fuse: allow fallocate(FALLOC_FL_ZERO_RANGE)

The current fuse module filters out fallocate(FALLOC_FL_ZERO_RANGE)
returning -EOPNOTSUPP.  libnbd's nbdfuse would like to translate
FALLOC_FL_ZERO_RANGE requests into the NBD command
NBD_CMD_WRITE_ZEROES which allows NBD servers that support it to do
zeroing efficiently.

This commit treats this flag exactly like FALLOC_FL_PUNCH_HOLE.

A way to test this, requiring fuse >= 3, nbdkit >= 1.8 and the latest
nbdfuse from https://gitlab.com/nbdkit/libnbd/-/tree/master/fuse is to
create a file containing some data and "mirror" it to a fuse file:

  $ dd if=/dev/urandom of=disk.img bs=1M count=1
  $ nbdkit file disk.img
  $ touch mirror.img
  $ nbdfuse mirror.img nbd://localhost &

(mirror.img -> nbdfuse -> NBD over loopback -> nbdkit -> disk.img)

You can then run commands such as:

  $ fallocate -z -o 1024 -l 1024 mirror.img

and check that the content of the original file ("disk.img") stays
synchronized.  To show NBD commands, export LIBNBD_DEBUG=1 before
running nbdfuse.  To clean up:

  $ fusermount3 -u mirror.img
  $ killall nbdkit

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: rwmjones

fs/fuse/file.c

@@ -2907,11 +2907,13 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	};
 	int err;
 	bool lock_inode = !(mode & FALLOC_FL_KEEP_SIZE) ||
-			   (mode & FALLOC_FL_PUNCH_HOLE);
+			   (mode & (FALLOC_FL_PUNCH_HOLE |
+				    FALLOC_FL_ZERO_RANGE));
 
 	bool block_faults = FUSE_IS_DAX(inode) && lock_inode;
 
-	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
+	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |
+		     FALLOC_FL_ZERO_RANGE))
 		return -EOPNOTSUPP;
 
 	if (fm->fc->no_fallocate)
@@ -2926,7 +2928,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 				goto out;
 		}
 
-		if (mode & FALLOC_FL_PUNCH_HOLE) {
+		if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)) {
 			loff_t endbyte = offset + length - 1;
 
 			err = fuse_writeback_range(inode, offset, endbyte);
@@ -2966,7 +2968,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 			file_update_time(file);
 	}
 
-	if (mode & FALLOC_FL_PUNCH_HOLE)
+	if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE))
 		truncate_pagecache_range(inode, offset, offset + length - 1);
 
 	fuse_invalidate_attr(inode);