Re: [PATCH] selftests: mm: restore settings from only parent process

[Joey Gouly <joey.gouly@arm.com>]

Hi again,

On Thu, Mar 14, 2024 at 02:40:45PM +0500, Muhammad Usama Anjum wrote:
> The atexit() is called from parent process as well as forked processes.
> Hence the child restores the settings at exit while the parent is still
> executing. Fix this by checking pid of atexit() calling process and only
> restore THP number from parent process.
> 
> Fixes: c23ea61726d5 ("selftests/mm: protection_keys: save/restore nr_hugepages settings")
> Tested-by: Joey Gouly <joey.gouly@arm.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
>  tools/testing/selftests/mm/protection_keys.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/protection_keys.c b/tools/testing/selftests/mm/protection_keys.c
> index f822ae31af22e..374a308174d2b 100644
> --- a/tools/testing/selftests/mm/protection_keys.c
> +++ b/tools/testing/selftests/mm/protection_keys.c
> @@ -1745,9 +1745,12 @@ void pkey_setup_shadow(void)
>  	shadow_pkey_reg = __read_pkey_reg();
>  }
>  
> +pid_t parent_pid;
> +
>  void restore_settings_atexit(void)
>  {
> -	cat_into_file(buf, "/proc/sys/vm/nr_hugepages");
> +	if (parent_pid == getpid())
> +		cat_into_file(buf, "/proc/sys/vm/nr_hugepages");
>  }
>  
>  void save_settings(void)
> @@ -1773,6 +1776,7 @@ void save_settings(void)
>  		exit(__LINE__);
>  	}
>  
> +	parent_pid = getpid();
>  	atexit(restore_settings_atexit);
>  	close(fd);
>  }

After more testing, this is not actually enough. It passes sometimes, which is
why I gave my Tested-by, but it can still fail the same way as I originally
said.

assert() at protection_keys.c::812 test_nr: 19 iteration: 1
running abort_hooks()...
errno at assert: 12

I think the parent process needs to wait for all it's child processes (recursively).
This is due to the test_pkey_alloc_exhaust() calling become_child(), where it
exits, and that exit may be from the original PID.

Thanks,
Joey