[PATCH] vfs: Merge sync_supers(), sync_filesystems() and sync_blockdevs()

[Jan Kara]

These three functions are quite similar so merge them to save superblock list
traversal code. As a bonus we get livelock avoidance for all these superblock
traversals. Also remove the condition that if wait == 0 and sb->s_dirt is
not set, then ->sync_fs() is not called. This does not really make much sence
since s_dirt is generally used by filesystem to mean that ->write_super() needs
to be called. But ->sync_fs() does different things. I even suspect that some
filesystems (btrfs?) sets s_dirt just to fool this logic.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/super.c          |  104 ++++++++++++++------------------------------------
 fs/sync.c           |   12 ++++--
 include/linux/fs.h  |   11 +++--
 mm/page-writeback.c |    2 +-
 4 files changed, 44 insertions(+), 85 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 4f56333..6d6209e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -413,58 +413,24 @@ void drop_super(struct super_block *sb)
 
 EXPORT_SYMBOL(drop_super);
 
-static inline void write_super(struct super_block *sb)
-{
-	lock_super(sb);
-	if (sb->s_root && sb->s_dirt)
-		if (sb->s_op->write_super)
-			sb->s_op->write_super(sb);
-	unlock_super(sb);
-}
-
-/*
- * Note: check the dirty flag before waiting, so we don't
- * hold up the sync while mounting a device. (The newly
- * mounted device won't need syncing.)
- */
-void sync_supers(void)
-{
-	struct super_block *sb;
-
-	spin_lock(&sb_lock);
-restart:
-	list_for_each_entry(sb, &super_blocks, s_list) {
-		if (sb->s_dirt) {
-			sb->s_count++;
-			spin_unlock(&sb_lock);
-			down_read(&sb->s_umount);
-			write_super(sb);
-			up_read(&sb->s_umount);
-			spin_lock(&sb_lock);
-			if (__put_super_and_need_restart(sb))
-				goto restart;
-		}
-	}
-	spin_unlock(&sb_lock);
-}
-
 /*
- * Call the ->sync_fs super_op against all filesystems which are r/w and
- * which implement it.
+ * Call the ->write_super, ->sync_fs, sync_blockdev() against all filesystems
+ * which are r/w and which implement it.
  *
- * This operation is careful to avoid the livelock which could easily happen
- * if two or more filesystems are being continuously dirtied.  s_need_sync_fs
- * is used only here.  We set it against all filesystems and then clear it as
- * we sync them.  So redirtied filesystems are skipped.
+ * This operation is careful to avoid the livelock. Currently it's not so easy
+ * to trigger as it used to be - we restart scanning the superblock list only
+ * if the superblock got unmounted under us - but it's better to be safe and it
+ * doesn't cost us much. s_need_sync is used only here.  We set it against all
+ * filesystems and then clear it as we sync them.  So redirtied filesystems are
+ * skipped.
  *
  * But if process A is currently running sync_filesystems and then process B
- * calls sync_filesystems as well, process B will set all the s_need_sync_fs
- * flags again, which will cause process A to resync everything.  Fix that with
- * a local mutex.
+ * calls sync_filesystems as well, process B will set all the s_need_sync flags
+ * again, which will cause process A to resync everything.  Fix that with a
+ * local mutex.
  *
- * (Fabian) Avoid sync_fs with clean fs & wait mode 0
  */
-void sync_filesystems(int wait)
+void sync_filesystems(int what, int wait)
 {
 	struct super_block *sb;
 	static DEFINE_MUTEX(mutex);
@@ -472,26 +438,37 @@ void sync_filesystems(int wait)
 	mutex_lock(&mutex);		/* Could be down_interruptible */
 	spin_lock(&sb_lock);
 	list_for_each_entry(sb, &super_blocks, s_list) {
-		if (!sb->s_op->sync_fs)
+		if ((!sb->s_op->sync_fs || !(what & FSSYNC_FS)) &&
+		    (!sb->s_op->write_super || !(what & FSSYNC_SUPER)) &&
+		    !(what & FSSYNC_BDEV))
 			continue;
 		if (sb->s_flags & MS_RDONLY)
 			continue;
-		sb->s_need_sync_fs = 1;
+		sb->s_need_sync = 1;
 	}
 
 restart:
 	list_for_each_entry(sb, &super_blocks, s_list) {
-		if (!sb->s_need_sync_fs)
+		if (!sb->s_need_sync)
 			continue;
-		sb->s_need_sync_fs = 0;
+		sb->s_need_sync = 0;
 		if (sb->s_flags & MS_RDONLY)
 			continue;	/* hm.  Was remounted r/o meanwhile */
 		sb->s_count++;
 		spin_unlock(&sb_lock);
 		down_read(&sb->s_umount);
 		async_synchronize_full_domain(&sb->s_async_list);
-		if (sb->s_root && (wait || sb->s_dirt))
-			sb->s_op->sync_fs(sb, wait);
+		if (sb->s_root) {
+			lock_super(sb);
+			if (what & FSSYNC_SUPER && sb->s_dirt &&
+			    sb->s_op->write_super)
+				sb->s_op->write_super(sb);
+			unlock_super(sb);
+			if (what & FSSYNC_FS && sb->s_op->sync_fs)
+				sb->s_op->sync_fs(sb, wait);
+			if (what & FSSYNC_BDEV)
+				sync_blockdev(sb->s_bdev);
+		}
 		up_read(&sb->s_umount);
 		/* restart only when sb is no longer on the list */
 		spin_lock(&sb_lock);
@@ -502,29 +479,6 @@ restart:
 	mutex_unlock(&mutex);
 }
 
