The test will set the global system THP setting to never, madvise
or always depending on the fixture variant 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
  with the prctl, including with MADV_HUGE and MADV_COLLAPSE.
- successfully reset the policy of the process.
- after reset, only get hugepages with:
  - MADV_COLLAPSE when policy is set to never.
  - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
  - always when policy is set to "always".
- repeat the above tests in a forked process to make sure
  the policy is carried across forks.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../testing/selftests/mm/prctl_thp_disable.c  | 241 ++++++++++++++++++
 tools/testing/selftests/mm/thp_settings.c     |   9 +-
 tools/testing/selftests/mm/thp_settings.h     |   1 +
 5 files changed, 252 insertions(+), 1 deletion(-)
 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..2f54e5e52274
--- /dev/null
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -0,0 +1,241 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic tests for PR_GET/SET_THP_DISABLE prctl calls
+ *
+ * Author(s): Usama Arif <usamaarif642@gmail.com>
+ */
+#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"
+
+static int sz2ord(size_t size, size_t pagesize)
+{
+	return __builtin_ctzll(size / pagesize);
+}
+
+enum thp_collapse_type {
+	THP_COLLAPSE_NONE,
+	THP_COLLAPSE_MADV_HUGEPAGE,	/* MADV_HUGEPAGE before access */
+	THP_COLLAPSE_MADV_COLLAPSE,	/* MADV_COLLAPSE after access */
+};
+
+enum thp_policy {
+	THP_POLICY_NEVER,
+	THP_POLICY_MADVISE,
+	THP_POLICY_ALWAYS,
+};
+
+struct test_results {
+	int prctl_get_thp_disable;
+	int prctl_applied_collapse_none;
+	int prctl_applied_collapse_madv_huge;
+	int prctl_applied_collapse_madv_collapse;
+	int prctl_removed_collapse_none;
+	int prctl_removed_collapse_madv_huge;
+	int prctl_removed_collapse_madv_collapse;
+};
+
+/*
+ * 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
+ * -errno if mmap fails
+ */
+static int test_mmap_thp(enum thp_collapse_type madvise_buf, size_t pmdsize)
+{
+	char *mem, *mmap_mem;
+	size_t mmap_size;
+	int ret;
+
+	/* For alignment purposes, we need twice the THP size. */
+	mmap_size = 2 * pmdsize;
+	mmap_mem = (char *)mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
+				    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (mmap_mem == MAP_FAILED)
+		return -errno;
+
+	/* We need a THP-aligned memory area. */
+	mem = (char *)(((uintptr_t)mmap_mem + pmdsize) & ~(pmdsize - 1));
+
+	if (madvise_buf == THP_COLLAPSE_MADV_HUGEPAGE)
+		madvise(mem, pmdsize, MADV_HUGEPAGE);
+
+	/* Ensure memory is allocated */
+	memset(mem, 1, pmdsize);
+
+	if (madvise_buf == THP_COLLAPSE_MADV_COLLAPSE)
+		madvise(mem, pmdsize, MADV_COLLAPSE);
+
+	/*
+	 * MADV_HUGEPAGE will create a new VMA at "mem", which is the address
+	 * pattern we want to check for to detect the presence of hugepage in
+	 * smaps.
+	 * MADV_COLLAPSE will not create a new VMA, therefore we need to check
+	 * for hugepage at "mmap_mem" in smaps.
+	 * Check for hugepage at both locations to ensure that
+	 * THP_COLLAPSE_NONE, THP_COLLAPSE_MADV_HUGEPAGE and
+	 * THP_COLLAPSE_MADV_COLLAPSE only gives a THP when expected
+	 * in the range [mmap_mem, mmap_mem + 2 * pmdsize].
+	 */
+	ret = check_huge_anon(mem, 1, pmdsize) ||
+	      check_huge_anon(mmap_mem, 1, pmdsize);
+	munmap(mmap_mem, mmap_size);
+	return ret;
+}
+
+static void prctl_thp_disable_test(struct __test_metadata *const _metadata,
+				   size_t pmdsize, struct test_results *results)
+{
+
+	ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL),
+		  results->prctl_get_thp_disable);
+
+	/* tests after prctl overrides global policy */
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
+		  results->prctl_applied_collapse_none);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
+		  results->prctl_applied_collapse_madv_huge);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize),
