selftests/vm: make charge_reserved_hugetlb.sh work with existing cgroup setting

The hugetlb cgroup reservation test charge_reserved_hugetlb.sh assume
that no cgroup filesystems are mounted before running the test. That is
not true in many cases. As a result, the test fails to run. Fix that
by querying the current cgroup mount setting and using the existing
cgroup setup instead before attempting to freshly mount a cgroup
filesystem.

Similar change is also made for hugetlb_reparenting_test.sh as well,
though it still has problem if cgroup v2 isn't used.

The patched test scripts were run on a centos 8 based system to verify
that they ran properly.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 29750f71a9b4 ("hugetlb_cgroup: add hugetlb_cgroup reservation tests")
Signed-off-by: Waiman Long <[e-mail]>
Acked-by: Mina Almasry <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/vm/charge_reserved_hugetlb.sh

···
   reservation_usage_file=rsvd.current
 fi
 
-cgroup_path=/dev/cgroup/memory
-if [[ ! -e $cgroup_path ]]; then
-  mkdir -p $cgroup_path
-  if [[ $cgroup2 ]]; then
+if [[ $cgroup2 ]]; then
+  cgroup_path=$(mount -t cgroup2 | head -1 | awk -e '{print $3}')
+  if [[ -z "$cgroup_path" ]]; then
+    cgroup_path=/dev/cgroup/memory
     mount -t cgroup2 none $cgroup_path
-  else
+    do_umount=1
+  fi
+  echo "+hugetlb" >$cgroup_path/cgroup.subtree_control
+else
+  cgroup_path=$(mount -t cgroup | grep ",hugetlb" | awk -e '{print $3}')
+  if [[ -z "$cgroup_path" ]]; then
+    cgroup_path=/dev/cgroup/memory
     mount -t cgroup memory,hugetlb $cgroup_path
+    do_umount=1
   fi
 fi
-
-if [[ $cgroup2 ]]; then
-  echo "+hugetlb" >/dev/cgroup/memory/cgroup.subtree_control
-fi
+export cgroup_path
 
 function cleanup() {
   if [[ $cgroup2 ]]; then
···
 
 function wait_for_hugetlb_memory_to_get_depleted() {
   local cgroup="$1"
-  local path="/dev/cgroup/memory/$cgroup/hugetlb.${MB}MB.$reservation_usage_file"
+  local path="$cgroup_path/$cgroup/hugetlb.${MB}MB.$reservation_usage_file"
   # Wait for hugetlbfs memory to get depleted.
   while [ $(cat $path) != 0 ]; do
     echo Waiting for hugetlb memory to get depleted.
···
   local cgroup="$1"
   local size="$2"
 
-  local path="/dev/cgroup/memory/$cgroup/hugetlb.${MB}MB.$reservation_usage_file"
+  local path="$cgroup_path/$cgroup/hugetlb.${MB}MB.$reservation_usage_file"
   # Wait for hugetlbfs memory to get written.
   while [ $(cat $path) != $size ]; do
     echo Waiting for hugetlb memory reservation to reach size $size.
···
   local cgroup="$1"
   local size="$2"
 
-  local path="/dev/cgroup/memory/$cgroup/hugetlb.${MB}MB.$fault_usage_file"
+  local path="$cgroup_path/$cgroup/hugetlb.${MB}MB.$fault_usage_file"
   # Wait for hugetlbfs memory to get written.
   while [ $(cat $path) != $size ]; do
     echo Waiting for hugetlb memory to reach size $size.
···
   done     # populate
 done       # method
 
-umount $cgroup_path
-rmdir $cgroup_path
+if [[ $do_umount ]]; then
+  umount $cgroup_path
+  rmdir $cgroup_path
+fi

tools/testing/selftests/vm/hugetlb_reparenting_test.sh

···
   usage_file=current
 fi
 
-CGROUP_ROOT='/dev/cgroup/memory'
-MNT='/mnt/huge/'
 
-if [[ ! -e $CGROUP_ROOT ]]; then
-  mkdir -p $CGROUP_ROOT
-  if [[ $cgroup2 ]]; then
+if [[ $cgroup2 ]]; then
+  CGROUP_ROOT=$(mount -t cgroup2 | head -1 | awk -e '{print $3}')
+  if [[ -z "$CGROUP_ROOT" ]]; then
+    CGROUP_ROOT=/dev/cgroup/memory
     mount -t cgroup2 none $CGROUP_ROOT
-    sleep 1
-    echo "+hugetlb +memory" >$CGROUP_ROOT/cgroup.subtree_control
-  else
+    do_umount=1
+  fi
+  echo "+hugetlb +memory" >$CGROUP_ROOT/cgroup.subtree_control
+else
+  CGROUP_ROOT=$(mount -t cgroup | grep ",hugetlb" | awk -e '{print $3}')
+  if [[ -z "$CGROUP_ROOT" ]]; then
+    CGROUP_ROOT=/dev/cgroup/memory
     mount -t cgroup memory,hugetlb $CGROUP_ROOT
+    do_umount=1
   fi
 fi
+MNT='/mnt/huge/'
 
 function get_machine_hugepage_size() {
   hpz=$(grep -i hugepagesize /proc/meminfo)

tools/testing/selftests/vm/write_hugetlb_memory.sh

···
 reserve=$9
 
 echo "Putting task in cgroup '$cgroup'"
-echo $$ > /dev/cgroup/memory/"$cgroup"/cgroup.procs
+echo $$ > ${cgroup_path:-/dev/cgroup/memory}/"$cgroup"/cgroup.procs
 
 echo "Method is $method"