selftests/mm/charge_reserved_hugetlb.sh: add waits with timeout helper

The hugetlb cgroup usage wait loops in charge_reserved_hugetlb.sh were
unbounded and could hang forever if the expected cgroup file value never
appears (e.g.  due to write_to_hugetlbfs in Error mapping).

=== Error log ===
  # uname -r
  6.12.0-xxx.el10.aarch64+64k

  # ls /sys/kernel/mm/hugepages/hugepages-*
  hugepages-16777216kB/  hugepages-2048kB/  hugepages-524288kB/

  #./charge_reserved_hugetlb.sh -cgroup-v2
  # -----------------------------------------
  ...
  # nr hugepages = 10
  # writing cgroup limit: 5368709120
  # writing reseravation limit: 5368709120
  ...
  # write_to_hugetlbfs: Error mapping the file: Cannot allocate memory
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  # Waiting for hugetlb memory reservation to reach size 2684354560.
  # 0
  ...

Introduce a small helper, wait_for_file_value(), and use it for:
  - waiting for reservation usage to drop to 0,
  - waiting for reservation usage to reach a given size,
  - waiting for fault usage to reach a given size.

This makes the waits consistent and adds a hard timeout (60 tries with 1s
sleep) so the test fails instead of stalling indefinitely.

Link: https://lkml.kernel.org/r/20251221122639.3168038-4-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 30 insertions, 21 deletions

diff --git a/tools/testing/selftests/mm/charge_reserved_hugetlb.sh b/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
index fa6713892d82..447769657634 100755
--- a/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
+++ b/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
@@ -100,7 +100,7 @@ function setup_cgroup() {
   echo writing cgroup limit: "$cgroup_limit"
   echo "$cgroup_limit" >$cgroup_path/$name/hugetlb.${MB}MB.$fault_limit_file
 
-  echo writing reseravation limit: "$reservation_limit"
+  echo writing reservation limit: "$reservation_limit"
   echo "$reservation_limit" > \
     $cgroup_path/$name/hugetlb.${MB}MB.$reservation_limit_file
 
@@ -112,41 +112,50 @@ function setup_cgroup() {
   fi
 }
 
+function wait_for_file_value() {
+  local path="$1"
+  local expect="$2"
+  local max_tries=60
+
+  if [[ ! -r "$path" ]]; then
+    echo "ERROR: cannot read '$path', missing or permission denied"
+    return 1
+  fi
+
+  for ((i=1; i<=max_tries; i++)); do
+    local cur="$(cat "$path")"
+    if [[ "$cur" == "$expect" ]]; then
+      return 0
+    fi
+    echo "Waiting for $path to become '$expect' (current: '$cur') (try $i/$max_tries)"
+    sleep 1
+  done
+
+  echo "ERROR: timeout waiting for $path to become '$expect'"
+  return 1
+}
+
 function wait_for_hugetlb_memory_to_get_depleted() {
   local cgroup="$1"
   local path="$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.
-    cat $path
-    sleep 0.5
-  done
+
+  wait_for_file_value "$path" "0"
 }
 
 function wait_for_hugetlb_memory_to_get_reserved() {
   local cgroup="$1"
   local size="$2"
-
   local path="$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.
-    cat $path
-    sleep 0.5
-  done
+
+  wait_for_file_value "$path" "$size"
 }
 
 function wait_for_hugetlb_memory_to_get_written() {
   local cgroup="$1"
   local size="$2"
-
   local path="$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.
-    cat $path
-    sleep 0.5
-  done
+
+  wait_for_file_value "$path" "$size"
 }
 
 function write_hugetlbfs_and_get_usage() {