writeback: fix periodic superblock dirty inode flushing

Current -mm tree has bucketful of bug fixes in periodic writeback path.
However, we still hit a glitch where dirty pages on a given inode aren't
completely flushed to the disk, and system will accumulate large amount of
dirty pages beyond what dirty_expire_interval is designed for.

The problem is __sync_single_inode() will move an inode to sb->s_dirty list
even when there are more pending dirty pages on that inode.  If there is
another inode with a small number of dirty pages, we hit a case where the loop
iteration in wb_kupdate() terminates prematurely because wbc.nr_to_write > 0.
Thus leaving the inode that has large amount of dirty pages behind and it has
to wait for another dirty_writeback_interval before we flush it again.  We
effectively only write out MAX_WRITEBACK_PAGES every dirty_writeback_interval.
If the rate of dirtying is sufficiently high, the system will start
accumulate a large number of dirty pages.

So fix it by having another sb->s_more_io list on which to park the inode
while we iterate through sb->s_io and to allow each dirty inode which resides
on that sb to have an equal chance of flushing some amount of dirty pages.

Signed-off-by: Ken Chen <kenchen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 30aca3399450..0b38a897c114 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1002,6 +1002,7 @@ struct super_block {
         struct list_head        s_inodes;       /* all inodes */
         struct list_head        s_dirty;        /* dirty inodes */
         struct list_head        s_io;           /* parked for writeback */
+        struct list_head        s_more_io;      /* parked for more writeback */
         struct hlist_head       s_anon;         /* anonymous dentries for (nfs) exporting */
         struct list_head        s_files;