selftests/mm/write_to_hugetlbfs: parse -s as size_t

selftests/mm/write_to_hugetlbfs: parse -s as size_t

Patch series "selftests/mm: hugetlb cgroup charging: robustness fixes", v3.

This series fixes a few issues in the hugetlb cgroup charging selftests
(write_to_hugetlbfs.c + charge_reserved_hugetlb.sh) that show up on
systems with large hugepages (e.g.  512MB) and when failures cause the
test to wait indefinitely.

On an aarch64 64k page kernel with 512MB hugepages, the test consistently
fails in write_to_hugetlbfs with ENOMEM and then hangs waiting for the
expected usage values.  The root cause is that charge_reserved_hugetlb.sh
mounts hugetlbfs with a fixed size=256M, which is smaller than a single
hugepage, resulting in a mount with size=0 capacity.

In addition, write_to_hugetlbfs previously parsed -s via atoi() into an
int, which can overflow and print negative sizes.

Reproducer / environment:
  - Kernel: 6.12.0-xxx.el10.aarch64+64k
  - Hugepagesize: 524288 kB (512MB)
  - ./charge_reserved_hugetlb.sh -cgroup-v2
  - Observed mount: pagesize=512M,size=0 before this series

After applying the series, the test completes successfully on the above
setup.

This patch (of 3):

write_to_hugetlbfs currently parses the -s size argument with atoi() into
an int.  This silently accepts malformed input, cannot report overflow,
and can truncate large sizes.

=== 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
 ...
 # Writing to this path: /mnt/huge/test
 # Writing this size: -1610612736        <--------

Switch the size variable to size_t and parse -s with sscanf("%zu", ...).
Also print the size using %zu.

This avoids incorrect behavior with large -s values and makes the utility
more robust.

Link: https://lkml.kernel.org/r/20251221122639.3168038-1-liwang@redhat.com
Link: https://lkml.kernel.org/r/20251221122639.3168038-2-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>