Re: [PATCH -next v4 3/7] arm64: add support for machine check error safe

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

On Thu, May 19, 2022 at 02:29:54PM +0800, Tong Tiangen wrote:
> 
> 
> 在 2022/5/13 23:26, Mark Rutland 写道:
> > On Wed, Apr 20, 2022 at 03:04:14AM +0000, Tong Tiangen wrote:
> > > During the processing of arm64 kernel hardware memory errors(do_sea()), if
> > > the errors is consumed in the kernel, the current processing is panic.
> > > However, it is not optimal.
> > > 
> > > Take uaccess for example, if the uaccess operation fails due to memory
> > > error, only the user process will be affected, kill the user process
> > > and isolate the user page with hardware memory errors is a better choice.
> > 
> > Conceptually, I'm fine with the idea of constraining what we do for a
> > true uaccess, but I don't like the implementation of this at all, and I
> > think we first need to clean up the arm64 extable usage to clearly
> > distinguish a uaccess from another access.
> 
> OK,using EX_TYPE_UACCESS and this extable type could be recover, this is
> more reasonable.

Great.

> For EX_TYPE_UACCESS_ERR_ZERO, today we use it for kernel accesses in a
> couple of cases, such as
> get_user/futex/__user_cache_maint()/__user_swpX_asm(), 

Those are all user accesses.

However, __get_kernel_nofault() and __put_kernel_nofault() use
EX_TYPE_UACCESS_ERR_ZERO by way of __{get,put}_mem_asm(), so we'd need to
refactor that code to split the user/kernel cases higher up the callchain.

> your suggestion is:
> get_user continues to use EX_TYPE_UACCESS_ERR_ZERO and the other cases use
> new type EX_TYPE_FIXUP_ERR_ZERO?

Yes, that's the rough shape. We could make the latter EX_TYPE_KACCESS_ERR_ZERO
to be clearly analogous to EX_TYPE_UACCESS_ERR_ZERO, and with that I susepct we
could remove EX_TYPE_FIXUP.

Thanks,
Mark.