Re: [PATCH] arm64: mm: use ptep_clear() instead of pte_clear() in clear_flush()

Qi Zheng
2023-08-22T02:24:23Z

On 2023/8/22 04:21, Andrew Morton wrote:
> On Fri, 11 Aug 2023 19:28:41 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>
>> Will Deacon <will@kernel.org>于2023年8月11日 周五19:21写道：
>>
>>> On Fri, Aug 11, 2023 at 07:16:20PM +0800, Qi Zheng wrote:
>>>> Will Deacon <[1]will@kernel.org>于2023年8月11日 周五19:03写道：
>>>>
>>>> On Thu, Aug 10, 2023 at 09:32:41AM +0000, Qi Zheng wrote:
>>>> > From: Qi Zheng <[2]zhengqi.arch@bytedance.com>
>>>> >
>>>> > In clear_flush(), the original pte may be a present entry, so we
>>>> should
>>>> > use ptep_clear() to let page_table_check track the pte clearing
>>>> operation,
>>>> > otherwise it may cause false positive in subsequent set_pte_at().
>>>>
>>>> Isn't this true for most users of pte_clear()? There are some in the
>>>> core
>>>> code, so could they trigger the false positive as well?
>>>>
>>>> No, the PTE entry in other places where pte_clear() is used is
>>> non-present
>>>> PTE.
>>>> The page_table_check does not does track the pte operation in this
>>> case,
>>>> so it will not cause false positives.
>>>
>>> Are you sure? For example, the call from flush_all_zero_pkmaps() in
>>> highmem.c really looks like it's clearing a valid entry. Not that arm64
>>> cares about highmem, but still.
>>
>>
>> Ah, this is init_mm, not user mm, page_table_check does not care about this
>> case.
>
> It's unclear where we stand with this patch. An ack or a nack, please?

Hi all,

Any comments or suggestions here?

Thanks,
Qi