[PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

[Ard Biesheuvel]

On Mon, 4 Jul 2022 at 18:38, Will Deacon <will at kernel.org> wrote:
>
> On Mon, Jul 04, 2022 at 10:34:07PM +0800, guanghui.fgh wrote:
> > Thanks.
> >
> > 在 2022/7/4 22:23, Will Deacon 写道:
> > > On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote:
...
> > > > Namely, it's need to use non block/section mapping for crashkernel mem
> > > > before shringking.
> > >
> > > Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do
> > > that if we're leaving the thing mapped, no?
> > >
> > I think we should use arch_kexec_[un]protect_crashkres for crashkernel mem.
> >
> > Because when invalid crashkernel mem pagetable, there is no chance to rd/wr
> > the crashkernel mem by mistake.
> >
> > If we don't use arch_kexec_[un]protect_crashkres to invalid crashkernel mem
> > pagetable, there maybe some write operations to these mem by mistake which
> > may cause crashkernel boot error and vmcore saving error.
>
> I don't really buy this line of reasoning. The entire main kernel is
> writable, so why do we care about protecting the crashkernel so much? The
> _code_ to launch the crash kernel is writable! If you care about preventing
> writes to memory which should not be writable, then you should use
> rodata=full.
>

This is not entirely true - the core kernel text and rodata are
remapped r/o in the linear map, whereas all module code and rodata are
left writable when rodata != full.

But the conclusion is the same, imo: if you can't be bothered to
protect a good chunk of the code and rodata that the kernel relies on,
why should the crashkernel be treated any differently?