btrfs-progs: fi resize: warn if new size is < 256M

The lower kernel limit is 256M otherwise it's considered an invalid
parameter.

Issue: #875
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-filesystem.rst

@@ -269,6 +269,11 @@ resize [options] [<devid>:][+/-]<size>[kKmMgGtTpPeE]|[<devid>:]max <path>
         take a long time if there are data in the device area that's beyond the new
         end. Relocation of the data takes time.
 
+        Note that there's a lower limit on the new size (either specified
+        as an absolute size or difference) that is checked by kernel and
+        rejected eventually as invalid. Lower values will print a warning but
+        still pass the request to kernel. The currently known value is 256MiB.
+
         See also section *EXAMPLES*.
 
         ``Options``

cmds/filesystem.c

@@ -1408,6 +1408,9 @@ static int check_resize_args(const char *amount, const char *path, u64 *devid_re
 		di_args[dev_idx].path,
 		pretty_size_mode(di_args[dev_idx].total_bytes, UNITS_DEFAULT),
 		res_str);
+	if (new_size < 256 * SZ_1M)
+		warning("the new size %lld (%s) is < 256MiB, this may be rejected by kernel",
+			new_size, pretty_size_mode(new_size, UNITS_DEFAULT));
 
 out:
 	free(di_args);