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

HORIGUCHI NAOYA(堀口 直也)
2020-05-29T02:12:32Z

On Thu, May 28, 2020 at 02:50:09PM +0800, wetp wrote:
>
> On 2020/5/28 上午10:22, HORIGUCHI NAOYA(堀口 直也) wrote:
> > 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).
>
> The flag is not problem for me.
>
> In my case, two processes share memory with no any flag setting, both will
> be killed when only one
>
> access the fail memory.

Thanks, now your problem seems clearer.

It seems that this happens because in "Action Required" case kill_proc()
takes the first branch for current process, while it takes the else branch
for other affected processes:

static int kill_proc(struct to_kill *tk, unsigned long pfn, int flags)
{
...

if ((flags & MF_ACTION_REQUIRED) && t->mm == current->mm) {
ret = force_sig_mceerr(BUS_MCEERR_AR, (void __user *)tk->addr,
addr_lsb);
} else {
/*
* Don't use force here, it's convenient if the signal
* can be temporarily blocked.
* 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? */
}

Sending SIGBUS with BUS_MCEERR_AO for action optional error is strange, so
maybe this logic should be like this:

if (flags & MF_ACTION_REQUIRED) {
if (t->mm == current->mm)
ret = force_sig_mceerr(BUS_MCEERR_AR, (void __user *)tk->addr,
addr_lsb);
/* send no signal to non-current processes */
} else {
/*
* Don't use force here, it's convenient if the signal
* can be temporarily blocked.
* 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? */
}

>
> > > ---
> > > 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;
>
> The force_early is rely the flag MF_ACTION_REQUIRED, so it is always true
> when MCE occurs.
>
> This leads always sending SIGBUS to processes even if those are not current
> or no flag setting.
>
>  I think it could keep the non-current processes which has no flag setting
> running.
>
>
> Besides, base on your recommendation I reorder the force_early check and
> find_early_kill_thread()
>
> check, to send the signal to the right thread.

Sorry, my previous comment around task_early_kill() is for a separate problem,
so I'll try some fix on this later.