+		  results->prctl_applied_collapse_madv_collapse);
+
+	/* Reset to global policy */
+	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL), 0);
+
+	/* tests after prctl is cleared, and only global policy is effective */
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
+		  results->prctl_removed_collapse_none);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
+		  results->prctl_removed_collapse_madv_huge);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize),
+		  results->prctl_removed_collapse_madv_collapse);
+}
+
+FIXTURE(prctl_thp_disable_completely)
+{
+	struct thp_settings settings;
+	struct test_results results;
+	size_t pmdsize;
+};
+
+FIXTURE_VARIANT(prctl_thp_disable_completely)
+{
+	enum thp_policy thp_global_policy;
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, never)
+{
+	.thp_global_policy = THP_POLICY_NEVER,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, madvise)
+{
+	.thp_global_policy = THP_POLICY_MADVISE,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, always)
+{
+	.thp_global_policy = THP_POLICY_ALWAYS,
+};
+
+FIXTURE_SETUP(prctl_thp_disable_completely)
+{
+	if (!thp_available())
+		SKIP(return, "Transparent Hugepages not available\n");
+
+	self->pmdsize = read_pmd_pagesize();
+	if (!self->pmdsize)
+		SKIP(return, "Unable to read PMD size\n");
+
+	thp_save_settings();
+	thp_read_settings(&self->settings);
+	switch (variant->thp_global_policy) {
+	case THP_POLICY_NEVER:
+		self->settings.thp_enabled = THP_NEVER;
+		self->results = (struct test_results) {
+			.prctl_get_thp_disable = 1,
+			.prctl_applied_collapse_none = 0,
+			.prctl_applied_collapse_madv_huge = 0,
+			.prctl_applied_collapse_madv_collapse = 0,
+			.prctl_removed_collapse_none = 0,
+			.prctl_removed_collapse_madv_huge = 0,
+			.prctl_removed_collapse_madv_collapse = 1,
+		};
+		break;
+	case THP_POLICY_MADVISE:
+		self->settings.thp_enabled = THP_MADVISE;
+		self->results = (struct test_results) {
+			.prctl_get_thp_disable = 1,
+			.prctl_applied_collapse_none = 0,
+			.prctl_applied_collapse_madv_huge = 0,
+			.prctl_applied_collapse_madv_collapse = 0,
+			.prctl_removed_collapse_none = 0,
+			.prctl_removed_collapse_madv_huge = 1,
+			.prctl_removed_collapse_madv_collapse = 1,
+		};
+		break;
+	case THP_POLICY_ALWAYS:
+		self->settings.thp_enabled = THP_ALWAYS;
+		self->results = (struct test_results) {
+			.prctl_get_thp_disable = 1,
+			.prctl_applied_collapse_none = 0,
+			.prctl_applied_collapse_madv_huge = 0,
+			.prctl_applied_collapse_madv_collapse = 0,
+			.prctl_removed_collapse_none = 1,
+			.prctl_removed_collapse_madv_huge = 1,
+			.prctl_removed_collapse_madv_collapse = 1,
+		};
+		break;
+	}
+	self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
+	thp_write_settings(&self->settings);
+}
+
+FIXTURE_TEARDOWN(prctl_thp_disable_completely)
+{
+	thp_restore_settings();
+}
+
+TEST_F(prctl_thp_disable_completely, nofork)
+{
+	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 1, NULL, NULL, NULL), 0);
+	prctl_thp_disable_test(_metadata, self->pmdsize, &self->results);
+}
+
+TEST_F(prctl_thp_disable_completely, fork)
+{
+	int ret = 0;
+	pid_t pid;
+
+	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 1, NULL, NULL, NULL), 0);
+
+	/* Make sure prctl changes are carried across fork */
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (!pid)
+		prctl_thp_disable_test(_metadata, self->pmdsize, &self->results);
+
+	wait(&ret);
+	if (WIFEXITED(ret))
+		ret = WEXITSTATUS(ret);
+	else
+		ret = -EINVAL;
+	ASSERT_EQ(ret, 0);
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/mm/thp_settings.c b/tools/testing/selftests/mm/thp_settings.c
index bad60ac52874..574bd0f8ae48 100644
--- a/tools/testing/selftests/mm/thp_settings.c
+++ b/tools/testing/selftests/mm/thp_settings.c
@@ -382,10 +382,17 @@ unsigned long thp_shmem_supported_orders(void)
 	return __thp_supported_orders(true);
 }
 
