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 to disable all THPs and skip if it fails.

This tests if the process can:
- successfully 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".
- never get a THP with MADV_NOHUGEPAGE.
- 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>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../testing/selftests/mm/prctl_thp_disable.c  | 175 ++++++++++++++++++
 tools/testing/selftests/mm/thp_settings.c     |   9 +-
 tools/testing/selftests/mm/thp_settings.h     |   1 +
 5 files changed, 186 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 e7b23a8a05fe2..eb023ea857b31 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 d75f1effcb791..bd5d17beafa64 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -87,6 +87,7 @@ 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 0000000000000..e9e519c85224c
--- /dev/null
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -0,0 +1,175 @@
+// 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"
+
+enum thp_collapse_type {
+	THP_COLLAPSE_NONE,
+	THP_COLLAPSE_MADV_NOHUGEPAGE,
+	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
+ * -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);
+	else if (madvise_buf == THP_COLLAPSE_MADV_NOHUGEPAGE)
+		madvise(mem, pmdsize, MADV_NOHUGEPAGE);
+
+	/* Ensure memory is allocated */
+	memset(mem, 1, pmdsize);
+
+	if (madvise_buf == THP_COLLAPSE_MADV_COLLAPSE)
+		madvise(mem, pmdsize, MADV_COLLAPSE);
+
+	/* HACK: make sure we have a separate VMA that we can check reliably. */
+	mprotect(mem, pmdsize, PROT_READ);
+
+	ret = check_huge_anon(mem, 1, pmdsize);
+	munmap(mmap_mem, mmap_size);
+	return ret;
+}
+
+static void prctl_thp_disable_completely_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), 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_NOHUGEPAGE, pmdsize), 0);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize), 0);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 0);
+
+	/* 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_NOHUGEPAGE, 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);
+}
+
+FIXTURE(prctl_thp_disable_completely)
+{
+	struct thp_settings settings;
+	size_t pmdsize;
+};
+
+FIXTURE_VARIANT(prctl_thp_disable_completely)
+{
+	enum thp_enabled thp_policy;
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, never)
+{
+	.thp_policy = THP_NEVER,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, madvise)
+{
+	.thp_policy = THP_MADVISE,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_completely, always)
+{
+	.thp_policy = THP_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");
+
+	if (prctl(PR_SET_THP_DISABLE, 1, NULL, NULL, NULL))
+		SKIP(return, "Unable to disable THPs completely for the process\n");
+
+	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_completely)
+{
+	thp_restore_settings();
+}
+
+TEST_F(prctl_thp_disable_completely, nofork)
+{
+	prctl_thp_disable_completely_test(_metadata, self->pmdsize, variant->thp_policy);
+}
+
+TEST_F(prctl_thp_disable_completely, 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_completely_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
diff --git a/tools/testing/selftests/mm/thp_settings.c b/tools/testing/selftests/mm/thp_settings.c
index bad60ac52874a..574bd0f8ae480 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 6c07f70beee97..76eeb712e5f10 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 Fri, Aug 15, 2025 at 02:54:58PM +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 to disable all THPs and skip if it fails.

I don't think this is an issue in this patch but with it we're seeing
build failures in -next on arm64 with:

  make KBUILD_BUILD_USER=KernelCI FORMAT=.xz ARCH=arm64 HOSTCC=gcc CROSS_COMPILE=aarch64-linux-gnu- CROSS_COMPILE_COMPAT=arm-linux-gnueabihf- CC="ccache aarch64-linux-gnu-gcc" O=/tmp/kci/linux/build -C/tmp/kci/linux -j98 kselftest-gen_tar

  ...

