mark struct file that had write access grabbed by open()

new flag in ->f_mode - FMODE_WRITER.  Set by do_dentry_open() in case
when it has grabbed write access, checked by __fput() to decide whether
it wants to drop the sucker.  Allows to stop bothering with mnt_clone_write()
in alloc_file(), along with fewer special_file() checks.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -177,43 +177,12 @@ struct file *alloc_file(struct path *path, fmode_t mode,
 	file->f_mapping = path->dentry->d_inode->i_mapping;
 	file->f_mode = mode;
 	file->f_op = fop;
-
-	/*
-	 * These mounts don't really matter in practice
-	 * for r/o bind mounts.  They aren't userspace-
-	 * visible.  We do this for consistency, and so
-	 * that we can do debugging checks at __fput()
-	 */
-	if ((mode & FMODE_WRITE) && !special_file(path->dentry->d_inode->i_mode)) {
-		WARN_ON(mnt_clone_write(path->mnt));
-	}
 	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		i_readcount_inc(path->dentry->d_inode);
 	return file;
 }
 EXPORT_SYMBOL(alloc_file);
 
-/**
- * drop_file_write_access - give up ability to write to a file
- * @file: the file to which we will stop writing
- *
- * This is a central place which will give up the ability
- * to write to @file, along with access to write through
- * its vfsmount.
- */
-static void drop_file_write_access(struct file *file)
-{
-	struct vfsmount *mnt = file->f_path.mnt;
-	struct dentry *dentry = file->f_path.dentry;
-	struct inode *inode = dentry->d_inode;
-
-	if (special_file(inode->i_mode))
-		return;
-
-	put_write_access(inode);
-	__mnt_drop_write(mnt);
-}
-
 /* the real guts of fput() - releasing the last reference to file
  */
 static void __fput(struct file *file)
@@ -248,8 +217,10 @@ static void __fput(struct file *file)
 	put_pid(file->f_owner.pid);
 	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		i_readcount_dec(inode);
-	if (file->f_mode & FMODE_WRITE)
-		drop_file_write_access(file);
+	if (file->f_mode & FMODE_WRITER) {
+		put_write_access(inode);
+		__mnt_drop_write(mnt);
+	}
 	file->f_path.dentry = NULL;
 	file->f_path.mnt = NULL;
 	file->f_inode = NULL;

fs/namespace.c

@@ -414,9 +414,7 @@ EXPORT_SYMBOL_GPL(mnt_clone_write);
  */
 int __mnt_want_write_file(struct file *file)
 {
-	struct inode *inode = file_inode(file);
-
-	if (!(file->f_mode & FMODE_WRITE) || special_file(inode->i_mode))
+	if (!(file->f_mode & FMODE_WRITER))
 		return __mnt_want_write(file->f_path.mnt);
 	else
 		return mnt_clone_write(file->f_path.mnt);

fs/open.c

@@ -670,6 +670,7 @@ static int do_dentry_open(struct file *f,
 			put_write_access(inode);
 			goto cleanup_file;
 		}
+		f->f_mode |= FMODE_WRITER;
 	}
 
 	f->f_mapping = inode->i_mapping;
@@ -715,11 +716,9 @@ static int do_dentry_open(struct file *f,
 
 cleanup_all:
 	fops_put(f->f_op);
-	if (f->f_mode & FMODE_WRITE) {
-		if (!special_file(inode->i_mode)) {
-			put_write_access(inode);
-			__mnt_drop_write(f->f_path.mnt);
-		}
+	if (f->f_mode & FMODE_WRITER) {
+		put_write_access(inode);
+		__mnt_drop_write(f->f_path.mnt);
 	}
 cleanup_file:
 	path_put(&f->f_path);

include/linux/fs.h

@@ -125,6 +125,8 @@ typedef void (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 
 /* File needs atomic accesses to f_pos */
 #define FMODE_ATOMIC_POS	((__force fmode_t)0x8000)
+/* Write access to underlying fs */
+#define FMODE_WRITER		((__force fmode_t)0x10000)
 
 /* File was opened by fanotify and shouldn't generate fanotify events */
 #define FMODE_NONOTIFY		((__force fmode_t)0x1000000)