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

Will Deacon
2023-08-22T09:58:56Z

On Mon, Aug 21, 2023 at 01:21:19PM -0700, 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?

Sorry Andrew, I saw you'd queued it so I marked it as "done" on my list. I
think it's fine:

Acked-by: Will Deacon <will@kernel.org>

Will