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

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Message-ID: <20230811112107.GE6993@willie-the-truck>
Date:   Fri, 11 Aug 2023 12:21:07 +0100
From:   Will Deacon <will@...nel.org>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     Qi Zheng <qi.zheng@...ux.dev>, akpm@...ux-foundation.org,
        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.co
Subject: Re: [PATCH] arm64: mm: use ptep_clear() instead of pte_clear() in
 clear_flush()

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.

Will