[RFC][PATCH v2 4/4] vfs: wrap write f_ops with file_{start,end}_write()

[Amir Goldstein <amir73il@gmail.com>]

Before calling write f_ops, call file_start_write() instead
of sb_start_write().

This ensures freeze protection for both overlay and upper fs
when file is open from an overlayfs mount.

Replace {sb,file}_start_write() for {copy,clone}_file_range() and
for fallocate().

For dedup_file_range() there is no need for mnt_want_write_file().
File is already open for write, so we already have mnt_want_write()
and we only need file_start_write().

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/open.c          |  4 ++--
 fs/read_write.c    | 12 ++++--------
 include/linux/fs.h | 26 +++++++++++++-------------
 3 files changed, 19 insertions(+), 23 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 9921f70..d3fe1a1 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -316,7 +316,7 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	if (!file->f_op->fallocate)
 		return -EOPNOTSUPP;
 
-	sb_start_write(inode->i_sb);
+	file_start_write(file);
 	ret = file->f_op->fallocate(file, mode, offset, len);
 
 	/*
@@ -329,7 +329,7 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	if (ret == 0)
 		fsnotify_modify(file);
 
-	sb_end_write(inode->i_sb);
+	file_end_write(file);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(vfs_fallocate);
diff --git a/fs/read_write.c b/fs/read_write.c
index 5816d4c..1e42487 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1538,7 +1538,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	if (len == 0)
 		return 0;
 
-	sb_start_write(inode_out->i_sb);
+	file_start_write(file_out);
 
 	/*
 	 * Try cloning first, this is supported by more file systems, and
@@ -1574,7 +1574,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	inc_syscr(current);
 	inc_syscw(current);
 
-	sb_end_write(inode_out->i_sb);
+	file_end_write(file_out);
 
 	return ret;
 }
@@ -1976,11 +1976,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 		}
 		dst = file_inode(dst_file);
 
-		ret = mnt_want_write_file(dst_file);
-		if (ret) {
-			info->status = ret;
-			goto next_loop;
-		}
+		file_start_write(dst_file);
 
 		dst_off = info->dest_offset;
 		ret = clone_verify_area(dst_file, dst_off, len, true);
@@ -2013,7 +2009,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 		}
 
 next_file:
-		mnt_drop_write_file(dst_file);
+		file_end_write(dst_file);
 next_loop:
 		fdput(dst_fd);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 71811be..8a4dbc6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1741,19 +1741,6 @@ extern int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
 extern int vfs_dedupe_file_range(struct file *file,
 				 struct file_dedupe_range *same);
 
-static inline int do_clone_file_range(struct file *file_in, loff_t pos_in,
-				      struct file *file_out, loff_t pos_out,
-				      u64 len)
-{
-	int ret;
-
-	sb_start_write(file_inode(file_out)->i_sb);
-	ret = vfs_clone_file_range(file_in, pos_in, file_out, pos_out, len);
-	sb_end_write(file_inode(file_out)->i_sb);
-
-	return ret;
-}
-
 struct super_operations {
    	struct inode *(*alloc_inode)(struct super_block *sb);
 	void (*destroy_inode)(struct inode *);
@@ -2579,6 +2566,19 @@ static inline void file_end_write(struct file *file)
 	__sb_end_write(locks_inode(file)->i_sb, SB_FREEZE_WRITE);
 }
 
+static inline int do_clone_file_range(struct file *file_in, loff_t pos_in,
+				      struct file *file_out, loff_t pos_out,
+				      u64 len)
+{
+	int ret;
+
+	file_start_write(file_out);
+	ret = vfs_clone_file_range(file_in, pos_in, file_out, pos_out, len);
+	file_end_write(file_out);
+
+	return ret;
+}
+
 /*
  * get_write_access() gets write permission for a file.
  * put_write_access() releases this write permission.
-- 
2.7.4