[PATCH v2 1/3] fs: get mnt_writers count for an open backing file's real path

[Amir Goldstein <amir73il@gmail.com>]

A writeable mapped backing file can perform writes to the real inode.
Therefore, the real path mount must be kept writable so long as the
writable map exists.

This may not be strictly needed for ovelrayfs private upper mount,
but it is correct to take the mnt_writers count in the vfs helper.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/internal.h | 15 +++++++++++++--
 fs/open.c     | 34 ++++++++++++++++++++++++++++------
 2 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/fs/internal.h b/fs/internal.h
index f08d8fe3ae5e..846d5133dd9c 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -96,13 +96,24 @@ struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred);
 struct file *alloc_empty_backing_file(int flags, const struct cred *cred);
 void release_empty_file(struct file *f);
 
+static inline void file_put_write_access(struct file *file)
+{
+	put_write_access(file->f_inode);
+	mnt_put_write_access(file->f_path.mnt);
+	if (unlikely(file->f_mode & FMODE_BACKING)) {
+		struct path *real_path = backing_file_real_path(file);
+
+		if (real_path->mnt)
+			mnt_put_write_access(real_path->mnt);
+	}
+}
+
 static inline void put_file_access(struct file *file)
 {
 	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
 		i_readcount_dec(file->f_inode);
 	} else if (file->f_mode & FMODE_WRITER) {
-		put_write_access(file->f_inode);
-		mnt_put_write_access(file->f_path.mnt);
+		file_put_write_access(file);
 	}
 }
 
diff --git a/fs/open.c b/fs/open.c
index a65ce47810cf..2f3e28512663 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -870,6 +870,33 @@ SYSCALL_DEFINE3(fchown, unsigned int, fd, uid_t, user, gid_t, group)
 	return ksys_fchown(fd, user, group);
 }
 
+static inline int file_get_write_access(struct file *f)
+{
+	int error;
+
+	error = get_write_access(f->f_inode);
+	if (unlikely(error))
+		return error;
+	error = mnt_get_write_access(f->f_path.mnt);
+	if (unlikely(error))
+		goto cleanup_inode;
+	if (unlikely(f->f_mode & FMODE_BACKING)) {
+		struct path *real_path = backing_file_real_path(f);
+
+		if (real_path->mnt)
+			error = mnt_get_write_access(real_path->mnt);
+		if (unlikely(error))
+			goto cleanup_mnt;
+	}
+	return 0;
+
+cleanup_mnt:
+	mnt_put_write_access(f->f_path.mnt);
+cleanup_inode:
+	put_write_access(f->f_inode);
+	return error;
+}
+
 static int do_dentry_open(struct file *f,
 			  struct inode *inode,
 			  int (*open)(struct inode *, struct file *))
@@ -892,14 +919,9 @@ static int do_dentry_open(struct file *f,
 	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
 		i_readcount_inc(inode);
 	} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
-		error = get_write_access(inode);
+		error = file_get_write_access(f);
 		if (unlikely(error))
 			goto cleanup_file;
-		error = mnt_get_write_access(f->f_path.mnt);
-		if (unlikely(error)) {
-			put_write_access(inode);
-			goto cleanup_file;
-		}
 		f->f_mode |= FMODE_WRITER;
 	}
 
-- 
2.34.1