Re: [PATCH v2 4/5] selftests: prctl: introduce tests for disabling THPs completely

[David Hildenbrand]

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@xxxxxxxxx>
> ---

[...]

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