Re: [PATCH v4] selftests/mm: add folio_split() and filemap_get_entry() race test

[David Hildenbrand (Arm)]

On 3/20/26 15:22, Zi Yan wrote:
> The added folio_split_race_test is a modified C port of the race condition
> test from [1]. The test creates shmem huge pages, where the main thread
> punches holes in the shmem to cause folio_split() in the kernel and
> a set of 16 threads reads the shmem to cause filemap_get_entry() in the
> kernel. filemap_get_entry() reads the folio and xarray split by
> folio_split() locklessly. The original test[2] is written in rust and uses
> memfd (shmem backed). This C port uses shmem directly and use a single
> process.
> 
> Note: the initial rust to C conversion is done by Cursor.
> 
> Link: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@xxxxxxxxxxxxxx/ [1]
> Link: https://github.com/dfinity/thp-madv-remove-test [2]
> Signed-off-by: Bas van Dijk <bas@xxxxxxxxxxx>
> Signed-off-by: Adam Bratschi-Kaye <adam.bratschikaye@xxxxxxxxxxx>

You are likely missing two Co-developed-by.

See Documentation/process/submitting-patches.rst on how to handle such
SOBs.

> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
> From V3:
> 1. fixed for loop stepping issue
> 2. used PRIu64 instead of %zu for uint64_t.
> 
> From V2:
> 1. simplied the program by removing fork.
> 
> From V1:
> 1. added prctl(PR_SET_PDEATHSIG, SIGTERM) to avoid child looping
>    forever.
> 2. removed page_idx % PUNCH_INTERVAL >= 0, since it is a nop. Added a
>    comment.
> 3. added a child process status check to prevent parent looping forever
>    and record that as a failure.
> 4. used ksft_exit_skip() instead of ksft_finished() when the program is
>    not running as root.
> 5. restored THP settings properly when the program exits abnormally.
>  tools/testing/selftests/mm/Makefile           |   1 +
>  .../selftests/mm/folio_split_race_test.c      | 293 ++++++++++++++++++
>  tools/testing/selftests/mm/run_vmtests.sh     |   2 +
>  3 files changed, 296 insertions(+)
>  create mode 100644 tools/testing/selftests/mm/folio_split_race_test.c
> 
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index 7a5de4e9bf520..cd24596cdd27e 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -105,6 +105,7 @@ TEST_GEN_FILES += droppable
>  TEST_GEN_FILES += guard-regions
>  TEST_GEN_FILES += merge
>  TEST_GEN_FILES += rmap
> +TEST_GEN_FILES += folio_split_race_test
>  
>  ifneq ($(ARCH),arm64)
>  TEST_GEN_FILES += soft-dirty
> diff --git a/tools/testing/selftests/mm/folio_split_race_test.c b/tools/testing/selftests/mm/folio_split_race_test.c
> new file mode 100644
> index 0000000000000..c264cc625a7cb
> --- /dev/null
> +++ b/tools/testing/selftests/mm/folio_split_race_test.c
> @@ -0,0 +1,293 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * The test creates shmem PMD huge pages, fills all pages with known patterns,
> + * then continuously verifies non-punched pages with 16 threads. Meanwhile, the
> + * main thread punches holes via MADV_REMOVE on the shmem.
> + *
> + * It tests the race condition between folio_split() and filemap_get_entry(),
> + * where the hole punches on shmem lead to folio_split() and reading the shmem
> + * lead to filemap_get_entry().
> + */
> +
> +#define _GNU_SOURCE
> +#include <errno.h>
> +#include <inttypes.h>
> +#include <linux/mman.h>
> +#include <pthread.h>
> +#include <stdatomic.h>
> +#include <stdbool.h>
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/mman.h>
> +#include <signal.h>
> +#include <unistd.h>
> +#include "vm_util.h"
> +#include "kselftest.h"
> +#include "thp_settings.h"


[...] some comment son the main() part :)

> +int main(void)
> +{
> +	struct thp_settings current_settings;
> +	bool failed = false;
> +	size_t failures;
> +	size_t iter;

Why are iterations a "size_t" ? Similarly for "failures". Just use int / unsigned long?

> +
> +	ksft_print_header();
> +
> +	if (!thp_is_enabled())
> +		ksft_exit_skip("Transparent Hugepages not available\n");

Is checking thp_is_enabled() the right thing to do when you perform your own setup below either way?

I think you should just use thp_available(). Then, configure THP accordingly below?

> +
> +	if (geteuid() != 0)
> +		ksft_exit_skip("Please run the test as root\n");
> +
> +	thp_save_settings();
> +	/* make sure thp settings are restored */
> +	if (atexit(thp_settings_cleanup) != 0)
> +		ksft_exit_fail_msg("atexit failed\n");
> +
> +	signal(SIGINT, thp_cleanup_handler);
> +	signal(SIGTERM, thp_cleanup_handler);
> +
> +	thp_read_settings(&current_settings);
> +	current_settings.shmem_enabled = SHMEM_ADVISE;
> +	thp_write_settings(&current_settings);
> +
> +	ksft_set_plan(1);
> +
> +	page_size = getpagesize();
> +	pmd_pagesize = read_pmd_pagesize();

I wonder whether we should check for 0 here and skip the test (older kernels?).

> +
> +	ksft_print_msg("folio split race test\n");
> +	ksft_print_msg("===================================================\n");
> +	ksft_print_msg("Shmem size:       %" PRIu64 " MiB\n", FILE_SIZE / 1024 / 1024);
> +	ksft_print_msg("Total pages:     %" PRIu64 "\n", TOTAL_PAGES);
> +	ksft_print_msg("Child readers:   %d\n", NUM_READER_THREADS);
> +	ksft_print_msg("Punching every %dth to %dth page\n", PUNCH_INTERVAL,
> +		       PUNCH_INTERVAL + PUNCH_SIZE_FACTOR);
> +	ksft_print_msg("Iterations:      %d\n", NUM_ITERATIONS);


I don't think printing static test information is that helpful.
Do we need all that at all?


> +
> +	for (iter = 1; iter <= NUM_ITERATIONS; iter++) {

Why not start at 0? You know, to confuse less people :)

	for (iter = 0; iter < NUM_ITERATIONS; iter++) {

> +		failures = run_iteration();

"corrupted_pages" ?

> +		if (failures > 0) {
> +			failed = true;

Do you really need that variable?

> +			ksft_print_msg(
> +				"FAILED on iteration %zu: %zu pages corrupted by MADV_REMOVE!\n",
> +				iter, failures);

Can that simply be printed below?

Like

if (iter < NUM_ITERATIONS) {
	ksft_test_result_fail("Test failed on iterations %zu: %zu pages ...\n",
			      iter + 1, corrupted_pages);
} else {
	ksft_test_result_pass ...
}

> +			break;
> +		}
> +	}
> +
> +	if (failed) {
> +		ksft_test_result_fail("Test failed\n");
> +		ksft_exit_fail();
> +	} else {
> +		ksft_test_result_pass("All %d iterations passed\n",
> +				      NUM_ITERATIONS);
> +		ksft_exit_pass();
> +	}
> +
> +	return 0;
-- 
Cheers,

David