Subject: Re: [PATCH v2] bootmem/sparsemem: remove limit constraint in alloc_bootmem_section

From: Andrew Morton <[email-masked]>
Date: Wed, 29 Feb 2012 15:28:30 -0800

On Wed, 29 Feb 2012 10:12:33 -0800
Nishanth Aravamudan <[email-masked]> wrote:

> While testing AMS (Active Memory Sharing) / CMO (Cooperative Memory
> Overcommit) on powerpc, we tripped the following:
> 
> kernel BUG at mm/bootmem.c:483!
>
> ...
> 
> 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 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
> unconditionally remove the limit condition in alloc_bootmem_section,
> meaning allocations are allowed to cross section boundaries (necessary
> for systems of this size).
> 
> Johannes Weiner pointed out that if alloc_bootmem_section() no longer
> guarantees section-locality, we need check_usemap_section_nr() to print
> possible cross-dependencies between node descriptors and the usemaps
> allocated through it. That makes the two loops in
> sparse_early_usemaps_alloc_node() identical, so re-factor the code a
> bit.

The patch is a bit scary now, so I think we should merge it into
3.4-rc1 and then backport it into 3.3.1 if nothing blows up.

Do you think it should be backported into 3.3.x?  Earlier kernels?

Also, this?

--- a/mm/bootmem.c~bootmem-sparsemem-remove-limit-constraint-in-alloc_bootmem_section-fix
+++ a/mm/bootmem.c
@@ -766,14 +766,13 @@ void * __init alloc_bootmem_section(unsi
 				    unsigned long section_nr)
 {
 	bootmem_data_t *bdata;
-	unsigned long pfn, goal, limit;
+	unsigned long pfn, goal;
 
 	pfn = section_nr_to_pfn(section_nr);
 	goal = pfn << PAGE_SHIFT;
-	limit = 0;
 	bdata = &bootmem_node_data[early_pfn_to_nid(pfn)];
 
-	return alloc_bootmem_core(bdata, size, SMP_CACHE_BYTES, goal, limit);
+	return alloc_bootmem_core(bdata, size, SMP_CACHE_BYTES, goal, 0);
 }
 #endif
 
_