Re: [PATCH 3/7] Generic Virtual Memmap support for SPARSEMEM

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 13 Jul 2007 15:42:30 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Fri, 13 Jul 2007, KAMEZAWA Hiroyuki wrote:
> 
> > On Fri, 13 Jul 2007 14:36:08 +0100
> > Andy Whitcroft <apw@shadowen.org> wrote:
> > 
> > > SPARSEMEM is a pretty nice framework that unifies quite a bit of
> > > code over all the arches. It would be great if it could be the
> > > default so that we can get rid of various forms of DISCONTIG and
> > > other variations on memory maps. So far what has hindered this are
> > > the additional lookups that SPARSEMEM introduces for virt_to_page
> > > and page_address. This goes so far that the code to do this has to
> > > be kept in a separate function and cannot be used inline.
> > > 
> > Maybe it will be our(my or Goto-san's) work to implement MEMORY_HOTADD support
> > for this. Could you add !MEMORY_HOTPLUG in Kconfig ? Then, we'll write
> > patch later.
> > Or..If you'll add memory hotplug support by yourself, It's great, 
> 
> Why would hotadd not work as is?
> 
Just because this patch takes care of boot path. Maybe small problem.
Basically, I welcome this patch. I like this.
If we can remove DISCONTIG+VMEMMAP after this is merged, we can say good-bye
to terrible CONFIG_HOLES_IN_ZONE :)

Note
>From memory hotplug development/enhancement view, I have following thinking now.
 
 1. memmap's section is *not* aligned to "big page size". We have to take care
    of this at adding support for memory_hotplug/unplug.

 2. With an appropriate patch, we can allocate new section's memmap from
    itself. This will reduce possibility of memory hotplug failure becasue of
    large size kmalloc/vmalloc. And it guarantees locality of memmap.
    But maybe need some amount of work for implementing this in clean way.
    This will depend on vmemmap.

 3. removin memmap code for memory unplug will be necessary. But there is no code
    for removing memmap in usual SPARSEMEM. So this is not real problem of vmemmap
    now. 

Thanks,
 -Kame

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