[PATCH v4] selftests/mm: hugetlb_madv_vs_map: add underflow test

[PATCH v4] selftests/mm: hugetlb_madv_vs_map: add underflow test

[Guillaume Morin]

Add a test that checks for underflows when a parent unmaps the page
first. Also check that when the child exits the reserve count is
correct.

Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
Signed-off-by: Guillaume Morin <guillaume@morinfr.org
Cc: Breno Leitao <leitao@debian.org>
Cc: Mike Rapoport <rppt@kernel.org>
---
v4: address Mike's comments
v3: address Mike's comments
v2: refactor tests into 2 functions

 .../testing/selftests/mm/hugepage_settings.c  |   9 ++
 .../testing/selftests/mm/hugepage_settings.h  |   1 +
 .../selftests/mm/hugetlb_madv_vs_map.c        | 145 +++++++++++++++---
 3 files changed, 134 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/mm/hugepage_settings.c b/tools/testing/selftests/mm/hugepage_settings.c
index d7917dce3aba..584054736ce9 100644
--- a/tools/testing/selftests/mm/hugepage_settings.c
+++ b/tools/testing/selftests/mm/hugepage_settings.c
@@ -449,6 +449,15 @@ unsigned long hugetlb_free_pages(unsigned long size)
 	return read_num(path);
 }
 
+unsigned long hugetlb_nr_resv_pages(unsigned long size)
+{
+	char path[PATH_MAX];
+
+	hugetlb_sysfs_path(path, sizeof(path), size, "resv_hugepages");
+
+	return read_num(path);
+}
+
 static bool __hugetlb_setup(unsigned long size, unsigned long nr)
 {
 	unsigned long free = hugetlb_free_pages(size);
diff --git a/tools/testing/selftests/mm/hugepage_settings.h b/tools/testing/selftests/mm/hugepage_settings.h
index 726c73c43c05..548e9d288d1d 100644
--- a/tools/testing/selftests/mm/hugepage_settings.h
+++ b/tools/testing/selftests/mm/hugepage_settings.h
@@ -98,6 +98,7 @@ unsigned long default_huge_page_size(void);
 unsigned long hugetlb_nr_pages(unsigned long size);
 void hugetlb_set_nr_pages(unsigned long size, unsigned long nr);
 unsigned long hugetlb_free_pages(unsigned long size);
+unsigned long hugetlb_nr_resv_pages(unsigned long size);
 
 static inline void hugetlb_save_settings(void)
 {
diff --git a/tools/testing/selftests/mm/hugetlb_madv_vs_map.c b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
index f94549efcc6f..d89b2c27c2ae 100644
--- a/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
+++ b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
@@ -3,25 +3,16 @@
  * A test case that must run on a system with one and only one huge page available.
  *	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
  *
- * During setup, the test allocates the only available page, and starts three threads:
- *  - thread1:
- *	* madvise(MADV_DONTNEED) on the allocated huge page
- *  - thread 2:
- *	* Write to the allocated huge page
- *  - thread 3:
- *	* Try to allocated an extra huge page (which must not available)
- *
- *  The test fails if thread3 is able to allocate a page.
- *
- *  Touching the first page after thread3's allocation will raise a SIGBUS
- *
  *  Author: Breno Leitao <leitao@debian.org>
  */
+#include <limits.h>
 #include <pthread.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <sys/types.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include "vm_util.h"
@@ -74,7 +65,21 @@ void *map_extra(void *unused)
 	return NULL;
 }
 
-int main(void)
+/* During setup in main, the only available page was allocated. This test then
+ * starts three threads:
+ *
+ * - thread1:
+ *	* madvise(MADV_DONTNEED) on the allocated huge page
+ *  - thread 2:
+ *	* Write to the allocated huge page
+ *  - thread 3:
+ *	* Try to allocated an extra huge page (which must not available)
+ *
+ *  The test fails if thread3 is able to allocate a page.
+ *
+ *  Touching the first page after thread3's allocation will raise a SIGBUS
+ */
+void test_madv_vs_map(void)
 {
 	pthread_t thread1, thread2, thread3;
 	void *ret;
@@ -85,13 +90,6 @@ int main(void)
 	 */
 	int max = 10;
 
