linux-kernel - Re: [PATCH] arm64: mm: use ptep_clear() instead of pte_clear() in clear

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Date:   Tue, 22 Aug 2023 10:23:49 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Will Deacon <will@...nel.org>, Qi Zheng <qi.zheng@...ux.dev>,
        catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, muchun.song@...ux.dev,
        pasha.tatashin@...een.com, wangkefeng.wang@...wei.com
Subject: Re: [PATCH] arm64: mm: use ptep_clear() instead of pte_clear() in
 clear_flush()



On 2023/8/22 04:21, Andrew Morton wrote:
> On Fri, 11 Aug 2023 19:28:41 +0800 Qi Zheng <zhengqi.arch@...edance.com> wrote:
> 
>> Will Deacon <will@...nel.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@...nel.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@...edance.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