Add a tunable to disable BRT support.

Copy the disable parameter that FreeBSD implemented, and extend it to
work on Linux as well, until we're sure this is stable.

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

include/os/freebsd/zfs/sys/zfs_vfsops_os.h

@@ -285,6 +285,7 @@ typedef struct zfid_long {
 #define	LONG_FID_LEN	(sizeof (zfid_long_t) - sizeof (uint16_t))
 
 extern int zfs_super_owner;
+extern int zfs_bclone_enabled;
 
 extern void zfs_init(void);
 extern void zfs_fini(void);

include/os/linux/zfs/sys/zfs_vfsops_os.h

@@ -45,6 +45,8 @@ extern "C" {
 typedef struct zfsvfs zfsvfs_t;
 struct znode;
 
+extern int zfs_bclone_enabled;
+
 /*
  * This structure emulates the vfs_t from other platforms.  It's purpose
  * is to facilitate the handling of mount options and minimize structural

man/man4/zfs.4

@@ -1154,6 +1154,11 @@ Selecting any option other than
 results in vector instructions
 from the respective CPU instruction set being used.
 .
+.It Sy zfs_bclone_enabled Ns = Ns Sy 0 Ns | Ns 1 Pq int
+Enable the experimental block cloning feature.
+If this setting is 0, then even if feature@block_cloning is enabled,
+attempts to clone blocks will act as though the feature is disabled.
+.
 .It Sy zfs_blake3_impl Ns = Ns Sy fastest Pq string
 Select a BLAKE3 implementation.
 .Pp

module/os/freebsd/zfs/zfs_vfsops.c

@@ -89,6 +89,10 @@ int zfs_debug_level;
 SYSCTL_INT(_vfs_zfs, OID_AUTO, debug, CTLFLAG_RWTUN, &zfs_debug_level, 0,
 	"Debug level");
 
+int zfs_bclone_enabled;
+SYSCTL_INT(_vfs_zfs, OID_AUTO, bclone_enabled, CTLFLAG_RWTUN,
+	&zfs_bclone_enabled, 0, "Enable block cloning");
+
 struct zfs_jailparam {
 	int mount_snapshot;
 };

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -6250,6 +6250,11 @@ zfs_freebsd_copy_file_range(struct vop_copy_file_range_args *ap)
 	int error;
 	uint64_t len = *ap->a_lenp;
 
+	if (!zfs_bclone_enabled) {
+		mp = NULL;
+		goto bad_write_fallback;
+	}
+
 	/*
 	 * TODO: If offset/length is not aligned to recordsize, use
 	 * vn_generic_copy_file_range() on this fragment.

module/os/linux/zfs/zfs_vnops_os.c

@@ -4249,4 +4249,8 @@ EXPORT_SYMBOL(zfs_map);
 module_param(zfs_delete_blocks, ulong, 0644);
 MODULE_PARM_DESC(zfs_delete_blocks, "Delete files larger than N blocks async");
 
+/* CSTYLED */
+module_param(zfs_bclone_enabled, uint, 0644);
+MODULE_PARM_DESC(zfs_bclone_enabled, "Enable block cloning");
+
 #endif

module/os/linux/zfs/zpl_file_range.c

@@ -31,6 +31,8 @@
 #include <sys/zfs_vnops.h>
 #include <sys/zfeature.h>
 
+int zfs_bclone_enabled = 0;
+
 /*
  * Clone part of a file via block cloning.
  *
@@ -50,6 +52,9 @@ __zpl_clone_file_range(struct file *src_file, loff_t src_off,
 	fstrans_cookie_t cookie;
 	int err;
 
+	if (!zfs_bclone_enabled)
+		return (-EOPNOTSUPP);
+
 	if (!spa_feature_is_enabled(
 	    dmu_objset_spa(ITOZSB(dst_i)->z_os), SPA_FEATURE_BLOCK_CLONING))
 		return (-EOPNOTSUPP);