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

Mykyta Yatsenko
2026-04-20T16:52:14Z

On 4/20/26 2:46 PM, Jerome Marchand wrote:
> 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@redhat.com>
> ---

Thank you for the patch, the change looks good. Does it fail
consistently on 4K page size?

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

> .../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;