> diff --git a/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
> b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
> new file mode 100644
> index 0000000000000..c76a7eb2f01d9
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
[ ... ]
> + /*
> + * Step 5: fault ARENA_FAULT of arena in, much more than
> + * HEADROOM. Once it hits memory.max every further page has to
> + * come from reclaiming the page cache. With the fix the
> + * fault-in reclaims and all of it succeeds; without it the
> + * allocation cannot reclaim and we die on a valid address.
> + */
> + for (i = 0; i < npages; i++)
> + base[(size_t)i * ps] = 1;
> + _exit(0); /* fd deliberately kept open until here */
> +}
> +
> + if (!ASSERT_EQ(waitpid(pid, &status, 0), pid, "waitpid"))
> + goto out;
> +
> + /* The working directory cannot hold a reclaimable page cache. */
> + if (WIFEXITED(status) && WEXITSTATUS(status) == CHILD_UNSUPPORTED) {
> + fprintf(stderr, "%s:SKIP:no disk-backed O_TMPFILE in cwd\n",
> __func__);
> + test__skip();
> + goto out;
> + }
> +
> + /* A non-zero exit means the child failed to set up; the code says
> where. */
> + if (WIFEXITED(status) && WEXITSTATUS(status)) {
> + ASSERT_OK(WEXITSTATUS(status), "child setup");
> + goto out;
> + }
> +
> + /*
> + * With the fix the arena fault-in reclaims the page cache and every
> + * fault succeeds, so the child exits 0. Without it the allocation
> + * cannot reclaim, fails once the headroom is used up, and the child
> + * dies with SIGSEGV on a valid arena address.
> + */
This isn't a bug, but since the Step 5 comment 25 lines above already
explains the fixed vs. unfixed outcome (fault-in reclaims and succeeds vs.
allocation cannot reclaim and dies on valid address), would it read better
to keep just one of the two explanations?
> + if (!ASSERT_TRUE(WIFEXITED(status) && !WEXITSTATUS(status),
> + "child faulted the arena in")) {
> + if (WIFSIGNALED(status))
> + fprintf(stderr, "child killed by signal %d\n",
> WTERMSIG(status));
> + dump_memcg();
> + }
[ ... ]
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/35716612692