    CC       prctl_thp_disable
  prctl_thp_disable.c: In function ‘test_mmap_thp’:
  prctl_thp_disable.c:64:39: error: ‘MADV_COLLAPSE’ undeclared (first use in this function); did you mean ‘MADV_COLD’?
     64 |                 madvise(mem, pmdsize, MADV_COLLAPSE);
        |                                       ^~~~~~~~~~~~~
        |                                       MADV_COLD

since the headers_install copy of asm-generic/mman-common.h doesn't
appear to being picked up with the above build invocation (most others
are fine).  I'm not clear why, it looks like an appropriate -isystem
ends up getting passed to the compiler:

  aarch64-linux-gnu-gcc -Wall -O2 -I /linux/tools/testing/selftests/../../..  -isystem /tmp/kci/linux/build/usr/include -isystem /linux/tools/testing/selftests/../../../tools/include/uapi -U_FORTIFY_SOURCE -D_GNU_SOURCE=     prctl_thp_disable.c vm_util.c thp_settings.c -lrt -lpthread -lm -o /tmp/kci/linux/build/kselftest/mm/prctl_thp_disable

but the header there is getting ignored AFAICT.  Probably the problem is
fairly obvious and I'm just being slow - I'm not quite 100% at the
minute.

Thanks to Aishwarya for confirming which patch triggered the issue.

On 5 Sep 2025, at 13:43, Mark Brown wrote:

> On Fri, Aug 15, 2025 at 02:54:58PM +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 to disable all THPs and skip if it fails.
>
> I don't think this is an issue in this patch but with it we're seeing
> build failures in -next on arm64 with:
>
>   make KBUILD_BUILD_USER=KernelCI FORMAT=.xz ARCH=arm64 HOSTCC=gcc CROSS_COMPILE=aarch64-linux-gnu- CROSS_COMPILE_COMPAT=arm-linux-gnueabihf- CC="ccache aarch64-linux-gnu-gcc" O=/tmp/kci/linux/build -C/tmp/kci/linux -j98 kselftest-gen_tar
>
>   ...
>
>     CC       prctl_thp_disable
>   prctl_thp_disable.c: In function ‘test_mmap_thp’:
>   prctl_thp_disable.c:64:39: error: ‘MADV_COLLAPSE’ undeclared (first use in this function); did you mean ‘MADV_COLD’?
>      64 |                 madvise(mem, pmdsize, MADV_COLLAPSE);
>         |                                       ^~~~~~~~~~~~~
>         |                                       MADV_COLD
>
> since the headers_install copy of asm-generic/mman-common.h doesn't
> appear to being picked up with the above build invocation (most others
> are fine).  I'm not clear why, it looks like an appropriate -isystem
> ends up getting passed to the compiler:
>
>   aarch64-linux-gnu-gcc -Wall -O2 -I /linux/tools/testing/selftests/../../..  -isystem /tmp/kci/linux/build/usr/include -isystem /linux/tools/testing/selftests/../../../tools/include/uapi -U_FORTIFY_SOURCE -D_GNU_SOURCE=     prctl_thp_disable.c vm_util.c thp_settings.c -lrt -lpthread -lm -o /tmp/kci/linux/build/kselftest/mm/prctl_thp_disable
>
> but the header there is getting ignored AFAICT.  Probably the problem is
> fairly obvious and I'm just being slow - I'm not quite 100% at the
> minute.

prctl_thp_disable.c uses “#include <sys/mman.h>” but asm-generic/mman-common.h
is included in asm/mman.h. And sys/mman.h gets MADV_COLLAPSE from
bits/mman-linux.h. Maybe that is why?

>
> Thanks to Aishwarya for confirming which patch triggered the issue.


Best Regards,
Yan, Zi

On Fri, Sep 05, 2025 at 01:55:53PM -0400, Zi Yan wrote:
> On 5 Sep 2025, at 13:43, Mark Brown wrote:

> > but the header there is getting ignored AFAICT.  Probably the problem is
> > fairly obvious and I'm just being slow - I'm not quite 100% at the
> > minute.

> prctl_thp_disable.c uses “#include <sys/mman.h>” but asm-generic/mman-common.h
> is included in asm/mman.h. And sys/mman.h gets MADV_COLLAPSE from
> bits/mman-linux.h. Maybe that is why?

Ah, of course - if glibc is reproducing the kernel definitions rather
than including the kernel headers to get them then that'd do it.
Probably the test needs to locally define the new MADV_COLLAPSE for
glibc compatibility, IME trying to directly include the kernel headers
when glibc doesn't normally use them tends to blow up on you sooner or
later.

I knew it'd be something simple, thanks.

