selftests/bpf: Fix intermittent failures in file_reader test

file_reader/on_open_expect_fault intermittently fails when test_progs
runs tests in parallel, because it expects a page fault on first read.
Another file_reader test running concurrently may have already pulled
the same pages into the page cache, eliminating the fault and causing a
spurious failure.

Make file_reader/on_open_expect_fault read from a file region that does
not overlap with other file_reader tests, so the initial access still
faults even under parallel execution.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Ihor Solodrai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: mykyta5

tools/testing/selftests/bpf/prog_tests/file_reader.c

@@ -52,7 +52,11 @@ static int initialize_file_contents(void)
 	/* page-align base file address */
 	addr = (void *)((unsigned long)addr & ~(page_sz - 1));
 
-	for (off = 0; off < sizeof(file_contents); off += page_sz) {
+	/*
+	 * Page out range 0..512K, use 0..256K for positive tests and
+	 * 256K..512K for negative tests expecting page faults
+	 */
+	for (off = 0; off < sizeof(file_contents) * 2; off += page_sz) {
 		if (!ASSERT_OK(madvise(addr + off, page_sz, MADV_PAGEOUT),
 			       "madvise pageout"))
 			return errno;

tools/testing/selftests/bpf/progs/file_reader.c

@@ -49,7 +49,7 @@ int on_open_expect_fault(void *c)
 	if (bpf_dynptr_from_file(file, 0, &dynptr))
 		goto out;
 
-	local_err = bpf_dynptr_read(tmp_buf, user_buf_sz, &dynptr, 0, 0);
+	local_err = bpf_dynptr_read(tmp_buf, user_buf_sz, &dynptr, user_buf_sz, 0);
 	if (local_err == -EFAULT) { /* Expect page fault */
 		local_err = 0;
 		run_success = 1;