-/*
- *  Sync all block devices underlying some superblock
- */
-void sync_blockdevs(void)
-{
-	struct super_block *sb;
-
-	spin_lock(&sb_lock);
-restart:
-	list_for_each_entry(sb, &super_blocks, s_list) {
-		sb->s_count++;
-		spin_unlock(&sb_lock);
-		down_read(&sb->s_umount);
-		if (sb->s_root)
-			sync_blockdev(sb->s_bdev);
-		up_read(&sb->s_umount);
-		spin_lock(&sb_lock);
-		if (__put_super_and_need_restart(sb))
-			goto restart;
-	}
-	spin_unlock(&sb_lock);
-}
-
 /**
  *	get_super - get the superblock of a device
  *	@bdev: device to get the superblock for
diff --git a/fs/sync.c b/fs/sync.c
index fa14e42..3c677ee 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -26,11 +26,15 @@ static void do_sync(unsigned long wait)
 	wakeup_pdflush(0);
 	sync_inodes(0);		/* All mappings, inodes and their blockdevs */
 	vfs_dq_sync(NULL);
+	sync_filesystems(FSSYNC_FS, 0);	/* Start syncing the filesystems */
+	/*
+	 * We have to reliably submit IO for all the inodes before writing
+	 * super blocks and calling sync_fs(). Otherwise superblock could miss
+	 * some updates or journal could still have uncommitted data.
+	 */
 	sync_inodes(wait);	/* Mappings, inodes and blockdevs, again. */
-	sync_supers();		/* Write the superblocks */
-	sync_filesystems(0);	/* Start syncing the filesystems */
-	sync_filesystems(wait);	/* Waitingly sync the filesystems */
-	sync_blockdevs();
+	/* Waitingly sync the filesystems */
+	sync_filesystems(FSSYNC_SUPER | FSSYNC_FS | FSSYNC_BDEV, wait);
 	if (!wait)
 		printk("Emergency Sync complete\n");
 	if (unlikely(laptop_mode))
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4bad02e..2758e75 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1321,7 +1321,7 @@ struct super_block {
 	struct rw_semaphore	s_umount;
 	struct mutex		s_lock;
 	int			s_count;
-	int			s_need_sync_fs;
+	int			s_need_sync;
 	atomic_t		s_active;
 #ifdef CONFIG_SECURITY
 	void                    *s_security;
@@ -1942,7 +1942,6 @@ extern void bdput(struct block_device *);
 extern struct block_device *open_by_devnum(dev_t, fmode_t);
 extern void invalidate_bdev(struct block_device *);
 extern int sync_blockdev(struct block_device *bdev);
-extern void sync_blockdevs(void);
 extern struct super_block *freeze_bdev(struct block_device *);
 extern void emergency_thaw_all(void);
 extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
@@ -1952,7 +1951,6 @@ extern int fsync_no_super(struct block_device *);
 #else
 static inline void bd_forget(struct inode *inode) {}
 static inline int sync_blockdev(struct block_device *bdev) { return 0; }
-static inline void sync_blockdevs(void) { }
 static inline void invalidate_bdev(struct block_device *bdev) {}
 
 static inline struct super_block *freeze_bdev(struct block_device *sb)
@@ -2082,8 +2080,11 @@ extern int filemap_fdatawrite_range(struct address_space *mapping,
 				loff_t start, loff_t end);
 
 extern int vfs_fsync(struct file *file, struct dentry *dentry, int datasync);
-extern void sync_supers(void);
-extern void sync_filesystems(int wait);
+/* Flags telling what should be synced by sync_filesystems() */
+#define FSSYNC_FS 1
+#define FSSYNC_SUPER 2
+#define FSSYNC_BDEV 4
+extern void sync_filesystems(int what, int wait);
 extern void __fsync_super(struct super_block *sb);
 extern void emergency_sync(void);
 extern void emergency_remount(void);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 30351f0..05a2948 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -768,7 +768,7 @@ static void wb_kupdate(unsigned long arg)
 		.range_cyclic	= 1,
 	};
 
-	sync_supers();
+	sync_filesystems(FSSYNC_SUPER, 0);
 
 	oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval);
 	start_jif = jiffies;
-- 
1.6.0.2

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