Re: [PATCH v3 3/3] selftests/mm/charge_reserved_hugetlb.sh: add waits with timeout helper

Waiman Long
2025-12-21T20:30:59Z

On 12/21/25 7:26 AM, Li Wang wrote:
> 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.
>
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> ---
> .../selftests/mm/charge_reserved_hugetlb.sh | 51 +++++++++++--------
> 1 file 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() {

wait_for_file_value() now return 0 onr success and 1 on timeout.
However, none of the callers of the wait_for_hugetlb_memory* are
checking their return values and acting accordingly. Are we expecting
that the test will show failure because the waiting isn't completed or
should we explicitly exit with ksft_fail (1) value?

Cheers,
Longman