Re: [PATCH part1 v6 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up

[Tejun Heo <tj@kernel.org>]

Hello,

On Wed, Oct 09, 2013 at 01:36:36AM +0800, Zhang Yanfei wrote:
> > I'm still seriously concerned about this.  This unconditionally
> > introduces new behavior which may very well break some classes of

This is an optional behavior which is triggered by a very specific
kernel boot param, which I suspect is gonna need to stick around to
support memory hotplug in the current setup unless we add another
layer of address translation to support memory hotplug.

> > systems -- the whole point of creating the page tables top down is
> > because the kernel tends to be allocated in lower memory, which is also
> > the memory that some devices need for DMA.

Would that really matter for the target use cases here?  These are
likely fairly huge highend machines.  ISA DMA limit is below the
kernel image and 32bit limit is pretty big in comparison and at this
point even that limit is likely to be irrelevant at least for the
target machines, which are gonna be almost inherently extremely niche.

> > so if we allocate memory close to the kernel image,
> >   it's likely that we don't contaminate hotpluggable node.  We're
> >   talking about few megs at most right after the kernel image.  I
> >   can't see how that would make any noticeable difference.
> 
> You meant that the memory size is about few megs. But here, page tables
> seems to be large enough in big memory machines, so that page tables will

Hmmm?  Even with 4k mappings and, say, 16Gigs of memory, it's still
somewhere above 32MiB, right?  And, these physical mappings don't
usually use 4k mappings to begin with.  Unless we're worrying about
ISA DMA limit, I don't think it'd be problematic.

> consume the precious lower memory. So I think we may really reorder
> the page table setup after we get the hotplug info in some way. Just like
> we have done in patch 5, we reorder reserve_crashkernel() to be called
> after initmem_init().
> 
> So do you still have any objection to the pagetable setup reorder?

I still feel quite uneasy about pulling SRAT parsing and ACPI initrd
overriding into early boot.

Thanks.

-- 
tejun

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