[PATCH] fs-writeback: writeback_sb_inodes: Do not increase 'total_wrote' when nothing is written

[Chunhai Guo]

I am encountering a deadlock issue as shown below. There is a commit
344150999b7f ("f2fs: fix to avoid potential deadlock") can fix this issue.
However, from log analysis, it appears that this is more likely a fake
progress issue similar to commit 68f4c6eba70d ("fs-writeback:
writeback_sb_inodes: Recalculate 'wrote' according skipped pages"). In each
writeback iteration, nothing is written, while writeback_sb_inodes()
increases 'total_wrote' each time, causing an infinite loop. This patch
fixes this issue by not increasing 'total_wrote' when nothing is written.

    wb_writeback        fsync (inode-Y)
blk_start_plug(&plug)
for (;;) {
  iter i-1: some reqs with page-X added into plug->mq_list // f2fs node page-X with PG_writeback
                        filemap_fdatawrite
                          __filemap_fdatawrite_range // write inode-Y with sync_mode WB_SYNC_ALL
                           do_writepages
                            f2fs_write_data_pages
                             __f2fs_write_data_pages // wb_sync_req[DATA]++ for WB_SYNC_ALL
                              f2fs_write_cache_pages
                               f2fs_write_single_data_page
                                f2fs_do_write_data_page
                                 f2fs_outplace_write_data
                                  f2fs_update_data_blkaddr
                                   f2fs_wait_on_page_writeback
                                     wait_on_page_writeback // wait for f2fs node page-X
  iter i:
    progress = __writeback_inodes_wb(wb, work)
    . writeback_sb_inodes
    .   __writeback_single_inode // write inode-Y with sync_mode WB_SYNC_NONE
    .   . do_writepages
    .   .   f2fs_write_data_pages
    .   .   .  __f2fs_write_data_pages // skip writepages due to (wb_sync_req[DATA]>0)
    .   .   .   wbc->pages_skipped += get_dirty_pages(inode) // wbc->pages_skipped = 1
    .   if (!(inode->i_state & I_DIRTY_ALL)) // i_state = I_SYNC | I_SYNC_QUEUED
    .    total_wrote++;  // total_wrote = 1
    .   requeue_inode // requeue inode-Y to wb->b_dirty queue due to non-zero pages_skipped
    if (progress) // progress = 1
      continue;
  iter i+1:
      queue_io
      // similar process with iter i, infinite for-loop !
}
blk_finish_plug(&plug)   // flush plug won't be called

Signed-off-by: Chunhai Guo <guochunhai@xxxxxxxx>
---
 fs/fs-writeback.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 969ce991b0b0..54cdee906be9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1820,6 +1820,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 		struct inode *inode = wb_inode(wb->b_io.prev);
 		struct bdi_writeback *tmp_wb;
 		long wrote;
+		bool is_dirty_before;
 
 		if (inode->i_sb != sb) {
 			if (work->sb) {
@@ -1881,6 +1882,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 			continue;
 		}
 		inode->i_state |= I_SYNC;
+		is_dirty_before = inode->i_state & I_DIRTY_ALL;
 		wbc_attach_and_unlock_inode(&wbc, inode);
 
 		write_chunk = writeback_chunk_size(wb, work);
@@ -1918,7 +1920,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 		 */
 		tmp_wb = inode_to_wb_and_lock_list(inode);
 		spin_lock(&inode->i_lock);
-		if (!(inode->i_state & I_DIRTY_ALL))
+		if (!(inode->i_state & I_DIRTY_ALL) && is_dirty_before)
 			total_wrote++;
 		requeue_inode(inode, tmp_wb, &wbc);
 		inode_sync_complete(inode);
-- 
2.25.1