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    SubjectRe: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation
    On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote:
    > On Mon, 4 Jul 2022 at 18:38, Will Deacon <will@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

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