Re: [PATCH v10 2/6] arm64: add support for machine check error safe

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jan 30, 2024 at 06:57:24PM +0800, Tong Tiangen wrote:
> 在 2024/1/30 1:51, Mark Rutland 写道:
> > On Mon, Jan 29, 2024 at 09:46:48PM +0800, Tong Tiangen wrote:

> > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > > index 55f6455a8284..312932dc100b 100644
> > > --- a/arch/arm64/mm/fault.c
> > > +++ b/arch/arm64/mm/fault.c
> > > @@ -730,6 +730,31 @@ static int do_bad(unsigned long far, unsigned long esr, struct pt_regs *regs)
> > >   	return 1; /* "fault" */
> > >   }
> > > +static bool arm64_do_kernel_sea(unsigned long addr, unsigned int esr,
> > > +				     struct pt_regs *regs, int sig, int code)
> > > +{
> > > +	if (!IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC))
> > > +		return false;
> > > +
> > > +	if (user_mode(regs))
> > > +		return false;
> > 
> > This function is called "arm64_do_kernel_sea"; surely the caller should *never*
> > call this for a SEA taken from user mode?
> 
> In do_sea(), the processing logic is as follows:
>   do_sea()
>   {
>     [...]
>     if (user_mode(regs) && apei_claim_sea(regs) == 0) {
>        return 0;
>     }
>     [...]
>     //[1]
>     if (!arm64_do_kernel_sea()) {
>        arm64_notify_die();
>     }
>   }
> 
> [1] user_mode() is still possible to go here,If user_mode() goes here,
>  it indicates that the impact caused by the memory error cannot be
>  processed correctly by apei_claim_sea().
> 
> 
> In this case, only arm64_notify_die() can be used, This also maintains
> the original logic of user_mode()'s processing.

My point is that either:

(a) The name means that this should *only* be called for SEAs from a kernel
    context, and the caller should be responsible for ensuring that.

(b) The name is misleading, and the 'kernel' part should be removed from the
    name.

I prefer (a), and if you head down that route it's clear that you can get rid
of a bunch of redundant logic and remove the need for do_kernel_sea(), anyway,
e.g.

| static int do_sea(unsigned long far, unsigned long esr, struct pt_regs *regs)
| {
|         const struct fault_info *inf = esr_to_fault_info(esr);
|         bool claimed = apei_claim_sea(regs) == 0;
|         unsigned long siaddr;
| 
|         if (claimed) {
|                 if (user_mode(regs)) {
|                         /*  
|                          * APEI claimed this as a firmware-first notification.
|                          * Some processing deferred to task_work before ret_to_user().
|                          */
|                         return 0;
|                 } else {
|                         /*
|                          * TODO: explain why this is correct.
|                          */
|                         if ((current->flags & PF_KTHREAD) &&
|                             fixup_exception_mc(regs))
|                                 return 0;
|                 }
|         }
| 
|         if (esr & ESR_ELx_FnV) {
|                 siaddr = 0;
|         } else {
|                 /*  
|                  * The architecture specifies that the tag bits of FAR_EL1 are
|                  * UNKNOWN for synchronous external aborts. Mask them out now
|                  * so that userspace doesn't see them.
|                  */
|                 siaddr  = untagged_addr(far);
|         }   
|         arm64_notify_die(inf->name, regs, inf->sig, inf->code, siaddr, esr);
| 
|         return 0;
| }

> > > +
> > > +	if (apei_claim_sea(regs) < 0)
> > > +		return false;
> > > +
> > > +	if (!fixup_exception_mc(regs))
> > > +		return false;
> > > +
> > > +	if (current->flags & PF_KTHREAD)
> > > +		return true;
> > 
> > I think this needs a comment; why do we allow kthreads to go on, yet kill user
> > threads? What about helper threads (e.g. for io_uring)?
> 
> If a memroy error occurs in the kernel thread, the problem is more
> serious than that of the user thread. As a result, related kernel
> functions, such as khugepaged, cannot run properly. kernel panic should
> be a better choice at this time.
> 
> Therefore, the processing scope of this framework is limited to the user
> thread.

That's reasonable, but needs to be explained in a comment.

Also, as above, I think you haven't conisderd helper threads (e.g. io_uring),
which don't have PF_KTHREAD set but do have PF_USER_WORKER set. I suspect those
need the same treatment as kthreads.

> > > +	set_thread_esr(0, esr);
> > 
> > Why do we set the ESR to 0?
> 
> The purpose is to reuse the logic of arm64_notify_die() and set the
> following parameters before sending signals to users:
>   current->thread.fault_address = 0;
>   current->thread.fault_code = err;

Ok, but there's no need to open-code that.

As per my above example, please continue to use the existing call to
arm64_notify_die() rather than open-coding bits of it.

Mark.