder if something more lightweight could be added to avoid this problem? For example, we only care about this case if it has been going on for more than the lazytime interval (about a day), so the inode could store a 16-bit i_dirtied_time_when that is approximately (jiffies >> bits_in_a_half_a_day) and only check time_after() that. The __u16 could fit into some existing hole (e.g. after i_bytes on my kernel) and avoid expanding the size of the inode at all.

The remaining high bits of i_dirtied_time_when would be irrelevant, since a __u16 of half-days is about 80 years, so it would be enough to compare:

    time_after(i_dirtied_time_when, (__u16)(jiffies >> bits_in_half_a_day))

Minor issue, is there a good reason why dirtied_time_when doesn't have an "i_" prefix?

Jan Kara pointed out that if there is an inode which is constantly getting dirtied with I_DIRTY_PAGES, an inode with an updated timestamp will never be written since inode->dirtied_when is constantly getting updated. We fix this by adding an extra field to the inode, dirtied_time_when, so inodes with a stale dirtytime can get detected and handled.

In addition, if we have a dirtytime inode caused by an atime update, and there is no write activity on the file system, we need to have a secondary system to make sure these inodes get written out. We do this by setting up a second delayed work structure which wakes up the CPU much more rarely compared to writeback_expire_centisecs.

Signed-off-by Theodore Ts'o <[email protected]>

Cc [email protected]

fs/fs-writeback.c +0 -2

@@ -505,12 +518,17 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 	spin_lock(&inode->i_lock);
 
 	dirty = inode->i_state & I_DIRTY;
-	if (((dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) &&
-	     (inode->i_state & I_DIRTY_TIME)) ||

Add a tuning knob so we can adjust the dirtytime expiration timeout, which is very useful for testing lazytime.

Signed-off-by Theodore Ts'o <[email protected]>

Cc [email protected]

---
 fs/fs-writeback.c         | 11 +++++++++++
 include/linux/writeback.h |  3 +++
 kernel/sysctl.c           |  8 ++++++++
 3 files changed, 22 insertions(+)

fs/fs-writeback.c +11 -0

index ae13fba..d6fa722 100644
@@ -1188,6 +1188,17 @@ static int __init start_dirtytime_writeback(void)
 }
 __initcall(start_dirtytime_writeback);
 
+int dirtytime_interval_handler(struct ctl_table *table, int write,
+			       void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	int ret;
+
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (ret == 0 && write)
+		mod_delayed_work(system_wq, &dirtytime_work, 0);
+	return ret;
+}
+
 static noinline void block_dump___mark_inode_dirty(struct inode *inode)
 {
 	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {

include/linux/writeback.h +3 -0

index 0004833..b2dd371e 100644
@@ -130,6 +130,7 @@ extern int vm_dirty_ratio;
 extern unsigned long vm_dirty_bytes;
 extern unsigned int dirty_writeback_interval;
 extern unsigned int dirty_expire_interval;
+extern unsigned int dirtytime_expire_interval;
 extern int vm_highmem_is_dirtyable;
 extern int block_dump;
 extern int laptop_mode;

@@ -146,6 +147,8 @@ extern int dirty_ratio_handler(struct ctl_table *table, int write,
 extern int dirty_bytes_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *lenp,
 		loff_t *ppos);
+int dirtytime_interval_handler(struct ctl_table *table, int write,
+			       void __user *buffer, size_t *lenp, loff_t *ppos);
 
 struct ctl_table;
 int dirty_writeback_centisecs_handler(struct ctl_table *, int,

kernel/sysctl.c +8 -0

index 88ea2d6..ce410bb 100644
@@ -1228,6 +1228,14 @@ static struct ctl_table vm_table[] = {
 		.extra1		= &zero,
 	},
 	{
+		.procname	= "dirtytime_expire_seconds",
+		.data		= &dirtytime_expire_interval,
+		.maxlen		= sizeof(dirty_expire_interval),
+		.mode		= 0644,
+		.proc_handler	= dirtytime_interval_handler,
+		.extra1		= &zero,
+	},
+	{
 		.procname       = "nr_pdflush_threads",
 		.mode           = 0444 /* read-only */,
 		.proc_handler   = pdflush_proc_obsolete,

Signature · 1 line

2.3.0