[PATCH 2/4] Unionfs: mmap fixes

[Erez Zadok <ezk@cs.sunysb.edu>]

Ensure we have lower inodes in prepare/commit_write.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
---
 fs/unionfs/mmap.c |   26 +++++++++++++++++++++-----
 1 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index a0e654b..ad770ac 100644
--- a/fs/unionfs/mmap.c
+++ b/fs/unionfs/mmap.c
@@ -224,13 +224,26 @@ out:
 static int unionfs_prepare_write(struct file *file, struct page *page,
 				 unsigned from, unsigned to)
 {
+	int err;
+
+	unionfs_read_lock(file->f_path.dentry->d_sb, UNIONFS_SMUTEX_PARENT);
 	/*
-	 * Just copy lower inode attributes and return success.  Not much
-	 * else to do here.  No need to lock either (lockdep won't like it).
-	 * Let commit_write do all the hard work instead.
+	 * This is the only place where we unconditionally copy the lower
+	 * attribute times before calling unionfs_file_revalidate.  The
+	 * reason is that our ->write calls do_sync_write which in turn will
+	 * call our ->prepare_write and then ->commit_write.  Before our
+	 * ->write is called, the lower mtimes are in sync, but by the time
+	 * the VFS calls our ->commit_write, the lower mtimes have changed.
+	 * Therefore, the only reasonable time for us to sync up from the
+	 * changed lower mtimes, and avoid an invariant violation warning,
+	 * is here, in ->prepare_write.
 	 */
 	unionfs_copy_attr_times(file->f_path.dentry->d_inode);
-	return 0;
+	err = unionfs_file_revalidate(file, true);
+	unionfs_check_file(file);
+	unionfs_read_unlock(file->f_path.dentry->d_sb);
+
+	return err;
 }
 
 static int unionfs_commit_write(struct file *file, struct page *page,
@@ -252,7 +265,6 @@ static int unionfs_commit_write(struct file *file, struct page *page,
 	unionfs_check_file(file);
 
 	inode = page->mapping->host;
-	lower_inode = unionfs_lower_inode(inode);
 
 	if (UNIONFS_F(file) != NULL)
 		lower_file = unionfs_lower_file(file);
@@ -282,6 +294,10 @@ static int unionfs_commit_write(struct file *file, struct page *page,
 		goto out;
 
 	/* if vfs_write succeeded above, sync up our times/sizes */
+	lower_inode = lower_file->f_path.dentry->d_inode;
+	if (!lower_inode)
+		lower_inode = unionfs_lower_inode(inode);
+	BUG_ON(!lower_inode);
 	fsstack_copy_inode_size(inode, lower_inode);
 	unionfs_copy_attr_times(inode);
 	mark_inode_dirty_sync(inode);
-- 
1.5.2.2