Message-ID: <CAEemH2ehOUyDqHhn5+Mu2B31dCpOhDv6AL7sEFU5Vt7nSST=Tw@mail.gmail.com>
Date: Mon, 22 Dec 2025 08:56:01 +0800
From: Li Wang <liwang@...hat.com>
To: Waiman Long <llong@...hat.com>
Cc: akpm@...ux-foundation.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	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 Mon, Dec 22, 2025 at 4:30 AM Waiman Long <llong@...hat.com> wrote:
>
>
> 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?

Hmm, it seems the test shouldn't exit too early.

As the wait_for_hugetlb_memory* is only trying 60s to examine the file
value, if timeouted, we still need to keep going because the test requires
CLEANUP work and exit/report from there.

The key point of each subtest is to save the '$write_result' value and
examine it
which controls the whole test to exit.

e.g.

This is an intentional error test:

# ./charge_reserved_hugetlb.sh -cgroup-v2
CLEANUP DONE
...
Writing to this path: /mnt/huge/test
Writing this size: 2684354560
Not populating.
Not writing to memory.
Using method=0
Shared mapping.
RESERVE mapping.
Allocating using HUGETLBFS.
write_to_hugetlbfs: Error mapping the file: Cannot allocate memory
Waiting for /sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current
to become '2684354560' (current: '0') (try 1/60)
Waiting for /sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current
to become '2684354560' (current: '0') (try 2/60)
Waiting for /sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current
to become '2684354560' (current: '0') (try 3/60)
Waiting for /sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current
to become '2684354560' (current: '0') (try 4/60)
...
Waiting for /sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current
to become '2684354560' (current: '0') (try 60/60)
ERROR: timeout waiting for
/sys/fs/cgroup/hugetlb_cgroup_test/hugetlb.512MB.rsvd.current to
become '2684354560'
After write:
hugetlb_usage=0
reserved_usage=0
0
0
Memory charged to hugtlb=0
Memory charged to reservation=0
expected (2684354560) != actual (0): Reserved memory not charged to
reservation usage.
CLEANUP DONE


-- 
Regards,
Li Wang

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