-bool thp_is_enabled(void)
+bool thp_available(void)
 {
 	if (access(THP_SYSFS, F_OK) != 0)
 		return false;
+	return true;
+}
+
+bool thp_is_enabled(void)
+{
+	if (!thp_available())
+		return false;
 
 	int mode = thp_read_string("enabled", thp_enabled_strings);
 
diff --git a/tools/testing/selftests/mm/thp_settings.h b/tools/testing/selftests/mm/thp_settings.h
index 6c07f70beee9..76eeb712e5f1 100644
--- a/tools/testing/selftests/mm/thp_settings.h
+++ b/tools/testing/selftests/mm/thp_settings.h
@@ -84,6 +84,7 @@ void thp_set_read_ahead_path(char *path);
 unsigned long thp_supported_orders(void);
 unsigned long thp_shmem_supported_orders(void);
 
+bool thp_available(void);
 bool thp_is_enabled(void);
 
 #endif /* __THP_SETTINGS_H__ */
-- 
2.47.3

On 31.07.25 14:27, Usama Arif wrote:
> The test will set the global system THP setting to never, madvise
> or always depending on the fixture variant 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
>    with the prctl, including with MADV_HUGE and MADV_COLLAPSE.
> - successfully reset the policy of the process.
> - after reset, only get hugepages with:
>    - MADV_COLLAPSE when policy is set to never.
>    - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
>    - always when policy is set to "always".
> - repeat the above tests in a forked process to make sure
>    the policy is carried across forks.
> 
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---

[...]

Looks much better already. Some quirks.

> +#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"
> +
> +static int sz2ord(size_t size, size_t pagesize)
> +{
> +	return __builtin_ctzll(size / pagesize);
> +}
> +
> +enum thp_collapse_type {
> +	THP_COLLAPSE_NONE,
> +	THP_COLLAPSE_MADV_HUGEPAGE,	/* MADV_HUGEPAGE before access */
> +	THP_COLLAPSE_MADV_COLLAPSE,	/* MADV_COLLAPSE after access */
> +};
> +
> +enum thp_policy {
> +	THP_POLICY_NEVER,
> +	THP_POLICY_MADVISE,
> +	THP_POLICY_ALWAYS,
> +};

Couldn't you have reused "enum thp_enabled" end simply never specified 
the "THP_INHERIT"? Then, you need to do less translation.

> +
> +struct test_results {
> +	int prctl_get_thp_disable;

The result is always one, does that here make sense?

> +	int prctl_applied_collapse_none;

"prctl_applied" is a bit confusing. And most of these always have the 
same value.

Can't we special case the remaining two cases on the current policy and 
avoid this struct compeltely?


> +	int prctl_applied_collapse_madv_huge;
> +	int prctl_applied_collapse_madv_collapse;
> +	int prctl_removed_collapse_none;
> +	int prctl_removed_collapse_madv_huge;
> +	int prctl_removed_collapse_madv_collapse;
> +};
> +
> +/*
> + * 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
> + * -errno if mmap fails
> + */
> +static int test_mmap_thp(enum thp_collapse_type madvise_buf, size_t pmdsize)
> +{
> +	char *mem, *mmap_mem;
> +	size_t mmap_size;
> +	int ret;
> +
> +	/* For alignment purposes, we need twice the THP size. */
> +	mmap_size = 2 * pmdsize;
> +	mmap_mem = (char *)mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
> +				    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	if (mmap_mem == MAP_FAILED)
> +		return -errno;
> +
> +	/* We need a THP-aligned memory area. */
> +	mem = (char *)(((uintptr_t)mmap_mem + pmdsize) & ~(pmdsize - 1));
> +
> +	if (madvise_buf == THP_COLLAPSE_MADV_HUGEPAGE)
> +		madvise(mem, pmdsize, MADV_HUGEPAGE);
> +
> +	/* Ensure memory is allocated */
> +	memset(mem, 1, pmdsize);
> +
> +	if (madvise_buf == THP_COLLAPSE_MADV_COLLAPSE)
> +		madvise(mem, pmdsize, MADV_COLLAPSE);
> +

To avoid even mmap_mem to get merged with some other VMA, maybe just do
before reading the smap here:

/* HACK: make sure we have a separate VMA that we can check reliably. */
mprotect(mem, pmdsize, PROT_READ);

or

madvise(mem, pmdsize, MADV_DONTFORK);

before reading smaps.

