[PATCH v2 1/6] fs/writeback: avoid to writeback non-expired inode in kupdate writeback

[Kemeng Shi]

In kupdate writeback, only expired inode (have been dirty for longer than
dirty_expire_interval) is supposed to be written back. However, kupdate
writeback will writeback non-expired inode left in b_io or b_more_io from
last wb_writeback. As a result, writeback will keep being triggered
unexpected when we keep dirtying pages even dirty memory is under
threshold and inode is not expired. To be more specific:
Assume dirty background threshold is > 1G and dirty_expire_centisecs is
> 60s. When we running fio -size=1G -invalidate=0 -ioengine=libaio
--time_based -runtime=60... (keep dirtying), the writeback will keep
being triggered as following:
wb_workfn
  wb_do_writeback
    wb_check_background_flush
      /*
       * Wb dirty background threshold starts at 0 if device was idle and
       * grows up when bandwidth of wb is updated. So a background
       * writeback is triggered.
       */
      wb_over_bg_thresh
      /*
       * Dirtied inode will be written back and added to b_more_io list
       * after slice used up (because we keep dirtying the inode).
       */
      wb_writeback

Writeback is triggered per dirty_writeback_centisecs as following:
wb_workfn
  wb_do_writeback
    wb_check_old_data_flush
      /*
       * Write back inode left in b_io and b_more_io from last wb_writeback
       * even the inode is non-expired and it will be added to b_more_io
       * again as slice will be used up (because we keep dirtying the
       * inode)
       */
      wb_writeback

Fix this by moving non-expired inode to dirty list instead of more io
list for kupdate writeback in requeue_inode.

Test as following:
/* make it more easier to observe the issue */
echo 300000 > /proc/sys/vm/dirty_expire_centisecs
echo 100 > /proc/sys/vm/dirty_writeback_centisecs
/* create a idle device */
mkfs.ext4 -F /dev/vdb
mount /dev/vdb /bdi1/
/* run buffer write with fio */
fio -name test -filename=/bdi1/file -size=800M -ioengine=libaio -bs=4K \
-iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0

Fio result before fix (run three tests):
1360MB/s
1329MB/s
1455MB/s

Fio result after fix (run three tests):
1737MB/s
1729MB/s
1789MB/s

Writeback for non-expired inode is gone as expeted. Observe this with trace
writeback_start and writeback_written as following:
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_start/enab
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_written/enable
cat /sys/kernel/tracing/trace_pipe

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 fs/fs-writeback.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5ab1aaf805f7..4e6166e07eaf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1561,7 +1561,8 @@ static void inode_sleep_on_writeback(struct inode *inode)
  * thread's back can have unexpected consequences.
  */
 static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
-			  struct writeback_control *wbc)
+			  struct writeback_control *wbc,
+			  unsigned long dirtied_before)
 {
 	if (inode->i_state & I_FREEING)
 		return;
@@ -1594,7 +1595,8 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
 		 * We didn't write back all the pages.  nfs_writepages()
 		 * sometimes bales out without doing anything.
 		 */
-		if (wbc->nr_to_write <= 0) {
+		if (wbc->nr_to_write <= 0 &&
+		    !inode_dirtied_after(inode, dirtied_before)) {
 			/* Slice used up. Queue for next turn. */
 			requeue_io(inode, wb);
 		} else {
@@ -1862,6 +1864,11 @@ static long writeback_sb_inodes(struct super_block *sb,
 	unsigned long start_time = jiffies;
 	long write_chunk;
 	long total_wrote = 0;  /* count both pages and inodes */
+	unsigned long dirtied_before = jiffies;
+
+	if (work->for_kupdate)
+		dirtied_before = jiffies -
+			msecs_to_jiffies(dirty_expire_interval * 10);
 
 	while (!list_empty(&wb->b_io)) {
 		struct inode *inode = wb_inode(wb->b_io.prev);
@@ -1967,7 +1974,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 		spin_lock(&inode->i_lock);
 		if (!(inode->i_state & I_DIRTY_ALL))
 			total_wrote++;
-		requeue_inode(inode, tmp_wb, &wbc);
+		requeue_inode(inode, tmp_wb, &wbc, dirtied_before);
 		inode_sync_complete(inode);
 		spin_unlock(&inode->i_lock);
 
-- 
2.30.0