diff --git a/dev-support/smart-apply-patch.sh b/dev-support/smart-apply-patch.sh
index f91745f4f01..449fc228b0c 100755
--- a/dev-support/smart-apply-patch.sh
+++ b/dev-support/smart-apply-patch.sh
@@ -11,14 +11,48 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
-set -e
+#
+# Determine if the patch file is a git diff file with prefixes.
+# These files are generated via "git diff" *without* the --no-prefix option.
+#
+# We can apply these patches more easily because we know that the a/ and b/
+# prefixes in the "diff" lines stands for the project root directory.
+# So we don't have to hunt for the project root.
+# And of course, we know that the patch file was generated using git, so we
+# know git apply can handle it properly.
+#
+# Arguments: file name.
+# Return: 0 if it is a git diff; 1 otherwise.
+#
+is_git_diff_with_prefix() {
+  DIFF_TYPE="unknown"
+  while read -r line; do
+    if [[ "$line" =~ ^diff\  ]]; then
+      if [[ "$line" =~ ^diff\ \-\-git ]]; then
+        DIFF_TYPE="git"
+      else
+        return 1 # All diff lines must be diff --git lines.
+      fi
+    fi
+    if [[ "$line" =~ ^\+\+\+\  ]] ||
+       [[ "$line" =~ ^\-\-\-\  ]]; then
+      if ! [[ "$line" =~ ^....[ab]/ || "$line" =~ ^..../dev/null ]]; then
+        return 1 # All +++ and --- lines must start with a/ or b/ or be /dev/null.
+      fi
+    fi
+  done < $1
+  [ x$DIFF_TYPE == x"git" ] || return 1
+  return 0 # return true (= 0 in bash)
+}
 
 PATCH_FILE=$1
+DRY_RUN=$2
 if [ -z "$PATCH_FILE" ]; then
   echo usage: $0 patch-file
   exit 1
 fi
 
+TMPDIR=${TMPDIR:-/tmp}
 PATCH=${PATCH:-patch} # allow overriding patch binary
 
 # Cleanup handler for temporary files
@@ -31,13 +65,56 @@ trap "cleanup 1" HUP INT QUIT TERM
 
 # Allow passing "-" for stdin patches
 if [ "$PATCH_FILE" == "-" ]; then
-  PATCH_FILE=/tmp/tmp.in.$$
+  PATCH_FILE="$TMPDIR/smart-apply.in.$RANDOM"
   cat /dev/fd/0 > $PATCH_FILE
   TOCLEAN="$TOCLEAN $PATCH_FILE"
 fi
 
+ISSUE_RE='^(HADOOP|YARN|MAPREDUCE|HDFS)-[0-9]+$'
+if [[ ${PATCH_FILE} =~ ^http || ${PATCH_FILE} =~ ${ISSUE_RE} ]]; then
+  # Allow downloading of patches
+  PFILE="$TMPDIR/smart-apply.in.$RANDOM"
+  TOCLEAN="$TOCLEAN $PFILE"
+  if [[ ${PATCH_FILE} =~ ^http ]]; then
+    patchURL="${PATCH_FILE}"
+  else # Get URL of patch from JIRA
+    wget -q -O "${PFILE}" "http://issues.apache.org/jira/browse/${PATCH_FILE}"
+    if [[ $? != 0 ]]; then
+      echo "Unable to determine what ${PATCH_FILE} may reference." 1>&2
+      cleanup 1
+    elif [[ $(grep -c 'Patch Available' "${PFILE}") == 0 ]]; then
+      echo "${PATCH_FILE} is not \"Patch Available\".  Exiting." 1>&2
+      cleanup 1
+    fi
+    relativePatchURL=$(grep -o '"/jira/secure/attachment/[0-9]*/[^"]*' "${PFILE}" | grep -v -e 'htm[l]*$' | sort | tail -1 | grep -o '/jira/secure/attachment/[0-9]*/[^"]*')
+    patchURL="http://issues.apache.org${relativePatchURL}"
+  fi
+  if [[ -n $DRY_RUN ]]; then
+    echo "Downloading ${patchURL}"
+  fi
+  wget -q -O "${PFILE}" "${patchURL}"
+  if [[ $? != 0 ]]; then
+    echo "${PATCH_FILE} could not be downloaded." 1>&2
+    cleanup 1
+  fi
+  PATCH_FILE="${PFILE}"
+fi
+
+# Special case for git-diff patches without --no-prefix
+if is_git_diff_with_prefix "$PATCH_FILE"; then
+  GIT_FLAGS="--binary -p1 -v"
+  if [[ -z $DRY_RUN ]]; then
+      GIT_FLAGS="$GIT_FLAGS --stat --apply "
+      echo Going to apply git patch with: git apply "${GIT_FLAGS}"
+  else
+      GIT_FLAGS="$GIT_FLAGS --check "
+  fi
+  git apply ${GIT_FLAGS} "${PATCH_FILE}"
+  exit $?
+fi
+
 # Come up with a list of changed files into $TMP
-TMP=/tmp/tmp.paths.$$
+TMP="$TMPDIR/smart-apply.paths.$RANDOM"
 TOCLEAN="$TOCLEAN $TMP"
 