On 05/09/2025 19:02, Mark Brown wrote:
> On Fri, Sep 05, 2025 at 01:55:53PM -0400, Zi Yan wrote:
>> On 5 Sep 2025, at 13:43, Mark Brown wrote:
> 
>>> but the header there is getting ignored AFAICT.  Probably the problem is
>>> fairly obvious and I'm just being slow - I'm not quite 100% at the
>>> minute.
> 
>> prctl_thp_disable.c uses “#include <sys/mman.h>” but asm-generic/mman-common.h
>> is included in asm/mman.h. And sys/mman.h gets MADV_COLLAPSE from
>> bits/mman-linux.h. Maybe that is why?
> 
> Ah, of course - if glibc is reproducing the kernel definitions rather
> than including the kernel headers to get them then that'd do it.
> Probably the test needs to locally define the new MADV_COLLAPSE for
> glibc compatibility, IME trying to directly include the kernel headers
> when glibc doesn't normally use them tends to blow up on you sooner or
> later.
> 
> I knew it'd be something simple, thanks.

Hi Mark,

Thanks for raising this. I think doing 

diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
index 89ed0d7db1c16..0afcdbad94f3d 100644
--- a/tools/testing/selftests/mm/prctl_thp_disable.c
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -9,6 +9,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <sys/mman.h>
+#include <linux/mman.h>
 #include <sys/prctl.h>
 #include <sys/wait.h>


should fix this issue?

Thanks
Usama

On Fri, Sep 05, 2025 at 08:40:25PM +0100, Usama Arif wrote:

> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> index 89ed0d7db1c16..0afcdbad94f3d 100644
> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -9,6 +9,7 @@
>  #include <string.h>
>  #include <unistd.h>
>  #include <sys/mman.h>
> +#include <linux/mman.h>
>  #include <sys/prctl.h>
>  #include <sys/wait.h>

> should fix this issue?

That seems to do the right thing for me, I'm slightly nervous about
collisions with glibc in future (it's been an issue in the past when
they're not using our headers) but I guess we can deal with that if it
actually comes up.

On 08/09/2025 16:38, Mark Brown wrote:
> On Fri, Sep 05, 2025 at 08:40:25PM +0100, Usama Arif wrote:
> 
>> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
>> index 89ed0d7db1c16..0afcdbad94f3d 100644
>> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
>> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
>> @@ -9,6 +9,7 @@
>>  #include <string.h>
>>  #include <unistd.h>
>>  #include <sys/mman.h>
>> +#include <linux/mman.h>
>>  #include <sys/prctl.h>
>>  #include <sys/wait.h>
> 
>> should fix this issue?
> 
> That seems to do the right thing for me, I'm slightly nervous about
> collisions with glibc in future (it's been an issue in the past when
> they're not using our headers) but I guess we can deal with that if it
> actually comes up.

I have seen this included in another selftest as well with MADV_COLLAPSE.

I think best to have it. I will send a patch with it.

Thanks for confirming!

On 05/09/2025 20:40, Usama Arif wrote:
> 
> 
> On 05/09/2025 19:02, Mark Brown wrote:
>> On Fri, Sep 05, 2025 at 01:55:53PM -0400, Zi Yan wrote:
>>> On 5 Sep 2025, at 13:43, Mark Brown wrote:
>>
>>>> but the header there is getting ignored AFAICT.  Probably the problem is
>>>> fairly obvious and I'm just being slow - I'm not quite 100% at the
>>>> minute.
>>
>>> prctl_thp_disable.c uses “#include <sys/mman.h>” but asm-generic/mman-common.h
>>> is included in asm/mman.h. And sys/mman.h gets MADV_COLLAPSE from
>>> bits/mman-linux.h. Maybe that is why?
>>
>> Ah, of course - if glibc is reproducing the kernel definitions rather
>> than including the kernel headers to get them then that'd do it.
>> Probably the test needs to locally define the new MADV_COLLAPSE for
>> glibc compatibility, IME trying to directly include the kernel headers
>> when glibc doesn't normally use them tends to blow up on you sooner or
>> later.
>>
>> I knew it'd be something simple, thanks.
> 
> Hi Mark,
> 
> Thanks for raising this. I think doing 
> 
> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> index 89ed0d7db1c16..0afcdbad94f3d 100644
> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -9,6 +9,7 @@
>  #include <string.h>
>  #include <unistd.h>
>  #include <sys/mman.h>
> +#include <linux/mman.h>
>  #include <sys/prctl.h>
>  #include <sys/wait.h>
> 
> 
> should fix this issue?
> 
> Thanks
> Usama


If that fixes it, please feel free to send a patch or let me know and I will send it.

