Re: [PATCH v4 4/4] selftests/hmm-tests: Add test for dirty bits

John Hubbard
2022-09-05T00:41:22Z

On 9/1/22 17:35, Alistair Popple wrote:
> We were not correctly copying PTE dirty bits to pages during
> migrate_vma_setup() calls. This could potentially lead to data loss, so
> add a test for this.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> tools/testing/selftests/vm/hmm-tests.c | 124 ++++++++++++++++++++++++++-
> 1 file changed, 124 insertions(+)
>
> diff --git a/tools/testing/selftests/vm/hmm-tests.c b/tools/testing/selftests/vm/hmm-tests.c
> index 529f53b..70fdb49 100644
> --- a/tools/testing/selftests/vm/hmm-tests.c
> +++ b/tools/testing/selftests/vm/hmm-tests.c
> @@ -1200,6 +1200,130 @@ TEST_F(hmm, migrate_multiple)
> }
> }
>
> +static char cgroup[] = "/sys/fs/cgroup/hmm-test-XXXXXX";
> +static int write_cgroup_param(char *cgroup_path, char *param, long value)
> +{
> + int ret;
> + FILE *f;
> + char *filename;
> +
> + if (asprintf(&filename, "%s/%s", cgroup_path, param) < 0)
> + return -1;
> +
> + f = fopen(filename, "w");
> + if (!f) {
> + ret = -1;
> + goto out;
> + }
> +
> + ret = fprintf(f, "%ld\n", value);
> + if (ret < 0)
> + goto out1;
> +
> + ret = 0;
> +
> +out1:
> + fclose(f);
> +out:
> + free(filename);
> +
> + return ret;
> +}
> +
> +static int setup_cgroup(void)
> +{
> + pid_t pid = getpid();
> + int ret;
> +
> + if (!mkdtemp(cgroup))
> + return -1;
> +
> + ret = write_cgroup_param(cgroup, "cgroup.procs", pid);
> + if (ret)
> + return ret;
> +
> + return 0;
> +}
> +
> +static int destroy_cgroup(void)
> +{
> + pid_t pid = getpid();
> + int ret;
> +
> + ret = write_cgroup_param("/sys/fs/cgroup/cgroup.procs",
> + "cgroup.proc", pid);
> + if (ret)
> + return ret;
> +
> + if (rmdir(cgroup))
> + return -1;
> +
> + return 0;
> +}
> +
> +/*
> + * Try and migrate a dirty page that has previously been swapped to disk. This
> + * checks that we don't loose dirty bits.

s/loose/lose/

> + */
> +TEST_F(hmm, migrate_dirty_page)
> +{
> + struct hmm_buffer *buffer;
> + unsigned long npages;
> + unsigned long size;
> + unsigned long i;
> + int *ptr;
> + int tmp = 0;
> +
> + npages = ALIGN(HMM_BUFFER_SIZE, self->page_size) >> self->page_shift;
> + ASSERT_NE(npages, 0);
> + size = npages << self->page_shift;
> +
> + buffer = malloc(sizeof(*buffer));
> + ASSERT_NE(buffer, NULL);
> +
> + buffer->fd = -1;
> + buffer->size = size;
> + buffer->mirror = malloc(size);
> + ASSERT_NE(buffer->mirror, NULL);
> +
> + ASSERT_EQ(setup_cgroup(), 0);
> +
> + buffer->ptr = mmap(NULL, size,
> + PROT_READ | PROT_WRITE,
> + MAP_PRIVATE | MAP_ANONYMOUS,
> + buffer->fd, 0);
> + ASSERT_NE(buffer->ptr, MAP_FAILED);
> +
> + /* Initialize buffer in system memory. */
> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
> + ptr[i] = 0;
> +
> + ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
> +
> + /* Fault pages back in from swap as clean pages */
> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
> + tmp += ptr[i];
> +
> + /* Dirty the pte */
> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
> + ptr[i] = i;
> +
> + /*
> + * Attempt to migrate memory to device, which should fail because
> + * hopefully some pages are backed by swap storage.
> + */
> + ASSERT_TRUE(hmm_migrate_sys_to_dev(self->fd, buffer, npages));

Are you really sure that you want to assert on that? Because doing so
guarantees a test failure if and when we every upgrade the kernel to
be able to migrate swap-backed pages. And I seem to recall that this
current inability to migrate swap-backed pages is considered a flaw
to be fixed, right?

> +
> + ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
> +
> + /* Check we still see the updated data after restoring from swap. */
> + for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
> + ASSERT_EQ(ptr[i], i);
> +
> + hmm_buffer_free(buffer);
> + destroy_cgroup();
> +}
> +
> /*
> * Read anonymous memory multiple times.
> */

thanks,

--
John Hubbard
NVIDIA