On 31/07/2025 20:42, David Hildenbrand wrote:

On 31.07.25 14:27, Usama Arif wrote:

quoted

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 <redacted>
---

[...]

Looks much better already. Some quirks.

quoted

+#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.> 

quoted

+
+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.

quoted

+    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);
}

quoted

+    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.

[...]