Re: [PATCH v2 2/2] selftests/mm: Add a new test for madv and hugetlb

[Ryan Roberts <ryan.roberts@arm.com>]

On 02/11/2023 12:24, Ryan Roberts wrote:
> Hi Breno,
> 
> 
> On 05/10/2023 17:39, Breno Leitao wrote:
>> Create a selftest that exercises the race between page faults and
>> madvise(MADV_DONTNEED) in the same huge page. Do it by running two
>> threads that touches the huge page and madvise(MADV_DONTNEED) at the same
>> time.
>>
>> In case of a SIGBUS coming at pagefault, the test should fail, since we
>> hit the bug.
>>
>> The test doesn't have a signal handler, and if it fails, it fails like
>> the following
>>
>>   ----------------------------------
>>   running ./hugetlb_fault_after_madv
>>   ----------------------------------
>>   ./run_vmtests.sh: line 186: 595563 Bus error    (core dumped) "$@"
>>   [FAIL]
>>
>> This selftest goes together with the fix of the bug[1] itself.
>>
>> [1] https://lore.kernel.org/all/20231001005659.2185316-1-riel@surriel.com/#r
>>
>> Signed-off-by: Breno Leitao <leitao@debian.org>
>> ---
>>  tools/testing/selftests/mm/Makefile           |  1 +
>>  .../selftests/mm/hugetlb_fault_after_madv.c   | 73 +++++++++++++++++++
>>  tools/testing/selftests/mm/run_vmtests.sh     |  4 +
>>  3 files changed, 78 insertions(+)
>>  create mode 100644 tools/testing/selftests/mm/hugetlb_fault_after_madv.c
>>
>> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
>> index 6a9fc5693145..e71ec9910c62 100644
>> --- a/tools/testing/selftests/mm/Makefile
>> +++ b/tools/testing/selftests/mm/Makefile
>> @@ -68,6 +68,7 @@ TEST_GEN_FILES += split_huge_page_test
>>  TEST_GEN_FILES += ksm_tests
>>  TEST_GEN_FILES += ksm_functional_tests
>>  TEST_GEN_FILES += mdwe_test
>> +TEST_GEN_FILES += hugetlb_fault_after_madv
>>  
>>  ifneq ($(ARCH),arm64)
>>  TEST_GEN_PROGS += soft-dirty
>> diff --git a/tools/testing/selftests/mm/hugetlb_fault_after_madv.c b/tools/testing/selftests/mm/hugetlb_fault_after_madv.c
>> new file mode 100644
>> index 000000000000..73b81c632366
>> --- /dev/null
>> +++ b/tools/testing/selftests/mm/hugetlb_fault_after_madv.c
>> @@ -0,0 +1,73 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#include <pthread.h>
>> +#include <stdio.h>
>> +#include <stdlib.h>
>> +#include <sys/mman.h>
>> +#include <sys/types.h>
>> +#include <unistd.h>
>> +
>> +#include "vm_util.h"
>> +#include "../kselftest.h"
>> +
>> +#define MMAP_SIZE (1 << 21)
>> +#define INLOOP_ITER 100
>> +
>> +char *huge_ptr;
>> +
>> +/* Touch the memory while it is being madvised() */
>> +void *touch(void *unused)
>> +{
>> +	char *ptr = (char *)huge_ptr;
>> +
>> +	for (int i = 0; i < INLOOP_ITER; i++)
>> +		ptr[0] = '.';
>> +
>> +	return NULL;
>> +}
>> +
>> +void *madv(void *unused)
>> +{
>> +	usleep(rand() % 10);
>> +
>> +	for (int i = 0; i < INLOOP_ITER; i++)
>> +		madvise(huge_ptr, MMAP_SIZE, MADV_DONTNEED);
>> +
>> +	return NULL;
>> +}
>> +
>> +int main(void)
>> +{
>> +	unsigned long free_hugepages;
>> +	pthread_t thread1, thread2;
>> +	/*
>> +	 * On kernel 6.4, we are able to reproduce the problem with ~1000
>> +	 * interactions
>> +	 */
>> +	int max = 10000;
>> +
>> +	srand(getpid());
>> +
>> +	free_hugepages = get_free_hugepages();
>> +	if (free_hugepages != 1) {
>> +		ksft_exit_skip("This test needs one and only one page to execute. Got %lu\n",
>> +			       free_hugepages);
>> +	}
>> +
>> +	while (max--) {
>> +		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_skip("Failed to allocated huge page\n");
>> +
>> +		pthread_create(&thread1, NULL, madv, NULL);
>> +		pthread_create(&thread2, NULL, touch, NULL);
>> +
>> +		pthread_join(thread1, NULL);
>> +		pthread_join(thread2, NULL);
>> +		munmap(huge_ptr, MMAP_SIZE);
>> +	}
>> +
>> +	return KSFT_PASS;
>> +}
>> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
>> index 3e2bc818d566..9f53f7318a38 100755
>> --- a/tools/testing/selftests/mm/run_vmtests.sh
>> +++ b/tools/testing/selftests/mm/run_vmtests.sh
>> @@ -221,6 +221,10 @@ CATEGORY="hugetlb" run_test ./hugepage-mremap
>>  CATEGORY="hugetlb" run_test ./hugepage-vmemmap
>>  CATEGORY="hugetlb" run_test ./hugetlb-madvise
>>  
>> +# For this test, we need one and just one huge page
>> +echo 1 > /proc/sys/vm/nr_hugepages
> 
> I've noticed that this change breaks some of the uffd-stress tests further down
> the file, because you have freed previously reserved hugepages that the test
> requires to run. I notice that the patch is already in mm-stable, so perhaps its
> possible to submit a patch that does a save and restore?
> 
> Although I'm not sure if that might be tricky because the previous reservation
> is per-size and per-node (our CI does this on the kernel command line), and I
> suspect if you want just 1 huge page in the entire system you won't be able to
> get back to the previous state by just restoring this value?

Actually on closer inspection, I don't think this will be a problem; simply
saving and restoring the value around the test will be sufficient.

I also notice that the binary for the new test is not added to the .gitignore,
which is a minor annoyance.

Thanks,
Ryan

> 
> These are the failing tests for reference:
> 
> # ------------------------------------
> # running ./uffd-stress hugetlb 128 32
> # ------------------------------------
> # nr_pages: 64, nr_pages_per_cpu: 8
> # ERROR: context init failed (errno=12, @uffd-stress.c:254)
> # [FAIL]
> # --------------------------------------------
> # running ./uffd-stress hugetlb-private 128 32
> # --------------------------------------------
> # nr_pages: 64, nr_pages_per_cpu: 8
> # ERROR: context init failed (errno=12, @uffd-stress.c:254)
> # [FAIL]
> 
> Thanks,
> Ryan
> 
> 
> 
>> +CATEGORY="hugetlb" run_test ./hugetlb_fault_after_madv
>> +
>>  if test_selected "hugetlb"; then
>>  	echo "NOTE: These hugetlb tests provide minimal coverage.  Use"
>>  	echo "      https://github.com/libhugetlbfs/libhugetlbfs.git for"
>