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Subject[PATCH 2 of 4] hotplug-memory: adding non-section-aligned memory is bad
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Adding non-section-aligned memory will cause bad results, because page
structures are only built on section-aligned boundaries. Also, memory
whose size isn't a multiple of the section size is silently ignored.

This patch adds a couple of WARN_ONs to help confused programmers work
out what's going wrong when they hotplug memory without being aware of
these constraints.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -784,6 +784,9 @@
#define PAGES_PER_SECTION (1UL << PFN_SECTION_SHIFT)
#define PAGE_SECTION_MASK (~(PAGES_PER_SECTION-1))

+#define SECTION_SIZE (1UL << SECTION_SIZE_BITS)
+#define SECTION_SIZE_MASK (SECTION_SIZE - 1)
+
#define SECTION_BLOCKFLAGS_BITS \
((1UL << (PFN_SECTION_SHIFT - pageblock_order)) * NR_PAGEBLOCK_BITS)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -300,6 +300,11 @@
int ret;
u64 start = res->start;
u64 size = res->end - res->start + 1;
+
+ /* Adding non-section-aligned memory will give unexpected
+ and unintuitive results. */
+ WARN_ON((start & SECTION_SIZE_MASK) != 0);
+ WARN_ON((size & SECTION_SIZE_MASK) != 0);

if (!node_online(nid)) {
pgdat = hotadd_new_pgdat(nid, start);



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