-	ksft_print_header();
-	ksft_set_plan(1);
-
-	if (!hugetlb_setup_default_exact(1))
-		ksft_exit_skip("This test needs one and only one page to execute. Got %lu\n",
-			       hugetlb_free_default_pages());
-
 	mmap_size = default_huge_page_size();
 
 	while (max--) {
@@ -100,7 +98,7 @@ int main(void)
 				-1, 0);
 
 		if ((unsigned long)huge_ptr == -1)
-			ksft_exit_fail_msg("Failed to allocate huge page\n");
+			ksft_exit_fail_perror("Failed to allocate huge page\n");
 
 		pthread_create(&thread1, NULL, madv, NULL);
 		pthread_create(&thread2, NULL, touch, NULL);
@@ -120,5 +118,110 @@ int main(void)
 	}
 
 	ksft_test_result_pass("No unexpected huge page allocations\n");
+}
+
+/*  We create a child process, then unmap the page in the parent while the child
+ *  waits and verify that there is no underflow of the reserved count. We also
+ *  verify that after the child exits, the reserved count is properly restored.
+ */
+void test_underflow(void)
+{
+	pid_t pid;
+	int pipe_fds[2];
+	unsigned long nr_reserved = 0;
+
+	huge_ptr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0);
+
+	if ((unsigned long)huge_ptr == -1)
+		ksft_exit_fail_perror("Failed to allocate huge page\n");
+
+	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
+	if (nr_reserved != 1)
+		ksft_exit_fail_msg("Unexpected number of reserved pages: %lu, expected 1\n",
+				   nr_reserved);
+
+	/* Force the fault to ensure the reservation is consumed */
+	*huge_ptr = 0;
+	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
+	if (nr_reserved != 0)
+		ksft_exit_fail_msg("Unexpected number of reserved pages: %lu, expected 0\n",
+				   nr_reserved);
+
+	if (pipe(pipe_fds) != 0)
+		ksft_exit_fail_perror("pipe failed");
+
+	pid = fork();
+	if (pid < 0)
+		ksft_exit_fail_perror("fork failed");
+
+	if (pid == 0) {
+		/* Child: Simply wait for the parent */
+		char b;
+
+		close(pipe_fds[1]);
+		if (read(pipe_fds[0], &b, 1) < 0)
+			ksft_perror("child read failed");
+		/* Let the parent do the cleanup */
+		_exit(0);
+	}
+
+	/* Parent */
+	close(pipe_fds[0]);
+
+	/* First unmap, this will close the vma */
+	if (munmap(huge_ptr, mmap_size) != 0) {
+		ksft_perror("munmap failed");
+		goto err_cleanup;
+	}
+
+	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
+	if (nr_reserved == 0) {
+		ksft_test_result_pass("Underflow not present!\n");
+	} else {
+		ksft_test_result_fail("Unexpected HugePages_Rsvd=%ld after munmap, should be 0\n",
+				      nr_reserved);
+		goto err_cleanup;
+	}
+	/* Make the child exit, this should restore HugePages_Rsvd to 0 */
+	if (write(pipe_fds[1], &nr_reserved, 1) < 0) {
+		/* If write failed, the child is likely already gone */
+		ksft_exit_fail_perror("write failed");
+	}
+	close(pipe_fds[1]);
+	if (waitpid(pid, NULL, 0) <= 0) {
+		ksft_exit_fail_msg("waitpid failed\n");
+	}
+
+	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
+	if (nr_reserved == 0) {
+		ksft_test_result_pass("After the child dies, HugePages_Rsvd is properly set to 0\n");
+	} else {
+		ksft_exit_fail_msg("Unexpected HugePages_Rsvd=%ld after the child termination munmap, should be 0\n", nr_reserved);
+	}
+	return;
+
+err_cleanup:
+	if (write(pipe_fds[1], &nr_reserved, 1) < 0) {
+		ksft_exit_fail_perror("write failed");
+	}
+	if (waitpid(pid, NULL, 0) <= 0) {
+		ksft_exit_fail_perror("waitpid failed");
+	}
+	ksft_exit_fail();
+}
+
+int main(void)
+{
+	ksft_print_header();
+	ksft_set_plan(3);
+
+	if (!hugetlb_setup_default_exact(1))
+		ksft_exit_skip("This test needs one and only one page to execute. Got %lu\n",
+			       hugetlb_free_default_pages());
+
+	test_madv_vs_map();
+	test_underflow();
+
 	ksft_finished();
 }
-- 
2.39.1


