[PATCH v2] fuse: Allow fallocate(FALLOC_FL_ZERO_RANGE)

["Richard W.M. Jones" <rjones@redhat.com>]

libnbd's nbdfuse utility would like to translate fallocate zero
requests into NBD_CMD_WRITE_ZEROES.  Currently the fuse module filters
these out, returning -EOPNOTSUPP.  This commit treats these almost the
same way as FALLOC_FL_PUNCH_HOLE except not calling
truncate_pagecache_range.

A way to test this is with the following script:

--------------------
  #!/bin/bash
  # Requires fuse >= 3, nbdkit >= 1.8, and latest nbdfuse from
  # https://gitlab.com/nbdkit/libnbd/-/tree/master/fuse
  set -e
  set -x

  export output=$PWD/output
  rm -f test.img $output

  # Create an nbdkit instance that prints the NBD requests seen.
  nbdkit sh - <<'EOF'
  case "$1" in
    get_size) echo 1M ;;
    can_write|can_trim|can_zero|can_fast_zero) ;;
    pread) echo "$@" >>$output; dd if=/dev/zero count=$3 iflag=count_bytes ;;
    pwrite) echo "$@" >>$output; cat >/dev/null ;;
    trim|zero) echo "$@" >>$output ;;
    *) exit 2 ;;
  esac
  EOF

  # Fuse-mount NBD instance as a file.
  touch test.img
  nbdfuse test.img nbd://localhost & sleep 2
  ls -lh test.img

  # Run a read, write, trim and zero request.
  dd if=test.img of=/dev/null bs=512 skip=1024 count=1
  dd if=/dev/zero of=test.img bs=512 skip=2048 count=1
  fallocate -p -l 512 -o 4096 test.img
  fallocate -z -l 512 -o 8192 test.img

  # Print the output from the NBD server.
  cat $output

  # Clean up.
  fusermount3 -u test.img
  killall nbdkit
  rm test.img $output
  --------------------

which will print:

  pread  4096 524288    # number depends on readahead
  pwrite  512 0
  trim  512 4096
  zero  512 8192 may_trim

The last line indicates that the FALLOC_FL_ZERO_RANGE request was
successfully passed through by the kernel module to nbdfuse,
translated to NBD_CMD_WRITE_ZEROES and sent through to the server.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
 fs/fuse/file.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 09ef2a4d25ed..22e8e88c78d4 100644
--- a/fs/fuse/file.c
+++ b/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) ||
+			   (mode & 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 +2928,8 @@ 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) ||
+		    (mode & FALLOC_FL_ZERO_RANGE)) {
 			loff_t endbyte = offset + length - 1;
 
 			err = fuse_writeback_range(inode, offset, endbyte);
-- 
2.31.1