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

[guanghui.fgh]

在 2022/7/5 17:52, Will Deacon 写道:
> On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote:
> > On Mon, 4 Jul 2022 at 18:38, Will Deacon <will@xxxxxxxxxx> 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
Thanks.

1.The rodata full is harm to the performance and has been disabled in-house.

2.When using crashkernel with rodata non full, the kernel also will use 
non block/section mapping which cause high d-TLB miss and degrade 
performance greatly.
This patch fix it to use block/section mapping as far as possible.

bool can_set_direct_map(void)
{
	return rodata_full || debug_pagealloc_enabled();
}

map_mem:
if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
	flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

3.When rodata full is disabled, crashkernel also need protect(keep 
arch_kexec_[un]protect_crashkres using).
I think crashkernel should't depend on radata full(Maybe other 
architecture don't support radata full now).

Thanks.