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

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

On 03/11/2023 13:59, Breno Leitao wrote:
> Hello Ryan,
> 
> On Thu, Nov 02, 2023 at 12:29:54PM +0000, Ryan Roberts wrote:
>> On 02/11/2023 12:24, Ryan Roberts wrote:
>>> 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.
> 
> Thanks for checking it, I will prepare a patch that will restore the number
> of huge pages allocated after the test.
> 
>> I also notice that the binary for the new test is not added to the .gitignore,
>> which is a minor annoyance.
> 
> I will add the file to .gitignore also.

Excellent - thanks!

> 
> Thanks for the heads-up.