[PATCH] selftests/bpf: Page out as late a possible in file_reader

[Jerome Marchand]

The file_reader/on_open_expect_fault fails consistently on my system.
It expects a page fault on first dynptr read of some range the exe
file of the current process because it has paged out that page range
earlier. However a lot can happen to that range (which depending on
the actual memory layout could contain text section, data section,
sections )related to dynamic linking...) between the moment it was
paged out and the moment the bpf program expected to hit a pagefault
actually run.

A bit of instrumentation with mincore() shows that pages from that
range were accessed several times before the program is run. In
particular the call of file_reader__load() seems to fault all the
range in.

Move the call to madvise(MADV_PAGEOUT) to just before attaching the
program to minimize the risk of having those page pulled back in from
under our feet.

Signed-off-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
---
 .../selftests/bpf/prog_tests/file_reader.c    | 22 +++++++++----------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/file_reader.c b/tools/testing/selftests/bpf/prog_tests/file_reader.c
index 5cde32b35da44..48aae7ea0e4bb 100644
--- a/tools/testing/selftests/bpf/prog_tests/file_reader.c
+++ b/tools/testing/selftests/bpf/prog_tests/file_reader.c
@@ -10,6 +10,7 @@
 
 const char *user_ptr = "hello world";
 char file_contents[256000];
+void *addr;
 
 void *get_executable_base_addr(void)
 {
@@ -26,8 +27,7 @@ void *get_executable_base_addr(void)
 static int initialize_file_contents(void)
 {
 	int fd, page_sz = sysconf(_SC_PAGESIZE);
-	ssize_t n = 0, cur, off;
-	void *addr;
+	ssize_t n = 0, cur;
 
 	fd = open("/proc/self/exe", O_RDONLY);
 	if (!ASSERT_OK_FD(fd, "Open /proc/self/exe\n"))
@@ -52,16 +52,6 @@ static int initialize_file_contents(void)
 	/* page-align base file address */
 	addr = (void *)((unsigned long)addr & ~(page_sz - 1));
 
-	/*
-	 * 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;
-	}
-
 	return 0;
 }
 
@@ -90,6 +80,14 @@ static void run_test(const char *prog_name)
 	if (!ASSERT_OK(err, "file_reader__load"))
 		goto cleanup;
 
+	/*
+	 * Page out range 0..512K, use 0..256K for positive tests and
+	 * 256K..512K for negative tests expecting page faults
+	 */
+	if (!ASSERT_OK(madvise(addr, sizeof(file_contents) * 2, MADV_PAGEOUT),
+		       "madvise pageout"))
+		goto cleanup;
+
 	err = file_reader__attach(skel);
 	if (!ASSERT_OK(err, "file_reader__attach"))
 		goto cleanup;
-- 
2.53.0