Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling THPs except for madvise

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Aug 13, 2025 at 02:55:42PM +0100, 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).
> The fixture setup will also test if PR_SET_THP_DISABLE prctl call can
> be made with PR_THP_DISABLE_EXCEPT_ADVISED and skip if it fails.
>
> This tests if the process can:
> - successfully get the policy to disable THPs expect for madvise.
> - get hugepages only on MADV_HUGE and MADV_COLLAPSE if the global policy
>   is madvise/always and only with MADV_COLLAPSE if the global policy is
>   never.
> - 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.
>
> Test results:
> ./prctl_thp_disable
> TAP version 13
> 1..12
> ok 1 prctl_thp_disable_completely.never.nofork
> ok 2 prctl_thp_disable_completely.never.fork
> ok 3 prctl_thp_disable_completely.madvise.nofork
> ok 4 prctl_thp_disable_completely.madvise.fork
> ok 5 prctl_thp_disable_completely.always.nofork
> ok 6 prctl_thp_disable_completely.always.fork
> ok 7 prctl_thp_disable_except_madvise.never.nofork
> ok 8 prctl_thp_disable_except_madvise.never.fork
> ok 9 prctl_thp_disable_except_madvise.madvise.nofork
> ok 10 prctl_thp_disable_except_madvise.madvise.fork
> ok 11 prctl_thp_disable_except_madvise.always.nofork
> ok 12 prctl_thp_disable_except_madvise.always.fork
>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>

I don't see any tests asserting VM_NOHUGEPAGE behaviour could you expand
this test to add that?

Other than that this is looking good.

Thanks!

> ---
>  .../testing/selftests/mm/prctl_thp_disable.c  | 107 ++++++++++++++++++
>  1 file changed, 107 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> index 8845e9f414560..9bfed4598a1a6 100644
> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -16,6 +16,10 @@
>  #include "thp_settings.h"
>  #include "vm_util.h"
>
> +#ifndef PR_THP_DISABLE_EXCEPT_ADVISED
> +#define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> +#endif
> +
>  enum thp_collapse_type {
>  	THP_COLLAPSE_NONE,
>  	THP_COLLAPSE_MADV_HUGEPAGE,	/* MADV_HUGEPAGE before access */
> @@ -165,4 +169,107 @@ TEST_F(prctl_thp_disable_completely, fork)
>  	ASSERT_EQ(ret, 0);
>  }
>
> +static void prctl_thp_disable_except_madvise_test(struct __test_metadata *const _metadata,
> +						  size_t pmdsize,
> +						  enum thp_enabled thp_policy)
> +{
> +	ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL), 3);
> +
> +	/* 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 ? 0 : 1);
> +
> +	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 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);
> +}
> +
> +FIXTURE(prctl_thp_disable_except_madvise)
> +{
> +	struct thp_settings settings;
> +	size_t pmdsize;
> +};
> +
> +FIXTURE_VARIANT(prctl_thp_disable_except_madvise)
> +{
> +	enum thp_enabled thp_policy;
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, never)
> +{
> +	.thp_policy = THP_NEVER,
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, madvise)
> +{
> +	.thp_policy = THP_MADVISE,
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, always)
> +{
> +	.thp_policy = THP_ALWAYS,
> +};

again the kselftest_harness stuff is really useful!

> +
> +FIXTURE_SETUP(prctl_thp_disable_except_madvise)
> +{
> +	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");
> +
> +	if (prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, NULL, NULL))
> +		SKIP(return, "Unable to set PR_THP_DISABLE_EXCEPT_ADVISED\n");

This should be a test fail I think, as the only ways this could fail are
invalid flags, or failure to obtain an mmap write lock.

> +
> +	thp_save_settings();
> +	thp_read_settings(&self->settings);
> +	self->settings.thp_enabled = variant->thp_policy;
> +	self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
> +	thp_write_settings(&self->settings);
> +}
> +
> +FIXTURE_TEARDOWN(prctl_thp_disable_except_madvise)
> +{
> +	thp_restore_settings();
> +}
> +
> +TEST_F(prctl_thp_disable_except_madvise, nofork)
> +{
> +	prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize, variant->thp_policy);
> +}
> +
> +TEST_F(prctl_thp_disable_except_madvise, fork)
> +{
> +	int ret = 0;
> +	pid_t pid;
> +
> +	/* Make sure prctl changes are carried across fork */
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (!pid)
> +		prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize,
> +						      variant->thp_policy);
> +
> +	wait(&ret);
> +	if (WIFEXITED(ret))
> +		ret = WEXITSTATUS(ret);
> +	else
> +		ret = -EINVAL;
> +	ASSERT_EQ(ret, 0);
> +}
> +
>  TEST_HARNESS_MAIN
> --
> 2.47.3
>