That is probably the easiest approach. The you can drop the lengthy 
comment and perform a single thp check.


[...]

> +
> +static void prctl_thp_disable_test(struct __test_metadata *const _metadata,
> +				   size_t pmdsize, struct test_results *results)
> +{
> +
> +	ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL),
> +		  results->prctl_get_thp_disable);
> +
> +	/* tests after prctl overrides global policy */
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
> +		  results->prctl_applied_collapse_none);
> +
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
> +		  results->prctl_applied_collapse_madv_huge);
> +
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize),
> +		  results->prctl_applied_collapse_madv_collapse);
> +
> +	/* Reset to global policy */
> +	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL), 0);
> +
> +	/* tests after prctl is cleared, and only global policy is effective */
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
> +		  results->prctl_removed_collapse_none);
> +
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
> +		  results->prctl_removed_collapse_madv_huge);
> +
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize),
> +		  results->prctl_removed_collapse_madv_collapse);
> +}
> +
> +FIXTURE(prctl_thp_disable_completely)
> +{
> +	struct thp_settings settings;
> +	struct test_results results;

Is this "expected_results" ?

But again, hopefully we can remove that and instead just base it on the 
polocy that we configured.

-- 
Cheers,

David / dhildenb

On 31/07/2025 20:42, David Hildenbrand wrote:
> On 31.07.25 14:27, Usama Arif wrote:
>> The test will set the global system THP setting to never, madvise
>> or always depending on the fixture variant 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
>>    with the prctl, including with MADV_HUGE and MADV_COLLAPSE.
>> - successfully reset the policy of the process.
>> - after reset, only get hugepages with:
>>    - MADV_COLLAPSE when policy is set to never.
>>    - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
>>    - always when policy is set to "always".
>> - repeat the above tests in a forked process to make sure
>>    the policy is carried across forks.
>>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> ---
> 
> [...]
> 
> Looks much better already. Some quirks.
> 
>> +#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"
>> +
>> +static int sz2ord(size_t size, size_t pagesize)
>> +{
>> +    return __builtin_ctzll(size / pagesize);
>> +}
>> +
>> +enum thp_collapse_type {
>> +    THP_COLLAPSE_NONE,
>> +    THP_COLLAPSE_MADV_HUGEPAGE,    /* MADV_HUGEPAGE before access */
>> +    THP_COLLAPSE_MADV_COLLAPSE,    /* MADV_COLLAPSE after access */
>> +};
>> +
>> +enum thp_policy {
>> +    THP_POLICY_NEVER,
>> +    THP_POLICY_MADVISE,
>> +    THP_POLICY_ALWAYS,
>> +};
> 
> Couldn't you have reused "enum thp_enabled" end simply never specified the "THP_INHERIT"? Then, you need to do less translation.

yes, introducing this enum was silly. Have removed it for next revision.> 
>> +
>> +struct test_results {
>> +    int prctl_get_thp_disable;
> 
> The result is always one, does that here make sense?

Its 3 in the next patch for PR_THP_DISABLE_EXCEPT_ADVISED :)

I will remove this struct, but I think maybe it might have been a good idea to squash this
with the next patch to show why the struct was useful.

> 
>> +    int prctl_applied_collapse_none;
> 
> "prctl_applied" is a bit confusing. And most of these always have the same value.
> 
> Can't we special case the remaining two cases on the current policy and avoid this struct compeltely?
> 

The values are different in the next patch when PR_THP_DISABLE_EXCEPT_ADVISED is used.

Just to explain how I came about using this struct test_results (though it doesnt matter as
I will remove it for the next revision):
I wanted to maximise code reuse and only wanted to have one instance of prctl_thp_disable_test.
I actually started with special casing, but went the brute force way of adding too many if else
statements and it was looking quite messy after I added the tests for PR_THP_DISABLE_EXCEPT_ADVISED.
I saw this struct test_results in another kselftest and thought this should make it much better and
extendable.

