VFS: allow filesystems to implement atomic open+truncate

Add a new attribute flag ATTR_OPEN, with the meaning: "truncation was
initiated by open() due to the O_TRUNC flag".

This way filesystems wanting to implement truncation within their ->open()
method can ignore such truncate requests.

This is a quick & dirty hack, but it comes for free.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -1659,8 +1659,10 @@ int may_open(struct nameidata *nd, int acc_mode, int flag)
 		error = locks_verify_locked(inode);
 		if (!error) {
 			DQUOT_INIT(inode);
-
-			error = do_truncate(dentry, 0, ATTR_MTIME|ATTR_CTIME, NULL);
+
+			error = do_truncate(dentry, 0,
+					    ATTR_MTIME|ATTR_CTIME|ATTR_OPEN,
+					    NULL);
 		}
 		put_write_access(inode);
 		if (error)

include/linux/fs.h

@@ -330,6 +330,7 @@ typedef void (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 #define ATTR_KILL_SGID	4096
 #define ATTR_FILE	8192
 #define ATTR_KILL_PRIV	16384
+#define ATTR_OPEN	32768	/* Truncating from open(O_TRUNC) */
 
 /*
  * This is the Inode Attributes structure, used for notify_change().  It