* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241205 14:07]:
> When we fork anonymous pages, apply a guard page then remove it, the
> previous CoW mapping is cleared.
>
> This might not be obvious to an outside observer without taking some time
> to think about how the overall process functions, so document that this is
> the case through a test, which also usefully asserts that the behaviour is
> as we expect.
>
> This is grouped with other, more important, fork tests that ensure that
> guard pages are correctly propagated on fork.
>
> Fix a typo in a nearby comment at the same time.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Nice to see some more testing going in.
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
> tools/testing/selftests/mm/guard-pages.c | 73 +++++++++++++++++++++++-
> 1 file changed, 72 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/guard-pages.c b/tools/testing/selftests/mm/guard-pages.c
> index 7cdf815d0d63..d8f8dee9ebbd 100644
> --- a/tools/testing/selftests/mm/guard-pages.c
> +++ b/tools/testing/selftests/mm/guard-pages.c
> @@ -990,7 +990,7 @@ TEST_F(guard_pages, fork)
> MAP_ANON | MAP_PRIVATE, -1, 0);
> ASSERT_NE(ptr, MAP_FAILED);
>
> - /* Establish guard apges in the first 5 pages. */
> + /* Establish guard pages in the first 5 pages. */
> ASSERT_EQ(madvise(ptr, 5 * page_size, MADV_GUARD_INSTALL), 0);
>
> pid = fork();
> @@ -1029,6 +1029,77 @@ TEST_F(guard_pages, fork)
> ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
> }
>
> +/*
> + * Assert expected behaviour after we fork populated ranges of anonymous memory
> + * and then guard and unguard the range.
> + */
> +TEST_F(guard_pages, fork_cow)
> +{
> + const unsigned long page_size = self->page_size;
> + char *ptr;
> + pid_t pid;
> + int i;
> +
> + /* Map 10 pages. */
> + ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE,
> + MAP_ANON | MAP_PRIVATE, -1, 0);
> + ASSERT_NE(ptr, MAP_FAILED);
> +
> + /* Populate range. */
> + for (i = 0; i < 10 * page_size; i++) {
> + char chr = 'a' + (i % 26);
> +
> + ptr[i] = chr;
> + }
> +
> + pid = fork();
> + ASSERT_NE(pid, -1);
> + if (!pid) {
> + /* This is the child process now. */
> +
> + /* Ensure the range is as expected. */
> + for (i = 0; i < 10 * page_size; i++) {
> + char expected = 'a' + (i % 26);
> + char actual = ptr[i];
> +
> + ASSERT_EQ(actual, expected);
> + }
> +
> + /* Establish guard pages across the whole range. */
> + ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
> + /* Remove it. */
> + ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_REMOVE), 0);
> +
> + /*
> + * By removing the guard pages, the page tables will be
> + * cleared. Assert that we are looking at the zero page now.
> + */
> + for (i = 0; i < 10 * page_size; i++) {
> + char actual = ptr[i];
> +
> + ASSERT_EQ(actual, '\0');
> + }
> +
> + exit(0);
> + }
> +
> + /* Parent process. */
> +
> + /* Parent simply waits on child. */
> + waitpid(pid, NULL, 0);
> +
> + /* Ensure the range is unchanged in parent anon range. */
> + for (i = 0; i < 10 * page_size; i++) {
> + char expected = 'a' + (i % 26);
> + char actual = ptr[i];
> +
> + ASSERT_EQ(actual, expected);
> + }
> +
> + /* Cleanup. */
> + ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
> +}
> +
> /*
> * Assert that forking a process with VMAs that do have VM_WIPEONFORK set
> * behave as expected.
> --
> 2.47.1
