Re: [PATCH RESEND] memory hotplug: fix a double register section info bug

[Mel Gorman <mgorman@suse.de>]

On Fri, Sep 14, 2012 at 11:43:27AM +0800, qiuxishi wrote:
> There may be a bug when registering section info. For example, on
> my Itanium platform, the pfn range of node0 includes the other nodes,
> so other nodes' section info will be double registered, and memmap's
> page count will equal to 3.
> 
> node0: start_pfn=0x100,    spanned_pfn=0x20fb00, present_pfn=0x7f8a3, => 0x000100-0x20fc00
> node1: start_pfn=0x80000,  spanned_pfn=0x80000,  present_pfn=0x80000, => 0x080000-0x100000
> node2: start_pfn=0x100000, spanned_pfn=0x80000,  present_pfn=0x80000, => 0x100000-0x180000
> node3: start_pfn=0x180000, spanned_pfn=0x80000,  present_pfn=0x80000, => 0x180000-0x200000
> 

This is an unusual configuration but it's not unheard of. PPC64 in rare
(and usually broken) configurations can have one node span another. Tony
should know if such a configuration is normally allowed on Itanium or if
this should be considered a platform bug. Tony?

> free_all_bootmem_node()
> 	register_page_bootmem_info_node()
> 		register_page_bootmem_info_section()
> 
> When hot remove memory, we can't free the memmap's page because
> page_count() is 2 after put_page_bootmem().
> 
> sparse_remove_one_section()
> 	free_section_usemap()
> 		free_map_bootmem()
> 			put_page_bootmem()
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
> ---
>  mm/memory_hotplug.c |   10 ++++------
>  1 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 2adbcac..cf493c7 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -126,9 +126,6 @@ static void register_page_bootmem_info_section(unsigned long start_pfn)
>  	struct mem_section *ms;
>  	struct page *page, *memmap;
> 
> -	if (!pfn_valid(start_pfn))
> -		return;
> -
>  	section_nr = pfn_to_section_nr(start_pfn);
>  	ms = __nr_to_section(section_nr);
> 
> @@ -187,9 +184,10 @@ void register_page_bootmem_info_node(struct pglist_data *pgdat)
>  	end_pfn = pfn + pgdat->node_spanned_pages;
> 
>  	/* register_section info */
> -	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION)
> -		register_page_bootmem_info_section(pfn);
> -
> +	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> +		if (pfn_valid(pfn) && (pfn_to_nid(pfn) == node))
> +			register_page_bootmem_info_section(pfn);
> +	}

Functionally what the patch does is check if the PFN is both valid *and*
belongs to the expected node to catch a situation where nodes overlap. As
there are no other callers of register_page_bootmem_info_section() this
patch seems reasonable to me so

Acked-by: Mel Gorman <mgorman@suse.de>

I think it would also be ok to consider this a -stable candidate.

>  }
>  #endif /* !CONFIG_SPARSEMEM_VMEMMAP */
> 
> -- 
> 1.7.1

-- 
Mel Gorman
SUSE Labs