[Libguestfs] [PATCH v2] fuse: Allow fallocate(FALLOC_FL_ZERO_RANGE)

[Shachar Sharon]

On Wed, May 12, 2021 at 11:37:04AM +0100, Richard W.M. Jones wrote:
>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.
>
Why don't you call '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 at 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);
To stay aligned with existing code style, consider:
-			   (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 +2928,8 @@ 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
>