diff --git a/epel-rpm-macros.spec b/epel-rpm-macros.spec
index 80c6781..f7cb5b3 100644
--- a/epel-rpm-macros.spec
+++ b/epel-rpm-macros.spec
@@ -1,6 +1,6 @@
 Name:           epel-rpm-macros
 Version:        8
-Release:        23
+Release:        24
 Summary:        Extra Packages for Enterprise Linux RPM macros
 
 Group:          System Environment/Base
@@ -72,6 +72,12 @@ install -Dpm 644 %{SOURCE150} \
 
 
 %changelog
+* Wed Sep  1 2021 Michel Alexandre Salim <salimma@fedoraproject.org> - 8-24
+- Fix macros.build-constraints' %%limit_build
+  - number of CPUs will never be set to less than 1
+  - this now outputs build flag overrides to be used with %%make_build etc.
+  - add documentation
+
 * Mon Aug  2 2021 Michel Alexandre Salim <salimma@fedoraproject.org> - 8-23
 - Add macros.build-constraints
 - Refactor to use standard macros, not hard-coded paths
diff --git a/macros.build-constraints b/macros.build-constraints
index 1343cd8..db99418 100644
--- a/macros.build-constraints
+++ b/macros.build-constraints
@@ -1,10 +1,42 @@
 # Macros to constrain resource use during the build process
 
-# m: memory limit in MBs per core; default is 1024
-%limit_build(m:) \
-  _mem_per_process="%{-m:%{-m*}}%{!-m:1024}" \
-  _max_mem=$(awk '/MemTotal/ { print $2 }' /proc/meminfo) \
-  _max_jobs="$(($_max_mem / ($_mem_per_process * 1024)))" \
-  _cur_max_jobs="%{_smp_build_ncpus}" \
-  test "${_cur_max_jobs}" -gt "${_max_jobs}" && _cur_max_jobs="${_max_jobs}" && echo "Warning: Reducing build parallelism to -j${_max_jobs} because of memory limits" \
-  %global _smp_build_ncpus "${_cur_max_jobs}"
+# outputs build flag overrides to be used in conjunction with
+# %%make_build, %%cmake_build etc.
+#
+# if no override is needed, this macro outputs nothing
+#
+# - m   memory limit in MBs per core; default is 1024
+#
+# Usage:
+# e.g. %make_build %{limit_build -m 2048}
+#   => /usr/bin/make -O -j16 V=1 VERBOSE=1
+#      %make_build %{limit_build -m 40960}
+#   => /usr/bin/make -O -j16 V=1 VERBOSE=1 -j1
+# 
+%limit_build(m:) %{lua:
+  local mem_per_process=rpm.expand("%{-m*}")
+  if mem_per_process == "" then
+    mem_per_process = 1024
+  else
+    mem_per_process = tonumber(mem_per_process)
+  end
+  local mem_total = 0
+  for line in io.lines('/proc/meminfo') do
+    if line:sub(1, 9) == "MemTotal:" then
+      local tokens = {}
+      for token in line:gmatch("%w+") do
+        tokens[#tokens + 1] = token
+      end
+      mem_total = tonumber(tokens[2])
+      break
+    end
+  end
+  local max_jobs = mem_total // (mem_per_process * 1024)
+  if max_jobs < 1 then
+    max_jobs = 1
+  end
+  cur_max_jobs=tonumber(rpm.expand("%{_smp_build_ncpus}"))
+  if cur_max_jobs > max_jobs then
+    print("-j" .. max_jobs)
+  end
+}