[POC PATCH 2/3] Reproducer for subpool usage leak

[Ackerley Tng <ackerleytng@google.com>]

(This reproducer was hacked up and not meant to be merged.)

The kernel leaks subpool usage and the subpool structure itself if a HugeTLBfs
mount specifying size (which sets max_hpages on the subpool) is created.

subpool_leak_max_size.sh reproduces this with the following steps:

1. Create mount, specifying size=2M (1 page). This sets max_hpages = 1 on the
   subpool, but does not reserve any pages.
2. Set nr_hugepages = 0 and nr_overcommit_hugepages = 0 so that physical
   allocations will fail.
3. Run fallocate -l 2M on a file in the mount.
     + This calls hugetlbfs_fallocate, which attempts to allocate a page by
       calling alloc_hugetlb_folio.
     + alloc_hugetlb_folio calls hugepage_subpool_get_pages to track the
       allocation against the subpool limit. This increments used_hpages to 1.
     + Physical allocation fails because nr_hugepages is 0.
     + Before patch (Buggy):
         + The error path in alloc_hugetlb_folio sees gbl_chg is 1 (indicating
           we tried to allocate a global page) and incorrectly skips calling
           hugepage_subpool_put_pages.
         + fallocate fails and returns to userspace, but the subpool used_hpages
           counter remains leaked at 1.
     + After patch:
         + The error path always calls hugepage_subpool_put_pages if map_chg is
           true, restoring used_hpages to 0.

4. Unmount the filesystem.
     + During unmount, the kernel calls unlock_or_release_subpool to clean up
       the subpool.
     + It checks if the subpool is free using subpool_is_free, which returns
       whether used_hpages is 0.
     + Before patch (Buggy):
         + Since used_hpages leaked and is 1, subpool_is_free returns false.
         + The kernel skips freeing the subpool structure, leaking the
           hugepage_subpool structure in kernel memory.
     + After patch:
         + Since used_hpages is 0, subpool_is_free returns true, and the subpool
           structure is correctly freed.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 subpool_leak_max_size.sh | 71 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100755 subpool_leak_max_size.sh

diff --git a/subpool_leak_max_size.sh b/subpool_leak_max_size.sh
new file mode 100755
index 0000000000000..bfafa1ba074ea
--- /dev/null
+++ b/subpool_leak_max_size.sh
@@ -0,0 +1,71 @@
+#!/bin/bash
+
+if [ "$EUID" -ne 0 ]; then
+    echo "Please run as root"
+    exit 1
+fi
+
+MNT_PATH="/tmp/mnt_hugetlb"
+FILE_PATH="$MNT_PATH/test_file"
+
+# Save original values
+orig_nr=$(cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages)
+orig_overcommit=$(cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_overcommit_hugepages)
+
+cleanup() {
+    echo "Cleaning up..."
+    rm -f "$FILE_PATH"
+    umount "$MNT_PATH" 2>/dev/null
+    rmdir "$MNT_PATH" 2>/dev/null
+    echo "$orig_nr" > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
+    echo "$orig_overcommit" > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_overcommit_hugepages
+    echo "Cleanup done."
+}
+trap cleanup EXIT
+
+# 1. Mount hugetlbfs with size=2M (1 page)
+mkdir -p "$MNT_PATH"
+if ! mount -t hugetlbfs -o size=2M none "$MNT_PATH"; then
+    echo "Failed to mount hugetlbfs"
+    exit 1
+fi
+
+# 2. Set nr_hugepages to 0, overcommit to 0
+echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
+echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_overcommit_hugepages
+
+# Check subpool usage before running
+read total free < <(stat -f -c "%b %f" "$MNT_PATH")
+used_before=$((total - free))
+echo "Before test - Subpool total blocks: $total"
+echo "Before test - Subpool free blocks:  $free"
+echo "Before test - Subpool used blocks:  $used_before"
+if [ "$used_before" -ne 0 ]; then
+    echo "ERROR: Subpool is not clean before test starts!"
+    exit 1
+fi
+
+# Run fallocate (expecting failure)
+echo "Running fallocate (expecting failure)..."
+if fallocate -l 2M "$FILE_PATH" 2>/dev/null; then
+    echo "ERROR: fallocate succeeded but should have failed (nr_hugepages is 0)"
+    exit 1
+fi
+
+# Check subpool usage via statfs
+# %b: Total blocks
+# %f: Free blocks
+read total free < <(stat -f -c "%b %f" "$MNT_PATH")
+used=$((total - free))
+
+echo "Subpool total blocks: $total"
+echo "Subpool free blocks:  $free"
+echo "Subpool used blocks (leaked if > 0): $used"
+
+if [ "$used" -gt 0 ]; then
+    echo "RESULT: LEAK DETECTED (FAIL)"
+    exit 1
+else
+    echo "RESULT: NO LEAK (PASS)"
+    exit 0
+fi
-- 
2.55.0.795.g602f6c329a-goog