From: "Kiryl Shutsemau (Meta)" <[email protected]> collapse_fork() checks that a fork-shared range collapses in the child while the parent keeps its own pages, but the parent sits still while that happens. Nothing checks that CoW isolation survives a collapse racing with writes to the shared source.
Add a case where the parent writes to the shared range throughout the child's collapse. CoW has to keep the two apart: the child must see the content from before the fork, and the parent only its own writes. The parent unshares one page every 10ms, starting only once the child says it is about to collapse. Writing the range in a burst would break CoW on all of it before the collapse begins, leaving the child to collapse pages that are already exclusive to it. Preparation for changing how collapse handles fork-shared sources. Assisted-by: LLM Tested-by: Muhammad Usama Anjum <[email protected]> Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]> --- tools/testing/selftests/mm/khugepaged.c | 100 ++++++++++++++++++++++++ 1 file changed, 100 insertions(+) diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c index b15cd07fc0b3..f5d4847c50cd 100644 --- a/tools/testing/selftests/mm/khugepaged.c +++ b/tools/testing/selftests/mm/khugepaged.c @@ -1164,6 +1164,103 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops ksft_test_result_report(exit_status, "%s\n", __func__); } +/* + * The parent writes to the fork-shared range throughout the child's + * collapse. CoW must keep the two apart: the child sees the pre-fork + * content, the parent only its own writes. + */ +static void collapse_fork_cow_race(struct collapse_context *c, struct mem_ops *ops) +{ + const int stride = page_size / sizeof(int); + int wstatus, child_status, i, n; + unsigned long shared; + volatile int *ip; + pid_t child; + int sync[2]; + char go = 1; + void *p; + + /* At a page per 10 ms, 64 pages spread the writes across the collapse */ + n = 64; + shared = n * page_size; + + p = ops->setup_area(1); + /* Shared prefix, with the pre-fork pattern */ + ops->fault(p, 0, shared); + if (pipe(sync)) + ksft_exit_fail_perror("pipe()"); + + /* A volatile pointer so the stores are not merged or dropped */ + ip = p; + + ksft_print_msg("Fork, collapse in the child while the parent rewrites..."); + child = fork(); + if (!child) { + int collapse_status; + + close(sync[0]); + /* Private remainder */ + ops->fault(p, shared, hpage_pmd_size); + /* Start the parent unsharing, and give it a head start */ + if (write(sync[1], &go, 1) != 1) + _exit(KSFT_FAIL); + usleep(5000); + c->collapse("Collapse a range the parent is writing to", + p, 1, ops, true); + collapse_status = exit_status; + for (i = 0; i < n; i++) + if (ip[i * stride] != i + 0xdead0000) + break; + if (i == n) + success("OK"); + else + fail("Fail: child content"); + /* The content check must not bury a failed collapse */ + if (exit_status != KSFT_FAIL) + exit_status = collapse_status; + ops->cleanup_area(p, hpage_pmd_size); + _exit(exit_status); + } + + close(sync[1]); + if (read(sync[0], &go, 1) != 1) + ksft_exit_fail_msg("child never reached the collapse\n"); + + /* + * Unshare one page at a time: a burst would break CoW on the whole + * range before the collapse starts, leaving nothing shared to collapse. + */ + i = 0; + for (;;) { + if (i < n) + ip[i * stride] = i + 0xbeef0000; + i++; + usleep(10 * 1000); + if (waitpid(child, &wstatus, WNOHANG)) + break; + } + + /* Finish whatever the paced sweep did not reach */ + for (; i < n; i++) + ip[i * stride] = i + 0xbeef0000; + /* A child that died reading the racing pages is a failure, not a zero */ + child_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL; + + ksft_print_msg("Check the parent sees only its own writes..."); + for (i = 0; i < n; i++) + if (ip[i * stride] != i + 0xbeef0000) + break; + if (i == n) + success("OK"); + else + fail("Fail: parent content"); + ops->cleanup_area(p, hpage_pmd_size); + /* The parent's check must not bury the child's verdict */ + if (exit_status != KSFT_FAIL) + exit_status = child_status; + ksft_test_result_report(exit_status, "%s\n", __func__); +} + static void madvise_collapse_existing_thps(struct collapse_context *c, struct mem_ops *ops) { @@ -1695,6 +1792,9 @@ int main(int argc, char **argv) TEST(collapse_max_ptes_shared, khugepaged_context, anon_ops); TEST(collapse_max_ptes_shared, madvise_context, anon_ops); + TEST(collapse_fork_cow_race, khugepaged_context, anon_ops); + TEST(collapse_fork_cow_race, madvise_context, anon_ops); + TEST(madvise_collapse_existing_thps, madvise_context, anon_ops); TEST(madvise_collapse_existing_thps, madvise_context, read_only_file_ops); TEST(madvise_collapse_existing_thps, madvise_context, read_write_file_read_ops); -- 2.54.0
