[PATCH 1/3] bdi: Use parent filesystem BDI for inodes not capable of writeback

[Jan Kara <jack@suse.cz>]

If inode->i_mapping->backing_dev_info is not capable of writeback
but the inode gets dirty (because inode itself has changed), use
inode lists of a bdi of a filesystem holding the device inode.
Forking the flusher thread just to do the writeout of a single
inode would be silly and generally bdi needn't be prepared to do
any writeback.

This is part of fixing a bug
https://bugzilla.kernel.org/show_bug.cgi?id=16312.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d5be169..00865aa 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -28,8 +28,6 @@
 #include <linux/buffer_head.h>
 #include "internal.h"
 
-#define inode_to_bdi(inode)	((inode)->i_mapping->backing_dev_info)
-
 /*
  * We don't actually have pdflush, but this one is exported though /proc...
  */
@@ -50,6 +48,21 @@ struct wb_writeback_work {
 	struct completion *done;	/* set if the caller waits */
 };
 
+static struct backing_dev_info *inode_to_bdi(struct inode *inode)
+{
+	struct backing_dev_info *bdi = inode->i_mapping->backing_dev_info;
+
+	/*
+	 * This is a hack but it solves a problem with device inode
+	 * for e.g. /dev/zero getting dirty (via touch or so) and confusing
+	 * writeback code. In such cases we return the "parent" filesystem's
+	 * bdi.
+	 */
+	if (bdi_cap_writeback_dirty(bdi))
+		return bdi;
+	return inode->i_sb->s_bdi;
+}
+
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.
-- 
1.6.4.2