I have removed struct test_results and changed prctl_thp_disable_test to the following for next revision:

static void prctl_thp_disable_test(struct __test_metadata *const _metadata,
				   size_t pmdsize, enum thp_enabled thp_policy,
				   int prctl_flags)
{
	ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL),
		  prctl_flags & PR_THP_DISABLE_EXCEPT_ADVISED ? 3 : 1);

	/* tests after prctl overrides global policy */
	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize), 0);

	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
		  thp_policy == THP_NEVER || !prctl_flags ? 0 : 1);

	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize),
		  !prctl_flags ? 0 : 1);

	/* Reset to global policy */
	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL), 0);

	/* tests after prctl is cleared, and only global policy is effective */
	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
		  thp_policy == THP_ALWAYS ? 1 : 0);

	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
		  thp_policy == THP_NEVER ? 0 : 1);

	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 1);
}




> 
>> +    int prctl_applied_collapse_madv_huge;
>> +    int prctl_applied_collapse_madv_collapse;
>> +    int prctl_removed_collapse_none;
>> +    int prctl_removed_collapse_madv_huge;
>> +    int prctl_removed_collapse_madv_collapse;
>> +};
>> +
>> +/*
>> + * 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
>> + * -errno if mmap fails
>> + */
>> +static int test_mmap_thp(enum thp_collapse_type madvise_buf, size_t pmdsize)
>> +{
>> +    char *mem, *mmap_mem;
>> +    size_t mmap_size;
>> +    int ret;
>> +
>> +    /* For alignment purposes, we need twice the THP size. */
>> +    mmap_size = 2 * pmdsize;
>> +    mmap_mem = (char *)mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
>> +                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>> +    if (mmap_mem == MAP_FAILED)
>> +        return -errno;
>> +
>> +    /* We need a THP-aligned memory area. */
>> +    mem = (char *)(((uintptr_t)mmap_mem + pmdsize) & ~(pmdsize - 1));
>> +
>> +    if (madvise_buf == THP_COLLAPSE_MADV_HUGEPAGE)
>> +        madvise(mem, pmdsize, MADV_HUGEPAGE);
>> +
>> +    /* Ensure memory is allocated */
>> +    memset(mem, 1, pmdsize);
>> +
>> +    if (madvise_buf == THP_COLLAPSE_MADV_COLLAPSE)
>> +        madvise(mem, pmdsize, MADV_COLLAPSE);
>> +
> 
> To avoid even mmap_mem to get merged with some other VMA, maybe just do
> before reading the smap here:
> 
> /* HACK: make sure we have a separate VMA that we can check reliably. */
> mprotect(mem, pmdsize, PROT_READ);
> 
Thanks! Yeah this is a nice hack, have used it in the next revision.

> or
> 
> madvise(mem, pmdsize, MADV_DONTFORK);
> 
> before reading smaps.
> 
> That is probably the easiest approach. The you can drop the lengthy comment and perform a single thp check.
> 
> 

[...]

>>> +
>>> +struct test_results {
>>> +    int prctl_get_thp_disable;
>>
>> The result is always one, does that here make sense?
> 
> Its 3 in the next patch for PR_THP_DISABLE_EXCEPT_ADVISED :)
> 
> I will remove this struct, but I think maybe it might have been a good idea to squash this
> with the next patch to show why the struct was useful.

I think it's reasonable to keep them separate.

> 
>>
>>> +    int prctl_applied_collapse_none;
>>
>> "prctl_applied" is a bit confusing. And most of these always have the same value.
>>
>> Can't we special case the remaining two cases on the current policy and avoid this struct compeltely?
>>
> 
> The values are different in the next patch when PR_THP_DISABLE_EXCEPT_ADVISED is used.
> 
> Just to explain how I came about using this struct test_results (though it doesnt matter as
> I will remove it for the next revision):
> I wanted to maximise code reuse and only wanted to have one instance of prctl_thp_disable_test.
> I actually started with special casing, but went the brute force way of adding too many if else
> statements and it was looking quite messy after I added the tests for PR_THP_DISABLE_EXCEPT_ADVISED.
> I saw this struct test_results in another kselftest and thought this should make it much better and
> extendable.
> 
> I have removed struct test_results and changed prctl_thp_disable_test to the following for next revision:

Yeah, or just duplicate that function and call it 
prctl_thp_disable_unless_advised_test() in the next patch.

Makes the code easier to read and the duplication is limited.

-- 
Cheers,

David / dhildenb