[PATCH v2 0/2] fix a kernel oops when reading sysfs valid_zones

[PATCH v2 0/2] fix a kernel oops when reading sysfs valid_zones

[Toshi Kani]

A sysfs memory file is created for each 2GiB memory block on x86-64
when the system has 64GiB or more memory. [1]  When the start address
of a memory block is not backed by struct page, i.e. a memory range is
not aligned by 2GiB, reading its 'valid_zones' attribute file leads to
a kernel oops.  This issue was observed on multiple x86-64 systems
with more than 64GiB of memory.  This patch-set fixes this issue.

Patch 1 first fixes an issue in test_pages_in_a_zone(), which does
not test the start section.

Patch 2 then fixes the kernel oops by extending test_pages_in_a_zone()
to return valid [start, end).

Note for stable kernels: The memory block size change was made by commit
bdee237c034, which was accepted to 3.9.  However, this patch-set depends
on (and fixes) the change to test_pages_in_a_zone() made by commit
5f0f2887f4, which was accepted to 4.4.  So, I recommend that we backport
it up to 4.4.

[1] 'Commit bdee237c0343 ("x86: mm: Use 2GB memory block size on
    large-memory x86-64 systems")'

v2:
 - Rebase to the -mm tree. (Andrew Morton)
 - Add more descriptions about the issue. (Andrew Morton)
 - Add cc to stable kernels. (Greg Kroah-Hartman, Andrew Morton)

---
Toshi Kani (2):
 1/2 mm/memory_hotplug.c: check start_pfn in test_pages_in_a_zone() 
 2/2 base/memory, hotplug: fix a kernel oops in show_valid_zones()

---
 drivers/base/memory.c          | 12 ++++++------
 include/linux/memory_hotplug.h |  3 ++-
 mm/memory_hotplug.c            | 28 +++++++++++++++++++++-------

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[PATCH v2 1/2] mm/memory_hotplug.c: check start_pfn in test_pages_in_a_zone()

[Toshi Kani]

test_pages_in_a_zone() does not check 'start_pfn' when it is
aligned by section since 'sec_end_pfn' is set equal to 'pfn'.
Since this function is called for testing the range of a sysfs
memory file, 'start_pfn' is always aligned by section.

Fix it by properly setting 'sec_end_pfn' to the next section pfn.

Also make sure that this function returns 1 only when the range
belongs to a zone.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrew Banman <abanman@sgi.com>
Cc: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # v4.4+
---
 mm/memory_hotplug.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3e3db7a..c845c5f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1489,7 +1489,7 @@ bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
 }
 
 /*
- * Confirm all pages in a range [start, end) is belongs to the same zone.
+ * Confirm all pages in a range [start, end) belong to the same zone.
  */
 int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
 {
@@ -1497,9 +1497,9 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
 	struct zone *zone = NULL;
 	struct page *page;
 	int i;
-	for (pfn = start_pfn, sec_end_pfn = SECTION_ALIGN_UP(start_pfn);
+	for (pfn = start_pfn, sec_end_pfn = SECTION_ALIGN_UP(start_pfn + 1);
 	     pfn < end_pfn;
-	     pfn = sec_end_pfn + 1, sec_end_pfn += PAGES_PER_SECTION) {
+	     pfn = sec_end_pfn, sec_end_pfn += PAGES_PER_SECTION) {
 		/* Make sure the memory section is present first */
 		if (!present_section_nr(pfn_to_section_nr(pfn)))
 			continue;
@@ -1518,7 +1518,11 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
 			zone = page_zone(page);
 		}
 	}
-	return 1;
+
+	if (zone)
+		return 1;
+	else
+		return 0;
 }
 
 /*

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[PATCH v2 2/2] base/memory, hotplug: fix a kernel oops in show_valid_zones()

[Toshi Kani]

Reading a sysfs "memoryN/valid_zones" file leads to the following
oops when the first page of a range is not backed by struct page.
show_valid_zones() assumes that 'start_pfn' is always valid for
page_zone().

 BUG: unable to handle kernel paging request at ffffea017a000000
 IP: show_valid_zones+0x6f/0x160

This issue may happen on x86-64 systems with 64GiB or more memory
since their memory block size is bumped up to 2GiB. [1]  An example
of such systems is desribed below.  0x3240000000 is only aligned by
1GiB and this memory block starts from 0x3200000000, which is not
backed by struct page.

 BIOS-e820: [mem 0x0000003240000000-0x000000603fffffff] usable

Since test_pages_in_a_zone() already checks holes, fix this issue
by extending this function to return 'valid_start' and 'valid_end'
for a given range.  show_valid_zones() then proceeds with the valid
range.

