Re: [PATCH 4/5] selftests: prctl: introduce tests for disabling THPs completely

[David Hildenbrand <david@redhat.com>]

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@gmail.com>
> ---
>   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@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"
> +
> +#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