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

[Will Deacon]

On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote:
> 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.

Yes, sorry, you're quite right. The kernel text is only writable if
rodata=off.

But I still think it makes sense to protect the crashkernel only if
rodata=full (which is the default on arm64) as this allows us to rely
on page mappings and I think fits well with what we do for modules.

> 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?

Thanks.

Will