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

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Message-ID: <33d5bdc7-0fde-4a97-aa1a-f8565e196ccd@redhat.com>
Date: Sun, 21 Dec 2025 15:30:50 -0500
From: Waiman Long <llong@...hat.com>
To: Li Wang <liwang@...hat.com>, akpm@...ux-foundation.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: David Hildenbrand <david@...nel.org>, Mark Brown <broonie@...nel.org>,
 Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v3 3/3] selftests/mm/charge_reserved_hugetlb.sh: add waits
 with timeout helper


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@...hat.com>
> Cc: David Hildenbrand <david@...nel.org>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: Waiman Long <longman@...hat.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