Subject: Re: [PATCH v2 2/2] Register bootmem pages

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 27 Aug 2013 13:44:51 +1000

On Mon, 2013-08-19 at 21:53 -0500, Nathan Fontenot wrote:
> Previous commit 46723bfa540... introduced a new config option
> HAVE_BOOTMEM_INFO_NODE that ended up breaking memory hot-remove for ppc
> when sparse vmemmap is not defined.
> 
> This patch defines HAVE_BOOTMEM_INFO_NODE for ppc and adds the call to
> register_page_bootmem_info_node. Without this we get a BUG_ON for memory
> hot remove in put_page_bootmem().
> 
> This also adds a stub for register_page_bootmem_memmap to allow ppc to build
> with sparse vmemmap defined.
> 
> Signed-off-by: Nathan Fontenot <[email-masked]>
> ---

So I still feel very uncomfortable with that stuff ....

For example, x86 calls register_page_bootmem_info_node() at boot time,
which does that strange "get_page_bootmem" on the NODE_DATA itself at
boot time, we don't. Should we ?

Since we don't, what do that mean ? We don't remove the node info pages
on unplug ? Is that ok ?

There's a whole pile of totally undocumented / uncommented generic code
with horrible function names in there whose sematic is very very
unclear.

Now, if we call that thing, are we expected to have
register_paqe_bootmem_memmap() to actually do something right? I assume
that means actually calling get_page_bootmem() on the various struct
page that comprise the vmemmap.

Well, we can probably implement that since we maintain a list of all the
vmemap pages... However, we don't implement vmemmap_free(). Should we ?

This all confuses me...

Cheers,
Ben.

> 
> ---
>  arch/powerpc/mm/init_64.c |    4 ++++
>  arch/powerpc/mm/mem.c     |    9 +++++++++
>  mm/Kconfig                |    2 +-
>  3 files changed, 14 insertions(+), 1 deletion(-)
> 
> Index: linux/arch/powerpc/mm/init_64.c
> ===================================================================
> --- linux.orig/arch/powerpc/mm/init_64.c
> +++ linux/arch/powerpc/mm/init_64.c
> @@ -300,5 +300,9 @@ void vmemmap_free(unsigned long start, u
>  {
>  }
> 
> +void register_page_bootmem_memmap(unsigned long section_nr,
> +				  struct page *start_page, unsigned long size)
> +{
> +}
>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
> 
> Index: linux/arch/powerpc/mm/mem.c
> ===================================================================
> --- linux.orig/arch/powerpc/mm/mem.c
> +++ linux/arch/powerpc/mm/mem.c
> @@ -297,12 +297,21 @@ void __init paging_init(void)
>  }
>  #endif /* ! CONFIG_NEED_MULTIPLE_NODES */
> 
> +static void __init register_page_bootmem_info(void)
> +{
> +	int i;
> +
> +	for_each_online_node(i)
> +		register_page_bootmem_info_node(NODE_DATA(i));
> +}
> +
>  void __init mem_init(void)
>  {
>  #ifdef CONFIG_SWIOTLB
>  	swiotlb_init(0);
>  #endif
> 
> +	register_page_bootmem_info();
>  	high_memory = (void *) __va(max_low_pfn * PAGE_SIZE);
>  	set_max_mapnr(max_pfn);
>  	free_all_bootmem();
> Index: linux/mm/Kconfig
> ===================================================================
> --- linux.orig/mm/Kconfig
> +++ linux/mm/Kconfig
> @@ -183,7 +183,7 @@ config MEMORY_HOTPLUG_SPARSE
>  config MEMORY_HOTREMOVE
>  	bool "Allow for memory hot remove"
>  	select MEMORY_ISOLATION
> -	select HAVE_BOOTMEM_INFO_NODE if X86_64
> +	select HAVE_BOOTMEM_INFO_NODE if (X86_64 || PPC64)
>  	depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
>  	depends on MIGRATION
> 
> 
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