fs: Fix busyloop in wb_writeback()

If all inodes are under writeback (e.g. in case when there's only one inode
with dirty pages), wb_writeback() with WB_SYNC_NONE work basically degrades
to busylooping until I_SYNC flags of the inode is cleared. Fix the problem by
waiting on I_SYNC flags of an inode on b_more_io list in case we failed to
write anything.

Tested-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 8e1e5e19d21e..c59d6737036c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -706,6 +706,7 @@ static long wb_writeback(struct bdi_writeback *wb,
         };
         unsigned long oldest_jif;
         long wrote = 0;
+        struct inode *inode;
 
         if (wbc.for_kupdate) {
                 wbc.older_than_this = &oldest_jif;
@@ -747,8 +748,24 @@ static long wb_writeback(struct bdi_writeback *wb,
                  * If we ran out of stuff to write, bail unless more_io got set
                  */
                 if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
-                        if (wbc.more_io && !wbc.for_kupdate)
+                        if (wbc.more_io && !wbc.for_kupdate) {
+                                if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
+                                        continue;
+                                /*
+                                 * Nothing written. Wait for some inode to
+                                 * become available for writeback. Otherwise
+                                 * we'll just busyloop.
+                                 */
+                                spin_lock(&inode_lock);
+                                if (!list_empty(&wb->b_more_io))  {
+                                        inode = list_entry(
+                                                        wb->b_more_io.prev,
+                                                        struct inode, i_list);
+                                        inode_wait_for_writeback(inode);
+                                }
+                                spin_unlock(&inode_lock);
                                 continue;
+                        }
                         break;
                 }
         }