Re: [PATCH] mm, memory_failure: only send BUS_MCEERR_AO to early-kill process

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

Hi Zhang,

Sorry for my late response.

On Tue, May 26, 2020 at 03:06:41PM +0800, Wetp Zhang wrote:
> From: Zhang Yi <wetpzy@gmail.com>
>
> If a process don't need early-kill, it may not care the BUS_MCEERR_AO.
> Let the process to be killed when it really access the corrupted memory.
>
> Signed-off-by: Zhang Yi <wetpzy@gmail.com>

Thank you for pointing this. This looks to me a bug (per-process flag
is ignored when system-wide flag is set).

> ---
>  mm/memory-failure.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index a96364be8ab4..2db13d48865c 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -210,7 +210,7 @@ static int kill_proc(struct to_kill *tk, unsigned long pfn, int flags)
>  {
>  	struct task_struct *t = tk->tsk;
>  	short addr_lsb = tk->size_shift;
> -	int ret;
> +	int ret = 0;
>
>  	pr_err("Memory failure: %#lx: Sending SIGBUS to %s:%d due to hardware memory corruption\n",
>  		pfn, t->comm, t->pid);
> @@ -225,8 +225,9 @@ static int kill_proc(struct to_kill *tk, unsigned long pfn, int flags)
>  		 * This could cause a loop when the user sets SIGBUS
>  		 * to SIG_IGN, but hopefully no one will do that?
>  		 */
> -		ret = send_sig_mceerr(BUS_MCEERR_AO, (void __user *)tk->addr,
> -				      addr_lsb, t);  /* synchronous? */
> +		if ((t->flags & PF_MCE_PROCESS) && (t->flags & PF_MCE_EARLY))
> +			ret = send_sig_mceerr(BUS_MCEERR_AO,
> +				(void __user *)tk->addr, addr_lsb, t);

kill_proc() could be called only for processes that are selected by
collect_procs() with task_early_kill().  So I think that we should fix
task_early_kill(), maybe by reordering sysctl_memory_failure_early_kill
check and find_early_kill_thread() check.

    static struct task_struct *task_early_kill(struct task_struct *tsk,
                                               int force_early)
    {
            struct task_struct *t;
            if (!tsk->mm)
                    return NULL;
            if (force_early)
                    return tsk;
            t = find_early_kill_thread(tsk);
            if (t)
                    return t;
            if (sysctl_memory_failure_early_kill)
                    return tsk;
            return NULL;
    }

One subtleness is to make sure that find_early_kill_thread() should distinguish
default value and explicitly set value, so we might need some modification
on find_early_kill_thread().

Can you try that?

Thanks,
Naoya Horiguchi