 if $PATCH -p0 -E --dry-run < $PATCH_FILE 2>&1 > $TMP; then
@@ -46,10 +123,10 @@ if $PATCH -p0 -E --dry-run < $PATCH_FILE 2>&1 > $TMP; then
   # is adding new files and they would apply anywhere. So try to guess the
   # correct place to put those files.
 
-  TMP2=/tmp/tmp.paths.2.$$
+  TMP2="$TMPDIR/smart-apply.paths.2.$RANDOM"
   TOCLEAN="$TOCLEAN $TMP2"
 
-  egrep '^patching file |^checking file ' $TMP | awk '{print $3}' | grep -v /dev/null | sort | uniq > $TMP2
+  egrep '^patching file |^checking file ' $TMP | awk '{print $3}' | grep -v /dev/null | sort -u > $TMP2
 
   if [ ! -s $TMP2 ]; then
     echo "Error: Patch dryrun couldn't detect changes the patch would make. Exiting."
@@ -77,8 +154,8 @@ if $PATCH -p0 -E --dry-run < $PATCH_FILE 2>&1 > $TMP; then
       sed -i -e 's,^[ab]/,,' $TMP2
     fi
 
-    PREFIX_DIRS_AND_FILES=$(cut -d '/' -f 1 | sort | uniq)
-
+    PREFIX_DIRS_AND_FILES=$(cut -d '/' -f 1 $TMP2 | sort -u)
+ 
     # if we are at the project root then nothing more to do
     if [[ -d hadoop-common-project ]]; then
       echo Looks like this is being run at project root
@@ -105,6 +182,11 @@ else
   cleanup 1;
 fi
 
+# If this is a dry run then exit instead of applying the patch
+if [[ -n $DRY_RUN ]]; then
+  cleanup 0;
+fi
+
 echo Going to apply patch with: $PATCH -p$PLEVEL
 $PATCH -p$PLEVEL -E < $PATCH_FILE
 
diff --git a/hadoop-common-project/hadoop-common/CHANGES.txt b/hadoop-common-project/hadoop-common/CHANGES.txt
index 3044aafcf76..c2e5fcb8a6f 100644
--- a/hadoop-common-project/hadoop-common/CHANGES.txt
+++ b/hadoop-common-project/hadoop-common/CHANGES.txt
@@ -78,6 +78,8 @@ Release 2.8.0 - UNRELEASED
     HADOOP-11917. test-patch.sh should work with ${BASEDIR}/patchprocess
     setups (aw)
 
+    HADOOP-11925. backport trunk's smart-apply-patch.sh to branch-2 (aw)
+
   OPTIMIZATIONS
 
     HADOOP-11785. Reduce the number of listStatus operation in distcp