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>

3 files changed, 16 insertions, 22 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 39fadfad86f7..c9d105ff7970 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -165,25 +165,11 @@ static void redirty_tail(struct inode *inode)
 }
 
 /*
- * Redirty an inode, but mark it as the very next-to-be-written inode on its
- * superblock's dirty-inode list.
- * We need to preserve s_dirty's reverse-time-orderedness, so we cheat by
- * setting this inode's dirtied_when to the same value as that of the inode
- * which is presently head-of-list, if present head-of-list is newer than this
- * inode. (head-of-list is the least-recently-dirtied inode: the oldest one).
+ * requeue inode for re-scanning after sb->s_io list is exhausted.
  */
-static void redirty_head(struct inode *inode)
+static void requeue_io(struct inode *inode)
 {
-        struct super_block *sb = inode->i_sb;
-
-        if (!list_empty(&sb->s_dirty)) {
-                struct inode *head_inode;
-
-                head_inode = list_entry(sb->s_dirty.prev, struct inode, i_list);
-                if (time_after(inode->dirtied_when, head_inode->dirtied_when))
-                        inode->dirtied_when = head_inode->dirtied_when;
-        }
-        list_move_tail(&inode->i_list, &sb->s_dirty);
+        list_move(&inode->i_list, &inode->i_sb->s_more_io);
 }
 
 /*
@@ -255,7 +241,7 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
                                  * uncongested.
                                  */
                                 inode->i_state |= I_DIRTY_PAGES;
-                                redirty_head(inode);
+                                requeue_io(inode);
                         } else {
                                 /*
                                  * Otherwise fully redirty the inode so that
@@ -315,7 +301,7 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
                  * on s_io.  We'll have another go at writing back this inode
                  * when the s_dirty iodes get moved back onto s_io.
                  */
-                redirty_head(inode);
+                requeue_io(inode);
 
                 /*
                  * Even if we don't actually write the inode itself here,
@@ -410,14 +396,14 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
                         wbc->encountered_congestion = 1;
                         if (!sb_is_blkdev_sb(sb))
                                 break;          /* Skip a congested fs */
-                        redirty_head(inode);
+                        requeue_io(inode);
                         continue;               /* Skip a congested blockdev */
                 }
 
                 if (wbc->bdi && bdi != wbc->bdi) {
                         if (!sb_is_blkdev_sb(sb))
                                 break;          /* fs has the wrong queue */
-                        redirty_head(inode);
+                        requeue_io(inode);
                         continue;               /* blockdev has wrong queue */
                 }
 
@@ -427,8 +413,10 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
 
                 /* Was this inode dirtied too recently? */
                 if (wbc->older_than_this && time_after(inode->dirtied_when,
-                                                *wbc->older_than_this))
+                                                *wbc->older_than_this)) {
+                        list_splice_init(&sb->s_io, sb->s_dirty.prev);
                         break;
+                }
 
                 /* Is another pdflush already flushing this queue? */
                 if (current_is_pdflush() && !writeback_acquire(bdi))
@@ -458,6 +446,10 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
                 if (wbc->nr_to_write <= 0)
                         break;
         }
+
+        if (list_empty(&sb->s_io))
+                list_splice_init(&sb->s_more_io, &sb->s_io);
+
         return;         /* Leave any unwritten inodes on s_io */
 }
 
diff --git a/fs/super.c b/fs/super.c
index fc8ebedc6bed..1bfcca2104be 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -67,6 +67,7 @@ static struct super_block *alloc_super(struct file_system_type *type)
                 }
                 INIT_LIST_HEAD(&s->s_dirty);
                 INIT_LIST_HEAD(&s->s_io);
+                INIT_LIST_HEAD(&s->s_more_io);
                 INIT_LIST_HEAD(&s->s_files);
                 INIT_LIST_HEAD(&s->s_instances);
                 INIT_HLIST_HEAD(&s->s_anon);
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;