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



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 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



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