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

[Breno Leitao <leitao@debian.org>]

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.

Thanks for the heads-up.