Subject: Re: [PATCH] sparsemem/bootmem: catch greater than section size allocations

From: Mel Gorman <[email-masked]>
Date: Tue, 28 Feb 2012 15:47:32 +0000

On Fri, Feb 24, 2012 at 11:33:58AM -0800, Nishanth Aravamudan wrote:
> While testing AMS (Active Memory Sharing) / CMO (Cooperative Memory
> Overcommit) on powerpc, we tripped the following:
> 
> kernel BUG at mm/bootmem.c:483!
> cpu 0x0: Vector: 700 (Program Check) at [c000000000c03940]
>     pc: c000000000a62bd8: .alloc_bootmem_core+0x90/0x39c
>     lr: c000000000a64bcc: .sparse_early_usemaps_alloc_node+0x84/0x29c
>     sp: c000000000c03bc0
>    msr: 8000000000021032
>   current = 0xc000000000b0cce0
>   paca    = 0xc000000001d80000
>     pid   = 0, comm = swapper
> kernel BUG at mm/bootmem.c:483!
> enter ? for help
> [c000000000c03c80] c000000000a64bcc
> .sparse_early_usemaps_alloc_node+0x84/0x29c
> [c000000000c03d50] c000000000a64f10 .sparse_init+0x12c/0x28c
> [c000000000c03e20] c000000000a474f4 .setup_arch+0x20c/0x294
> [c000000000c03ee0] c000000000a4079c .start_kernel+0xb4/0x460
> [c000000000c03f90] c000000000009670 .start_here_common+0x1c/0x2c
> 
> This is
> 
>         BUG_ON(limit && goal + size > limit);
> 
> and after some debugging, it seems that
> 
> 	goal = 0x7ffff000000
> 	limit = 0x80000000000
> 
> and sparse_early_usemaps_alloc_node ->
> sparse_early_usemaps_alloc_pgdat_section -> alloc_bootmem_section calls
> 
> 	return alloc_bootmem_section(usemap_size() * count, section_nr);
> 
> This is on a system with 8TB available via the AMS pool, and as a quirk
> of AMS in firmware, all of that memory shows up in node 0. So, we end up
> with an allocation that will fail the goal/limit constraints. In theory,
> we could "fall-back" to alloc_bootmem_node() in
> sparse_early_usemaps_alloc_node(), but since we actually have HOTREMOVE
> defined, we'll BUG_ON() instead. A simple solution appears to be to
> disable the limit check if the size of the allocation in
> alloc_bootmem_secition exceeds the section size.
> 
> Signed-off-by: Nishanth Aravamudan <[email-masked]>
> Cc: Dave Hansen <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Ben Herrenschmidt <[email-masked]>
> Cc: Robert Jennings <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> ---
>  include/linux/mmzone.h |    2 ++
>  mm/bootmem.c           |    5 ++++-
>  2 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 650ba2f..4176834 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -967,6 +967,8 @@ static inline unsigned long early_pfn_to_nid(unsigned long pfn)
>   * PA_SECTION_SHIFT		physical address to/from section number
>   * PFN_SECTION_SHIFT		pfn to/from section number
>   */
> +#define BYTES_PER_SECTION	(1UL << SECTION_SIZE_BITS)
> +
>  #define SECTIONS_SHIFT		(MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
>  
>  #define PA_SECTION_SHIFT	(SECTION_SIZE_BITS)
> diff --git a/mm/bootmem.c b/mm/bootmem.c
> index 668e94d..5cbbc76 100644
> --- a/mm/bootmem.c
> +++ b/mm/bootmem.c
> @@ -770,7 +770,10 @@ void * __init alloc_bootmem_section(unsigned long size,
>  
>  	pfn = section_nr_to_pfn(section_nr);
>  	goal = pfn << PAGE_SHIFT;
> -	limit = section_nr_to_pfn(section_nr + 1) << PAGE_SHIFT;
> +	if (size > BYTES_PER_SECTION)
> +		limit = 0;
> +	else
> +		limit = section_nr_to_pfn(section_nr + 1) << PAGE_SHIFT;

As it's ok to spill the allocation over to an adjacent section, why not
just make limit==0 unconditionally. That would avoid defining
BYTES_PER_SECTION.

-- 
Mel Gorman
SUSE Labs