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>

2 files changed, 5 insertions, 2 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 464eeccb675b..1e5c71669164 100644
--- a/fs/namei.c
+++ b/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)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e3fc5dbb2246..6a4d170ad9a5 100644
--- a/include/linux/fs.h
+++ b/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