linux-kernel - [PATCH 2 of 4] hotplug-memory: adding non-section-aligned memory is bad

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Message-Id: <e0b16cc8349b38a5f3f9.1206664118@localhost>
Date:	Thu, 27 Mar 2008 17:28:38 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Christoph Lameter <clameter@....com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 2 of 4] hotplug-memory: adding non-section-aligned memory is
	bad

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@...rix.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Yasunori Goto <y-goto@...fujitsu.com>
Cc: Christoph Lameter <clameter@....com>
Cc: Dave Hansen <dave@...ux.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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