writeback: fix WB_SYNC_NONE writeback from umount

When umount calls sync_filesystem(), we first do a WB_SYNC_NONE
writeback to kick off writeback of pending dirty inodes, then follow
that up with a WB_SYNC_ALL to wait for it. Since umount already holds
the sb s_umount mutex, WB_SYNC_NONE ends up doing nothing and all
writeback happens as WB_SYNC_ALL. This can greatly slow down umount,
since WB_SYNC_ALL writeback is a data integrity operation and thus
a bigger hammer than simple WB_SYNC_NONE. For barrier aware file systems
it's a lot slower.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index eb38a2c645f6..47e1c686cb02 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -65,6 +65,15 @@ struct writeback_control {
 	 * so we use a single control to update them
 	 */
 	unsigned no_nrwrite_index_update:1;
+
+	/*
+	 * For WB_SYNC_ALL, the sb must always be pinned. For WB_SYNC_NONE,
+	 * the writeback code will pin the sb for the caller. However,
+	 * for eg umount, the caller does WB_SYNC_NONE but already has
+	 * the sb pinned. If the below is set, caller already has the
+	 * sb pinned.
+	 */
+	unsigned sb_pinned:1;
 };
 
 /*
@@ -73,6 +82,7 @@ struct writeback_control {
 struct bdi_writeback;
 int inode_wait(void *);
 void writeback_inodes_sb(struct super_block *);
+void writeback_inodes_sb_locked(struct super_block *);
 int writeback_inodes_sb_if_idle(struct super_block *);
 void sync_inodes_sb(struct super_block *);
 void writeback_inodes_wbc(struct writeback_control *wbc);