Thanks!
Usama

On 15/08/2025 14:54, 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 to disable all THPs and skip if it fails.
> 
> This tests if the process can:
> - successfully 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".
> - never get a THP with MADV_NOHUGEPAGE.
> - 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>
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  tools/testing/selftests/mm/.gitignore         |   1 +
>  tools/testing/selftests/mm/Makefile           |   1 +
>  .../testing/selftests/mm/prctl_thp_disable.c  | 175 ++++++++++++++++++
>  tools/testing/selftests/mm/thp_settings.c     |   9 +-
>  tools/testing/selftests/mm/thp_settings.h     |   1 +
>  5 files changed, 186 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/mm/prctl_thp_disable.c
>

Andrew, could you please apply the below fixlet on top of this patch as suggested
by David in https://lore.kernel.org/all/a385e09f-f582-4ede-9e60-1d85cee02a3c@redhat.com/?

Thanks!

From db9306c06cbd6057c2a8839e5d4c1d2559b58b70 Mon Sep 17 00:00:00 2001
From: Usama Arif <usamaarif642@gmail.com>
Date: Mon, 18 Aug 2025 11:27:04 +0100
Subject: [PATCH 2/4] [fixlet] selftests: prctl: return after executing test in
 child process

The next step after executing the test is a wait, but there is nothing to wait
for in the child, so just return.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 tools/testing/selftests/mm/prctl_thp_disable.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
index e9e519c85224c..df3cce278e10a 100644
--- a/tools/testing/selftests/mm/prctl_thp_disable.c
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -161,8 +161,10 @@ TEST_F(prctl_thp_disable_completely, fork)
 	pid = fork();
 	ASSERT_GE(pid, 0);
 
-	if (!pid)
+	if (!pid) {
 		prctl_thp_disable_completely_test(_metadata, self->pmdsize, variant->thp_policy);
+		return;
+	}
 
 	wait(&ret);
 	if (WIFEXITED(ret))
-- 
2.47.3

> +
> +TEST_F(prctl_thp_disable_completely, 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_completely_test(_metadata, self->pmdsize, variant->thp_policy);
> +

Skimming over this once more ... this raises two questions

(a) There is nothing to wait for in the child
(b) Does it work when we return in the child from this function?

I think (b) works by design of the kselftest_harness, as this function is
itself executed from a child process.

Regarding (a), it might be cleaner to just

index 77c53a91124f1..1a48bcf2e9160 100644
--- a/tools/testing/selftests/mm/prctl_thp_disable.c
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -271,9 +271,11 @@ TEST_F(prctl_thp_disable_except_madvise, fork)
         pid = fork();
         ASSERT_GE(pid, 0);
  
-       if (!pid)
+       if (!pid) {
                 prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize,
                                                       variant->thp_policy);
+               return;
+       }
  
         wait(&ret);
         if (WIFEXITED(ret))



Same probably applies to patch #7. Feel free to send simple fixup patches
that Andrew can squash. No need for a full resend.

-- 
Cheers

David / dhildenb

On 18/08/2025 10:36, David Hildenbrand wrote:
>> +
>> +TEST_F(prctl_thp_disable_completely, 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_completely_test(_metadata, self->pmdsize, variant->thp_policy);
>> +
> 
> Skimming over this once more ... this raises two questions
> 
> (a) There is nothing to wait for in the child
> (b) Does it work when we return in the child from this function?
> 
> I think (b) works by design of the kselftest_harness, as this function is
> itself executed from a child process.
> 
> Regarding (a), it might be cleaner to just
> 

Makes sense, thanks for pointing this out! Have sent the fixlets.