fusefs: enable write clustering

Enable write clustering in fusefs whenever cache mode is set to writeback
and the "async" mount option is used.  With default values for MAXPHYS,
DFLTPHYS, and the fuse max_write mount parameter, that means sequential
writes will now be written 128KB at a time instead of 64KB.

Also, add a regression test for PR 238565, a panic during unmount that
probably affects UFS, ext2, and msdosfs as well as fusefs.

PR:		238565
Sponsored by:	The FreeBSD Foundation

2 files changed, 10 insertions, 5 deletions

diff --git a/sbin/mount_fusefs/mount_fusefs.8 b/sbin/mount_fusefs/mount_fusefs.8
index 30eb7c0acb74..259101a38630 100644
--- a/sbin/mount_fusefs/mount_fusefs.8
+++ b/sbin/mount_fusefs/mount_fusefs.8
@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 19, 2019
+.Dd June 14, 2019
 .Dt MOUNT_FUSEFS 8
 .Os
 .Sh NAME
@@ -136,21 +136,24 @@ The following options are available (and also their negated versions,
 by prefixing them with
 .Dq no ) :
 .Bl -tag -width indent
-.It Cm default_permissions
-Enable traditional (file mode based) permission checking in kernel
 .It Cm allow_other
 Do not apply
 .Sx STRICT ACCESS POLICY .
 Only root can use this option
+.It Cm async
+I/O to the file system may be done asynchronously.
+Writes may delayed and/or reordered.
+.It Cm default_permissions
+Enable traditional (file mode based) permission checking in kernel
 .It Cm max_read Ns = Ns Ar n
 Limit size of read requests to
 .Ar n
+.It Cm neglect_shares
+Do not refuse unmounting if there are secondary mounts
 .It Cm private
 Refuse shared mounting of the daemon.
 This is the default behaviour, to allow sharing, expicitly use
 .Fl o Cm noprivate
-.It Cm neglect_shares
-Do not refuse unmounting if there are secondary mounts
 .It Cm push_symlinks_in
 Prefix absolute symlinks with the mountpoint
 .It Cm subtype Ns = Ns Ar fsname
diff --git a/sbin/mount_fusefs/mount_fusefs.c b/sbin/mount_fusefs/mount_fusefs.c
index e6131ec8b063..b5dda5077dc4 100644
--- a/sbin/mount_fusefs/mount_fusefs.c
+++ b/sbin/mount_fusefs/mount_fusefs.c
@@ -88,6 +88,8 @@ static struct mntopt mopts[] = {
 	{ "large_read",          0, 0x00, 1 },
 	/* "nonempty", just the first two chars are stripped off during parsing */
 	{ "nempty",              0, 0x00, 1 },
+	{ "async",               0, MNT_ASYNC, 0},
+	{ "noasync",             1, MNT_ASYNC, 0},
 	MOPT_STDOPTS,
 	MOPT_END
 };