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

Will Deacon
2023-08-11T11:22:06Z

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.

Will