fuse: add support for SEEK_HOLE and SEEK_DATA in lseek

A useful performance improvement for accessing virtual machine images
via FUSE mount.

See https://bugzilla.redhat.com/show_bug.cgi?id=1220173 for a use-case
for glusterFS.
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

fs/fuse/file.c

@@ -2231,20 +2231,77 @@ static sector_t fuse_bmap(struct address_space *mapping, sector_t block)
 	return err ? 0 : outarg.block;
 }
 
+static loff_t fuse_lseek(struct file *file, loff_t offset, int whence)
+{
+	struct inode *inode = file->f_mapping->host;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_file *ff = file->private_data;
+	FUSE_ARGS(args);
+	struct fuse_lseek_in inarg = {
+		.fh = ff->fh,
+		.offset = offset,
+		.whence = whence
+	};
+	struct fuse_lseek_out outarg;
+	int err;
+
+	if (fc->no_lseek)
+		goto fallback;
+
+	args.in.h.opcode = FUSE_LSEEK;
+	args.in.h.nodeid = ff->nodeid;
+	args.in.numargs = 1;
+	args.in.args[0].size = sizeof(inarg);
+	args.in.args[0].value = &inarg;
+	args.out.numargs = 1;
+	args.out.args[0].size = sizeof(outarg);
+	args.out.args[0].value = &outarg;
+	err = fuse_simple_request(fc, &args);
+	if (err) {
+		if (err == -ENOSYS) {
+			fc->no_lseek = 1;
+			goto fallback;
+		}
+		return err;
+	}
+
+	return vfs_setpos(file, outarg.offset, inode->i_sb->s_maxbytes);
+
+fallback:
+	err = fuse_update_attributes(inode, NULL, file, NULL);
+	if (!err)
+		return generic_file_llseek(file, offset, whence);
+	else
+		return err;
+}
+
 static loff_t fuse_file_llseek(struct file *file, loff_t offset, int whence)
 {
 	loff_t retval;
 	struct inode *inode = file_inode(file);
 
-	/* No i_mutex protection necessary for SEEK_CUR and SEEK_SET */
-	if (whence == SEEK_CUR || whence == SEEK_SET)
-		return generic_file_llseek(file, offset, whence);
-
-	mutex_lock(&inode->i_mutex);
-	retval = fuse_update_attributes(inode, NULL, file, NULL);
-	if (!retval)
+	switch (whence) {
+	case SEEK_SET:
+	case SEEK_CUR:
+		 /* No i_mutex protection necessary for SEEK_CUR and SEEK_SET */
 		retval = generic_file_llseek(file, offset, whence);
-	mutex_unlock(&inode->i_mutex);
+		break;
+	case SEEK_END:
+		mutex_lock(&inode->i_mutex);
+		retval = fuse_update_attributes(inode, NULL, file, NULL);
+		if (!retval)
+			retval = generic_file_llseek(file, offset, whence);
+		mutex_unlock(&inode->i_mutex);
+		break;
+	case SEEK_HOLE:
+	case SEEK_DATA:
+		mutex_lock(&inode->i_mutex);
+		retval = fuse_lseek(file, offset, whence);
+		mutex_unlock(&inode->i_mutex);
+		break;
+	default:
+		retval = -EINVAL;
+	}
 
 	return retval;
 }

fs/fuse/fuse_i.h

@@ -605,6 +605,9 @@ struct fuse_conn {
 	/** Does the filesystem support asynchronous direct-IO submission? */
 	unsigned async_dio:1;
 
+	/** Is lseek not implemented by fs? */
+	unsigned no_lseek:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 

include/uapi/linux/fuse.h

@@ -102,6 +102,9 @@
  *  - add ctime and ctimensec to fuse_setattr_in
  *  - add FUSE_RENAME2 request
  *  - add FUSE_NO_OPEN_SUPPORT flag
+ *
+ *  7.24
+ *  - add FUSE_LSEEK for SEEK_HOLE and SEEK_DATA support
  */
 
 #ifndef _LINUX_FUSE_H
@@ -137,7 +140,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 23
+#define FUSE_KERNEL_MINOR_VERSION 24
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -358,6 +361,7 @@ enum fuse_opcode {
 	FUSE_FALLOCATE     = 43,
 	FUSE_READDIRPLUS   = 44,
 	FUSE_RENAME2       = 45,
+	FUSE_LSEEK         = 46,
 
 	/* CUSE specific operations */
 	CUSE_INIT          = 4096,
@@ -758,4 +762,15 @@ struct fuse_notify_retrieve_in {
 /* Device ioctls: */
 #define FUSE_DEV_IOC_CLONE	_IOR(229, 0, uint32_t)
 
+struct fuse_lseek_in {
+	uint64_t	fh;
+	uint64_t	offset;
+	uint32_t	whence;
+	uint32_t	padding;
+};
+
+struct fuse_lseek_out {
+	uint64_t	offset;
+};
+
 #endif /* _LINUX_FUSE_H */