-- 
Guillaume Morin <guillaume@morinfr.org>

Re: [PATCH v4] selftests/mm: hugetlb_madv_vs_map: add underflow test

[Mike Rapoport]

On Fri, Sep 04, 2026 at 09:34:19PM +0200, Guillaume Morin wrote:
> Add a test that checks for underflows when a parent unmaps the page
> first. Also check that when the child exits the reserve count is
> correct.
> 
> Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
> Signed-off-by: Guillaume Morin <guillaume@morinfr.org
> Cc: Breno Leitao <leitao@debian.org>
> Cc: Mike Rapoport <rppt@kernel.org>

With some small nits below fixed

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> v4: address Mike's comments
> v3: address Mike's comments
> v2: refactor tests into 2 functions
> 
> @@ -100,7 +98,7 @@ int main(void)
>  				-1, 0);
>  
>  		if ((unsigned long)huge_ptr == -1)
> -			ksft_exit_fail_msg("Failed to allocate huge page\n");
> +			ksft_exit_fail_perror("Failed to allocate huge page\n");

Nit: _perror() does not need "\n".

>  		pthread_create(&thread1, NULL, madv, NULL);
>  		pthread_create(&thread2, NULL, touch, NULL);
> @@ -120,5 +118,110 @@ int main(void)
>  	}
>  
>  	ksft_test_result_pass("No unexpected huge page allocations\n");
> +}
> +
> +/*  We create a child process, then unmap the page in the parent while the child
> + *  waits and verify that there is no underflow of the reserved count. We also
> + *  verify that after the child exits, the reserved count is properly restored.
> + */
> +void test_underflow(void)
> +{
> +	pid_t pid;
> +	int pipe_fds[2];
> +	unsigned long nr_reserved = 0;
> +
> +	huge_ptr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
> +			MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0);
> +
> +	if ((unsigned long)huge_ptr == -1)
> +		ksft_exit_fail_perror("Failed to allocate huge page\n");

Ditto.

> +
> +	nr_reserved = hugetlb_nr_resv_pages(default_huge_page_size());
> +	if (nr_reserved != 1)
> +		ksft_exit_fail_msg("Unexpected number of reserved pages: %lu, expected 1\n",
> +				   nr_reserved);
> +

-- 
Sincerely yours,
Mike.

Re: [PATCH v4] selftests/mm: hugetlb_madv_vs_map: add underflow test

[Breno Leitao]

On Fri, Sep 04, 2026 at 09:34:19PM +0200, Guillaume Morin wrote:
> Add a test that checks for underflows when a parent unmaps the page
> first. Also check that when the child exits the reserve count is
> correct.
> 
> Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
> Signed-off-by: Guillaume Morin <guillaume@morinfr.org

it seems you are missing a > at the end of your email.

> Cc: Breno Leitao <leitao@debian.org>
> Cc: Mike Rapoport <rppt@kernel.org>

Reviewed-by: Breno Leitao <leitao@debian.org>

> diff --git a/tools/testing/selftests/mm/hugetlb_madv_vs_map.c b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
> index f94549efcc6f..d89b2c27c2ae 100644
> --- a/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
> +++ b/tools/testing/selftests/mm/hugetlb_madv_vs_map.c
> @@ -3,25 +3,16 @@
>   * A test case that must run on a system with one and only one huge page available.
>   *	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>   *
> - * During setup, the test allocates the only available page, and starts three threads:
> - *  - thread1:
> - *	* madvise(MADV_DONTNEED) on the allocated huge page
> - *  - thread 2:
> - *	* Write to the allocated huge page
> - *  - thread 3:
> - *	* Try to allocated an extra huge page (which must not available)
> - *
> - *  The test fails if thread3 is able to allocate a page.
> - *
> - *  Touching the first page after thread3's allocation will raise a SIGBUS
> - *
>   *  Author: Breno Leitao <leitao@debian.org>
>   */
> +#include <limits.h>
>  #include <pthread.h>
> +#include <signal.h>

nit: Are limits.h and signal.h ever used by yor new code?