[1] 'Commit bdee237c0343 ("x86: mm: Use 2GB memory block size on
    large-memory x86-64 systems")'

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Zhang Zhen <zhenzhang.zhang@huawei.com>
Cc: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: <stable@vger.kernel.org> # v4.4+
---
 drivers/base/memory.c          |   12 ++++++------
 include/linux/memory_hotplug.h |    3 ++-
 mm/memory_hotplug.c            |   20 +++++++++++++++-----
 3 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 730d1cf..fb38146 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -389,33 +389,33 @@ static ssize_t show_valid_zones(struct device *dev,
 {
 	struct memory_block *mem = to_memory_block(dev);
 	unsigned long start_pfn, end_pfn;
+	unsigned long valid_start, valid_end, valid_pages;
 	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
-	struct page *first_page;
 	struct zone *zone;
 	int zone_shift = 0;
 
 	start_pfn = section_nr_to_pfn(mem->start_section_nr);
 	end_pfn = start_pfn + nr_pages;
-	first_page = pfn_to_page(start_pfn);
 
 	/* The block contains more than one zone can not be offlined. */
-	if (!test_pages_in_a_zone(start_pfn, end_pfn))
+	if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start, &valid_end))
 		return sprintf(buf, "none\n");
 
-	zone = page_zone(first_page);
+	zone = page_zone(pfn_to_page(valid_start));
+	valid_pages = valid_end - valid_start;
 
 	/* MMOP_ONLINE_KEEP */
 	sprintf(buf, "%s", zone->name);
 
 	/* MMOP_ONLINE_KERNEL */
-	zone_can_shift(start_pfn, nr_pages, ZONE_NORMAL, &zone_shift);
+	zone_can_shift(valid_start, valid_pages, ZONE_NORMAL, &zone_shift);
 	if (zone_shift) {
 		strcat(buf, " ");
 		strcat(buf, (zone + zone_shift)->name);
 	}
 
 	/* MMOP_ONLINE_MOVABLE */
-	zone_can_shift(start_pfn, nr_pages, ZONE_MOVABLE, &zone_shift);
+	zone_can_shift(valid_start, valid_pages, ZONE_MOVABLE, &zone_shift);
 	if (zone_shift) {
 		strcat(buf, " ");
 		strcat(buf, (zone + zone_shift)->name);
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 08856a6..2f2c0d1 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -85,7 +85,8 @@ extern int zone_grow_waitqueues(struct zone *zone, unsigned long nr_pages);
 extern int add_one_highpage(struct page *page, int pfn, int bad_ppro);
 /* VM interface that may be used by firmware interface */
 extern int online_pages(unsigned long, unsigned long, int);
-extern int test_pages_in_a_zone(unsigned long, unsigned long);
+extern int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
+	unsigned long *valid_start, unsigned long *valid_end);
 extern void __offline_isolated_pages(unsigned long, unsigned long);
 
 typedef void (*online_page_callback_t)(struct page *page);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c845c5f..5005e5f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1490,10 +1490,13 @@ bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
 
 /*
  * Confirm all pages in a range [start, end) belong to the same zone.
+ * When true, return its valid [start, end).
  */
-int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
+int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
+			 unsigned long *valid_start, unsigned long *valid_end)
 {
 	unsigned long pfn, sec_end_pfn;
+	unsigned long start, end;
 	struct zone *zone = NULL;
 	struct page *page;
 	int i;
@@ -1515,14 +1518,20 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
 			page = pfn_to_page(pfn + i);
 			if (zone && page_zone(page) != zone)
 				return 0;
+			if (!zone)
+				start = pfn + i;
 			zone = page_zone(page);
+			end = pfn + MAX_ORDER_NR_PAGES;
 		}
 	}
 
-	if (zone)
+	if (zone) {
+		*valid_start = start;
+		*valid_end = end;
 		return 1;
-	else
+	} else {
 		return 0;
+	}
 }
 
 /*
@@ -1849,6 +1858,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	long offlined_pages;
 	int ret, drain, retry_max, node;
 	unsigned long flags;
+	unsigned long valid_start, valid_end;
 	struct zone *zone;
 	struct memory_notify arg;
 
@@ -1859,10 +1869,10 @@ static int __ref __offline_pages(unsigned long start_pfn,
 		return -EINVAL;
 	/* This makes hotplug much easier...and readable.
 	   we assume this for now. .*/
-	if (!test_pages_in_a_zone(start_pfn, end_pfn))
+	if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start, &valid_end))
 		return -EINVAL;
 
-	zone = page_zone(pfn_to_page(start_pfn));
+	zone = page_zone(pfn_to_page(valid_start));
 	node = zone_to_nid(zone);
 	nr_pages = end_pfn - start_pfn;
 

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