[PATCH v4 3/6] fs/writeback: in wait_sb_inodes(), skip wait for AS_WRITEBACK_MAY_BLOCK mappings

[Joanne Koong <joannelkoong@gmail.com>]

For filesystems with the AS_WRITEBACK_MAY_BLOCK flag set, writeback
operations may block or take an indeterminate time to complete. For
example, writing data back to disk in FUSE filesystems depends on the
userspace server successfully completing writeback.

In this commit, wait_sb_inodes() skips waiting on writeback if the
inode's mapping has AS_WRITEBACK_MAY_BLOCK set, else sync(2) may take an
indeterminate amount of time to complete.

If the caller wishes to ensure the data for a mapping with the
AS_WRITEBACK_MAY_BLOCK flag set has actually been written back to disk,
they should use fsync(2)/fdatasync(2) instead.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/fs-writeback.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d8bec3c1bb1f..c80c45972162 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2659,6 +2659,9 @@ static void wait_sb_inodes(struct super_block *sb)
 		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
 			continue;
 
+		if (mapping_writeback_may_block(mapping))
+			continue;
+
 		spin_unlock_irq(&sb->s_inode_wblist_lock);
 
 		spin_lock(&inode->i_lock);
-- 
2.43.5