test(memtrack): reproduce fork-inherited unmap removes-exceed-adds

crates/memtrack/testdata/rss/fork_unmap_inherited.c

@@ -0,0 +1,26 @@
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main(void) {
+    sleep(1);
+    size_t len = 0x20 * 1024 * 1024;
+    void* region = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    if (region == MAP_FAILED) return 1;
+    memset(region, 0x42, len);
+    pid_t pid = fork();
+    if (pid < 0) return 1;
+    if (pid == 0) {
+        /* Unmap the inherited region without ever touching it: fork copies
+         * page tables through the folio_*_dup_* helpers, which have no rmap
+         * hooks, so the removes fired here have no matching adds under the
+         * child's pid. */
+        munmap(region, len);
+        _exit(0);
+    }
+    int status;
+    if (waitpid(pid, &status, 0) < 0) return 1;
+    munmap(region, len);
+    return !WIFEXITED(status) || WEXITSTATUS(status) != 0;
+}