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Date: Fri, 3 Nov 2023 06:59:56 -0700
From: Breno Leitao <leitao@...ian.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: mike.kravetz@...cle.com, muchun.song@...ux.dev,
akpm@...ux-foundation.org, Shuah Khan <shuah@...nel.org>,
linux-mm@...ck.org, riel@...riel.com,
open list <linux-kernel@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] selftests/mm: Add a new test for madv and hugetlb
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) "$@"
> >> [FAIL]
> >>
> >> This selftest goes together with the fix of the bug[1] itself.
> >>
> >> [1] https://lore.kernel.org/all/20231001005659.2185316-1-riel@surriel.com/#r
> >>
> >> Signed-off-by: Breno Leitao <leitao@...ian.org>
> >> ---
> >> tools/testing/selftests/mm/Makefile | 1 +
> >> .../selftests/mm/hugetlb_fault_after_madv.c | 73 +++++++++++++++++++
> >> tools/testing/selftests/mm/run_vmtests.sh | 4 +
> >> 3 files changed, 78 insertions(+)
> >> create mode 100644 tools/testing/selftests/mm/hugetlb_fault_after_madv.c
> >>
> >> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> >> index 6a9fc5693145..e71ec9910c62 100644
> >> --- a/tools/testing/selftests/mm/Makefile
> >> +++ b/tools/testing/selftests/mm/Makefile
> >> @@ -68,6 +68,7 @@ TEST_GEN_FILES += split_huge_page_test
> >> TEST_GEN_FILES += ksm_tests
> >> TEST_GEN_FILES += ksm_functional_tests
> >> TEST_GEN_FILES += mdwe_test
> >> +TEST_GEN_FILES += hugetlb_fault_after_madv
> >>
> >> ifneq ($(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.
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