Commit 6b1bdb56 authored May 12, 2021 by Richard W.M. Jones Committed by Miklos Szeredi Jun 22, 2021

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>

parent 1b539917

fs/fuse/file.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -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);