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Subject[patch 4/5] VFS: allow filesystems to implement atomic open+truncate
From: Miklos Szeredi <mszeredi@suse.cz>

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>
---
Index: linux/fs/namei.c
===================================================================
--- linux.orig/fs/namei.c 2007-09-21 13:44:53.000000000 +0200
+++ linux/fs/namei.c 2007-09-21 13:45:14.000000000 +0200
@@ -1656,8 +1656,10 @@ int may_open(struct nameidata *nd, int a
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)
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h 2007-09-21 13:45:11.000000000 +0200
+++ linux/include/linux/fs.h 2007-09-21 13:45:14.000000000 +0200
@@ -337,6 +337,7 @@ typedef void (dio_iodone_t)(struct kiocb
#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
@@ -1526,7 +1527,7 @@ static inline int break_lease(struct ino

/* fs/open.c */

-extern int do_truncate(struct dentry *, loff_t start, unsigned int time_attrs,
+extern int do_truncate(struct dentry *, loff_t start, unsigned int attrs,
struct file *filp);
extern long do_sys_open(int fdf, const char __user *filename, int flags,
int mode);
Index: linux/fs/open.c
===================================================================
--- linux.orig/fs/open.c 2007-09-21 13:45:04.000000000 +0200
+++ linux/fs/open.c 2007-09-21 13:45:14.000000000 +0200
@@ -194,7 +194,7 @@ out:
return error;
}

-int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
+int do_truncate(struct dentry *dentry, loff_t length, unsigned int attrs,
struct file *filp)
{
int err;
@@ -205,7 +205,7 @@ int do_truncate(struct dentry *dentry, l
return -EINVAL;

newattrs.ia_size = length;
- newattrs.ia_valid = ATTR_SIZE | time_attrs;
+ newattrs.ia_valid = ATTR_SIZE | attrs;
if (filp) {
newattrs.ia_file = filp;
newattrs.ia_valid |= ATTR_FILE;
--
-
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