From: Miklos Szeredi <mszeredi@suse.cz>

This patch allows fuse filesystems to implement open(..., O_TRUNC) as a single
request, instead of separate truncate and open requests.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


diff -puN fs/fuse/dir.c~fuse-add-atomic-opentruncate-support fs/fuse/dir.c
--- a/fs/fuse/dir.c~fuse-add-atomic-opentruncate-support
+++ a/fs/fuse/dir.c
@@ -1083,6 +1083,9 @@ static int fuse_do_setattr(struct dentry
 			return err;
 	}
 
+	if ((attr->ia_valid & ATTR_OPEN) && fc->atomic_o_trunc)
+		return 0;
+
 	if (attr->ia_valid & ATTR_SIZE) {
 		unsigned long limit;
 		if (IS_SWAPFILE(inode))
diff -puN fs/fuse/file.c~fuse-add-atomic-opentruncate-support fs/fuse/file.c
--- a/fs/fuse/file.c~fuse-add-atomic-opentruncate-support
+++ a/fs/fuse/file.c
@@ -28,7 +28,9 @@ static int fuse_send_open(struct inode *
 		return PTR_ERR(req);
 
 	memset(&inarg, 0, sizeof(inarg));
-	inarg.flags = file->f_flags & ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
+	inarg.flags = file->f_flags & ~(O_CREAT | O_EXCL | O_NOCTTY);
+	if (!fc->atomic_o_trunc)
+		inarg.flags &= ~O_TRUNC;
 	req->in.h.opcode = isdir ? FUSE_OPENDIR : FUSE_OPEN;
 	req->in.h.nodeid = get_node_id(inode);
 	req->in.numargs = 1;
diff -puN fs/fuse/fuse_i.h~fuse-add-atomic-opentruncate-support fs/fuse/fuse_i.h
--- a/fs/fuse/fuse_i.h~fuse-add-atomic-opentruncate-support
+++ a/fs/fuse/fuse_i.h
@@ -320,6 +320,9 @@ struct fuse_conn {
 	/** Do readpages asynchronously?  Only set in INIT */
 	unsigned async_read : 1;
 
+	/** Do not send separate SETATTR request before open(O_TRUNC)  */
+	unsigned atomic_o_trunc : 1;
+
 	/*
 	 * The following bitfields are only for optimization purposes
 	 * and hence races in setting them will not cause malfunction
diff -puN fs/fuse/inode.c~fuse-add-atomic-opentruncate-support fs/fuse/inode.c
--- a/fs/fuse/inode.c~fuse-add-atomic-opentruncate-support
+++ a/fs/fuse/inode.c
@@ -548,6 +548,8 @@ static void process_init_reply(struct fu
 				fc->async_read = 1;
 			if (!(arg->flags & FUSE_POSIX_LOCKS))
 				fc->no_lock = 1;
+			if (arg->flags & FUSE_ATOMIC_O_TRUNC)
+				fc->atomic_o_trunc = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_CACHE_SIZE;
 			fc->no_lock = 1;
@@ -570,7 +572,8 @@ static void fuse_send_init(struct fuse_c
 	arg->major = FUSE_KERNEL_VERSION;
 	arg->minor = FUSE_KERNEL_MINOR_VERSION;
 	arg->max_readahead = fc->bdi.ra_pages * PAGE_CACHE_SIZE;
-	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_FILE_OPS;
+	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_FILE_OPS |
+		FUSE_ATOMIC_O_TRUNC;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);
diff -puN include/linux/fuse.h~fuse-add-atomic-opentruncate-support include/linux/fuse.h
--- a/include/linux/fuse.h~fuse-add-atomic-opentruncate-support
+++ a/include/linux/fuse.h
@@ -101,6 +101,7 @@ struct fuse_file_lock {
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
 #define FUSE_FILE_OPS		(1 << 2)
+#define FUSE_ATOMIC_O_TRUNC	(1 << 3)
 
 /**
  * Release flags
_