fs: sync_sb_inodes fix

Fix data integrity semantics required by sys_sync, by iterating over all
inodes and waiting for any writeback pages after the initial writeout.
Comments explain the exact problem.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -440,6 +440,7 @@ void generic_sync_sb_inodes(struct super_block *sb,
 				struct writeback_control *wbc)
 {
 	const unsigned long start = jiffies;	/* livelock avoidance */
+	int sync = wbc->sync_mode == WB_SYNC_ALL;
 
 	spin_lock(&inode_lock);
 	if (!wbc->for_kupdate || list_empty(&sb->s_io))
@@ -516,7 +517,49 @@ void generic_sync_sb_inodes(struct super_block *sb,
 		if (!list_empty(&sb->s_more_io))
 			wbc->more_io = 1;
 	}
-	spin_unlock(&inode_lock);
+
+	if (sync) {
+		struct inode *inode, *old_inode = NULL;
+
+		/*
+		 * Data integrity sync. Must wait for all pages under writeback,
+		 * because there may have been pages dirtied before our sync
+		 * call, but which had writeout started before we write it out.
+		 * In which case, the inode may not be on the dirty list, but
+		 * we still have to wait for that writeout.
+		 */
+		list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+			struct address_space *mapping;
+
+			if (inode->i_state & (I_FREEING|I_WILL_FREE))
+				continue;
+			mapping = inode->i_mapping;
+			if (mapping->nrpages == 0)
+				continue;
+			__iget(inode);
+			spin_unlock(&inode_lock);
+			/*
+			 * We hold a reference to 'inode' so it couldn't have
+			 * been removed from s_inodes list while we dropped the
+			 * inode_lock.  We cannot iput the inode now as we can
+			 * be holding the last reference and we cannot iput it
+			 * under inode_lock. So we keep the reference and iput
+			 * it later.
+			 */
+			iput(old_inode);
+			old_inode = inode;
+
+			filemap_fdatawait(mapping);
+
+			cond_resched();
+
+			spin_lock(&inode_lock);
+		}
+		spin_unlock(&inode_lock);
+		iput(old_inode);
+	} else
+		spin_unlock(&inode_lock);
+
 	return;		/* Leave any unwritten inodes on s_io */
 }
 EXPORT_SYMBOL_GPL(generic_sync_sb_inodes);
@@ -596,13 +639,16 @@ void sync_inodes_sb(struct super_block *sb, int wait)
 		.range_start	= 0,
 		.range_end	= LLONG_MAX,
 	};
-	unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
-	unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
 
-	wbc.nr_to_write = nr_dirty + nr_unstable +
-			(inodes_stat.nr_inodes - inodes_stat.nr_unused) +
-			nr_dirty + nr_unstable;
-	wbc.nr_to_write += wbc.nr_to_write / 2;		/* Bit more for luck */
+	if (!wait) {
+		unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
+		unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
+
+		wbc.nr_to_write = nr_dirty + nr_unstable +
+			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
+	} else
+		wbc.nr_to_write = LONG_MAX; /* doesn't actually matter */
+
 	sync_sb_inodes(sb, &wbc);
 }