Re: [PATCH 4/5] selftests: prctl: introduce tests for disabling THPs completely
From: David Hildenbrand
Date: Mon Jul 28 2025 - 11:06:59 EST
On 25.07.25 18:22, Usama Arif wrote:
The test will set the global system THP setting to madvise and
the 2M setting to inherit before it starts (and reset to original
at teardown)
This tests if the process can:
- successfully set and get the policy to disable THPs completely.
- never get a hugepage when the THPs are completely disabled,
including with MADV_HUGE and MADV_COLLAPSE.
- successfully reset the policy of the process.
- get hugepages only on MADV_HUGE and MADV_COLLAPSE after reset.
- repeat the above tests in a forked process to make sure
the policy is carried across forks.
Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
tools/testing/selftests/mm/.gitignore | 1 +
tools/testing/selftests/mm/Makefile | 1 +
.../testing/selftests/mm/prctl_thp_disable.c | 162 ++++++++++++++++++
3 files changed, 164 insertions(+)
create mode 100644 tools/testing/selftests/mm/prctl_thp_disable.c
diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
index e7b23a8a05fe..eb023ea857b3 100644
--- a/tools/testing/selftests/mm/.gitignore
+++ b/tools/testing/selftests/mm/.gitignore
@@ -58,3 +58,4 @@ pkey_sighandler_tests_32
pkey_sighandler_tests_64
guard-regions
merge
+prctl_thp_disable
diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index d13b3cef2a2b..2bb8d3ebc17c 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -86,6 +86,7 @@ TEST_GEN_FILES += on-fault-limit
TEST_GEN_FILES += pagemap_ioctl
TEST_GEN_FILES += pfnmap
TEST_GEN_FILES += process_madv
+TEST_GEN_FILES += prctl_thp_disable
TEST_GEN_FILES += thuge-gen
TEST_GEN_FILES += transhuge-stress
TEST_GEN_FILES += uffd-stress
diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
new file mode 100644
index 000000000000..52f7e6659b1f
--- /dev/null
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -0,0 +1,162 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic tests for PR_GET/SET_THP_DISABLE prctl calls
+ *
+ * Author(s): Usama Arif <usamaarif642@xxxxxxxxx>
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+
+#include "../kselftest_harness.h"
+#include "thp_settings.h"
+#include "vm_util.h"
+
+#ifndef PR_THP_DISABLE_EXCEPT_ADVISED
+#define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
+#endif
Into patch #2 I guess.
+
+#define NR_HUGEPAGES 6
+
+static int sz2ord(size_t size, size_t pagesize)
+{
+ return __builtin_ctzll(size / pagesize);
+}
+
+enum madvise_buffer {
+ NONE,
+ HUGE,
+ COLLAPSE
+};
Is that rather something like:
enum thp_collapse_type {
THP_COLLAPSE_NONE,
THP_COLLAPSE_MADV_HUGEPAGE, /* MADV_HUGEPAGE before access */
THP_COLLAPSE_MADV_COLLAPSE, /* MADV_COLLAPSE after access */
};
+
+/*
+ * Function to mmap a buffer, fault it in, madvise it appropriately (before
+ * page fault for MADV_HUGE, and after for MADV_COLLAPSE), and check if the
+ * mmap region is huge.
+ * returns:
+ * 0 if test doesn't give hugepage
+ * 1 if test gives a hugepage
+ * -1 if mmap fails
+ */
+static int test_mmap_thp(enum madvise_buffer madvise_buf, size_t pmdsize)
+{
+ int ret;
+ int buf_size = NR_HUGEPAGES * pmdsize;
+
+ char *buffer = (char *)mmap(NULL, buf_size, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
Can we get rid of NR_HUGEPAGES and just use a single one, aligning in a bigger area? This is similar to what we do in cow.c
/* For alignment purposes, we need twice the thp size. */
mmap_size = 2 * pmdsize;
mmap_area = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (mmap_area == MAP_FAILED)
return -errno; /* todo, document that above */
buffer = (char *)(((uintptr_t)mmap_area + pmdsize) & ~(pmdsize - 1));
...
ret = check_huge_anon(buffer, NR_HUGEPAGES, pmdsize);
...
munmap(mmap_area, mmap_size);
+ if (buffer == MAP_FAILED)
+ return -1;
+
+ if (madvise_buf == HUGE)
+ madvise(buffer, buf_size, MADV_HUGEPAGE);
+
+ /* Ensure memory is allocated */
+ memset(buffer, 1, buf_size);
+
+ if (madvise_buf == COLLAPSE)
+ madvise(buffer, buf_size, MADV_COLLAPSE);
+
+ ret = check_huge_anon(buffer, NR_HUGEPAGES, pmdsize);
+ munmap(buffer, buf_size);
+ return ret;
+}
Empty line missing :)
+FIXTURE(prctl_thp_disable_completely)
+{
+ struct thp_settings settings;
+ size_t pmdsize;
+};
+
+FIXTURE_SETUP(prctl_thp_disable_completely)
+{
+ if (!thp_is_enabled())
+ SKIP(return, "Transparent Hugepages not available\n");
Heh, not completely correct. enabled != available.
Do we want a thp_available() that is essentially the first part of thp_is_enabled() ?
+
+ self->pmdsize = read_pmd_pagesize();
+ if (!self->pmdsize)
+ SKIP(return, "Unable to read PMD size\n");
+
+ thp_read_settings(&self->settings);
+ self->settings.thp_enabled = THP_MADVISE;
+ self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
+ thp_save_settings();
+ thp_push_settings(&self->settings);
push without pop, should that be alarming? :)
Can we just use thp_write_settings()? (not sure why that push/pop is required ... is it?)
+}
+
+FIXTURE_TEARDOWN(prctl_thp_disable_completely)
+{> + thp_restore_settings();
+}
+
+/* prctl_thp_disable_except_madvise fixture sets system THP setting to madvise */
+static void prctl_thp_disable_completely(struct __test_metadata *const _metadata,
+ size_t pmdsize)
+{
+ int res = 0;
+
+ res = prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL);
+ ASSERT_EQ(res, 1);
+
+ /* global = madvise, process = never, we shouldn't get HPs even with madvise */
s/HPs/THPs/
+ res = test_mmap_thp(NONE, pmdsize);
+ ASSERT_EQ(res, 0);
+
+ res = test_mmap_thp(HUGE, pmdsize);
+ ASSERT_EQ(res, 0);
+
+ res = test_mmap_thp(COLLAPSE, pmdsize);
+ ASSERT_EQ(res, 0);
+
+ /* Reset to system policy */
+ res = prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL);
+ ASSERT_EQ(res, 0);
+
+ /* global = madvise */
+ res = test_mmap_thp(NONE, pmdsize);
+ ASSERT_EQ(res, 0);
+
+ res = test_mmap_thp(HUGE, pmdsize);
+ ASSERT_EQ(res, 1);
+
+ res = test_mmap_thp(COLLAPSE, pmdsize);
+ ASSERT_EQ(res, 1);
Makes me wonder: should we test for global=always and global=always?
(or simply for all possible values, including global=never if easily possible?)
At least testing with global=always should exercise more possible paths
than global=always (esp., test_mmap_thp(NONE, pmdsize) which would
never apply in madvise mode).
--
Cheers,
David / dhildenb