Re: [PATCH v13 2/5] arm64: add support for ARCH_HAS_COPY_MC

[Tong Tiangen <tongtiangen@huawei.com>]

在 2025/3/25 0:54, Luck, Tony 写道:
> On Fri, Feb 14, 2025 at 09:44:02AM +0800, Tong Tiangen wrote:
>>
>>
>> 在 2025/2/13 0:21, Catalin Marinas 写道:
>>> (catching up with old threads)
>>>
>>> On Mon, Dec 09, 2024 at 10:42:54AM +0800, Tong Tiangen wrote:
>>>> For the arm64 kernel, when it processes hardware memory errors for
>>>> synchronize notifications(do_sea()), if the errors is consumed within the
>>>> kernel, the current processing is panic. However, it is not optimal.
>>>>
>>>> Take copy_from/to_user for example, If ld* triggers a memory error, even in
>>>> kernel mode, only the associated process is affected. Killing the user
>>>> process and isolating the corrupt page is a better choice.
>>>
>>> I agree that killing the user process and isolating the page is a better
>>> choice but I don't see how the latter happens after this patch. Which
>>> page would be isolated?
>>
>> The SEA is triggered when the page with hardware error is read. After
>> that, the page is isolated in memory_failure() (mf). The processing of
>> mf is mentioned in the comments of do_sea().
>>
>> /*
>>   * APEI claimed this as a firmware-first notification.
>>   * Some processing deferred to task_work before ret_to_user().
>>   */
>>
>> Some processing include mf.
>>
>>>
>>>> Add new fixup type EX_TYPE_KACCESS_ERR_ZERO_MEM_ERR to identify insn
>>>> that can recover from memory errors triggered by access to kernel memory,
>>>> and this fixup type is used in __arch_copy_to_user(), This make the regular
>>>> copy_to_user() will handle kernel memory errors.
>>>
>>> Is the assumption that the error on accessing kernel memory is
>>> transient? There's no way to isolate the kernel page and also no point
>>> in isolating the destination page either.
>>
>> Yes, it's transient, the kernel page in mf can't be isolated, the
>> transient access (ld) of this kernel page is currently expected to kill
>> the user-mode process to avoid error spread.
>>
>>
>> The SEA processes synchronization errors. Only hardware errors on the
>> source page can be detected (Through synchronous ld insn) and processed.
>> The destination page cannot be processed.
> 
> I've considered the copy_to_user() case as only partially fixable. There
> are lots of cases to consider:
> 
> 1) Many places where drivers copy to user in ioctl(2) calls.
>     Killing the application solves the immediate problem, but if
>     the problem with kernel memory is not transient, then you
>     may run into it again.
> 
> 2) Copy from Linux page cache to user for a read(2) system call.
>     This one is a candidate for recovery. Might need help from the
>     file system code. If the kernel page is a clean copy of data in
>     the file system, then drop this page and re-read from storage
>     into a new page. Then resume the copy_to_user().
>     If the page is modified, then need some file system action to
>     somehow mark this range of addresses in the file as lost forever.
>     First step in tackling this case is identifying that the source
>     address is a page cache page.
> 
> 3) Probably many other places where the kernel copies to user for
>     other system calls. Would need to look at these on a case by case
>     basis. Likely most have the same issue as ioctl(2) above.

1) 3)
Yes, in extreme cases, user-mode processes may be killed all the time.
The hardware error that repeatedly triggered in the same page, in this
case, firmware maybe report a fatal error, if yes, this problem can be
solved.

2)
This is indeed a workaround, somewhat complex, but it seems worthwhile
to avoid kernel panic.

Sorry for didn't catch your reply in time:)

Thanks,
Tong.

> 
> -Tony
> 
> .