charge_reserved_hugetlb.sh mounts a hugetlbfs instance at /mnt/huge with
a fixed size of 256M. On systems with large base hugepages (e.g. 512MB),
this is smaller than a single hugepage, so the hugetlbfs mount ends up
with effectively zero capacity (often visible as size=0 in mount output).

As a result, write_to_hugetlbfs fails with ENOMEM and the test can hang
waiting for progress.

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

  #./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
  ...

  # mount |grep /mnt/huge
  none on /mnt/huge type hugetlbfs (rw,relatime,seclabel,pagesize=512M,size=0)

  # grep -i huge /proc/meminfo
  ...
  HugePages_Total:      10
  HugePages_Free:       10
  HugePages_Rsvd:        0
  HugePages_Surp:        0
  Hugepagesize:     524288 kB
  Hugetlb:         5242880 kB

Mount hugetlbfs with size=${size}, the number of bytes the test will
reserve/write, so the filesystem capacity is sufficient regardless of
HugeTLB page size.

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>
---
 tools/testing/selftests/mm/charge_reserved_hugetlb.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/charge_reserved_hugetlb.sh b/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
index 249a5776c074..ac2744dbc0bd 100755
--- a/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
+++ b/tools/testing/selftests/mm/charge_reserved_hugetlb.sh
@@ -295,7 +295,7 @@ function run_test() {
   setup_cgroup "hugetlb_cgroup_test" "$cgroup_limit" "$reservation_limit"
 
   mkdir -p /mnt/huge
-  mount -t hugetlbfs -o pagesize=${MB}M,size=256M none /mnt/huge
+  mount -t hugetlbfs -o pagesize=${MB}M,size=${size} none /mnt/huge
 
   write_hugetlbfs_and_get_usage "hugetlb_cgroup_test" "$size" "$populate" \
     "$write" "/mnt/huge/test" "$method" "$private" "$expect_failure" \
@@ -349,7 +349,7 @@ function run_multiple_cgroup_test() {
   setup_cgroup "hugetlb_cgroup_test2" "$cgroup_limit2" "$reservation_limit2"
 
   mkdir -p /mnt/huge
-  mount -t hugetlbfs -o pagesize=${MB}M,size=256M none /mnt/huge
+  mount -t hugetlbfs -o pagesize=${MB}M,size=${size} none /mnt/huge
 
   write_hugetlbfs_and_get_usage "hugetlb_cgroup_test1" "$size1" \
     "$populate1" "$write1" "/mnt/huge/test1" "$method" "$private" \
-- 
2.49.0

On 12/21/25 09:58, Li Wang wrote:
> charge_reserved_hugetlb.sh mounts a hugetlbfs instance at /mnt/huge with
> a fixed size of 256M. On systems with large base hugepages (e.g. 512MB),
> this is smaller than a single hugepage, so the hugetlbfs mount ends up
> with effectively zero capacity (often visible as size=0 in mount output).
> 
> As a result, write_to_hugetlbfs fails with ENOMEM and the test can hang
> waiting for progress.

I'm curious, what's the history of using "256MB" in the first place (or 
specifying any size?).

-- 
Cheers

David

David Hildenbrand (Red Hat) <david@kernel.org> wrote:
> On 12/21/25 09:58, Li Wang wrote:
> > charge_reserved_hugetlb.sh mounts a hugetlbfs instance at /mnt/huge with
> > a fixed size of 256M. On systems with large base hugepages (e.g. 512MB),
> > this is smaller than a single hugepage, so the hugetlbfs mount ends up
> > with effectively zero capacity (often visible as size=0 in mount output).
> >
> > As a result, write_to_hugetlbfs fails with ENOMEM and the test can hang
> > waiting for progress.
>
> I'm curious, what's the history of using "256MB" in the first place (or
> specifying any size?).

Seems the script initializes it with "256MB" from:

commit 29750f71a9b4cfae57cdddfbd8ca287eddca5503
Author: Mina Almasry <almasrymina@google.com>
Date:   Wed Apr 1 21:11:38 2020 -0700

    hugetlb_cgroup: add hugetlb_cgroup reservation tests


-- 
Regards,
Li Wang

On 12/21/25 10:44, Li Wang wrote:
> David Hildenbrand (Red Hat) <david@kernel.org> wrote:
>> On 12/21/25 09:58, Li Wang wrote:
>>> charge_reserved_hugetlb.sh mounts a hugetlbfs instance at /mnt/huge with
>>> a fixed size of 256M. On systems with large base hugepages (e.g. 512MB),
>>> this is smaller than a single hugepage, so the hugetlbfs mount ends up
>>> with effectively zero capacity (often visible as size=0 in mount output).
>>>
>>> As a result, write_to_hugetlbfs fails with ENOMEM and the test can hang
>>> waiting for progress.
>>
>> I'm curious, what's the history of using "256MB" in the first place (or
>> specifying any size?).
> 
> Seems the script initializes it with "256MB" from:
> 
> commit 29750f71a9b4cfae57cdddfbd8ca287eddca5503
> Author: Mina Almasry <almasrymina@google.com>
> Date:   Wed Apr 1 21:11:38 2020 -0700
> 
>      hugetlb_cgroup: add hugetlb_cgroup reservation tests

What would happen if we don't specify a size at all?

-- 
Cheers

David

On Sun, Dec 21, 2025 at 5:49 PM David Hildenbrand (Red Hat)
<david@kernel.org> wrote:
>
> On 12/21/25 10:44, Li Wang wrote:
> > David Hildenbrand (Red Hat) <david@kernel.org> wrote:
> >> On 12/21/25 09:58, Li Wang wrote:
> >>> charge_reserved_hugetlb.sh mounts a hugetlbfs instance at /mnt/huge with
> >>> a fixed size of 256M. On systems with large base hugepages (e.g. 512MB),
> >>> this is smaller than a single hugepage, so the hugetlbfs mount ends up
> >>> with effectively zero capacity (often visible as size=0 in mount output).
> >>>
> >>> As a result, write_to_hugetlbfs fails with ENOMEM and the test can hang
> >>> waiting for progress.
> >>
> >> I'm curious, what's the history of using "256MB" in the first place (or
> >> specifying any size?).
> >
> > Seems the script initializes it with "256MB" from:
> >
> > commit 29750f71a9b4cfae57cdddfbd8ca287eddca5503
> > Author: Mina Almasry <almasrymina@google.com>
> > Date:   Wed Apr 1 21:11:38 2020 -0700
> >
> >      hugetlb_cgroup: add hugetlb_cgroup reservation tests
>
> What would happen if we don't specify a size at all?

It still works well, I have gone through the whole file and
there is no subtest that relies on the 256M capability.

So we could just:

    mount -t hugetlbfs -o pagesize=${MB}M none /mnt/huge

-- 
Regards,
Li Wang