memory-hotplug: add sysfs removable attribute for hotplug memory remove

Memory may be hot-removed on a per-memory-block basis, particularly on
POWER where the SPARSEMEM section size often matches the memory-block
size.  A user-level agent must be able to identify which sections of
memory are likely to be removable before attempting the potentially
expensive operation.  This patch adds a file called "removable" to the
memory directory in sysfs to help such an agent.  In this patch, a memory
block is considered removable if;

o It contains only MOVABLE pageblocks
o It contains only pageblocks with free pages regardless of pageblock type

On the other hand, a memory block starting with a PageReserved() page will
never be considered removable.  Without this patch, the user-agent is
forced to choose a memory block to remove randomly.

Sample output of the sysfs files:

./memory/memory0/removable: 0
./memory/memory1/removable: 0
./memory/memory2/removable: 0
./memory/memory3/removable: 0
./memory/memory4/removable: 0
./memory/memory5/removable: 0
./memory/memory6/removable: 0
./memory/memory7/removable: 1
./memory/memory8/removable: 0
./memory/memory9/removable: 0
./memory/memory10/removable: 0
./memory/memory11/removable: 0
./memory/memory12/removable: 0
./memory/memory13/removable: 0
./memory/memory14/removable: 0
./memory/memory15/removable: 0
./memory/memory16/removable: 0
./memory/memory17/removable: 1
./memory/memory18/removable: 1
./memory/memory19/removable: 1
./memory/memory20/removable: 1
./memory/memory21/removable: 1
./memory/memory22/removable: 1

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 115 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-memory b/Documentation/ABI/testing/sysfs-devices-memory
new file mode 100644
index 000000000000..7a16fe1e2270
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-devices-memory
@@ -0,0 +1,24 @@
+What:           /sys/devices/system/memory
+Date:           June 2008
+Contact:        Badari Pulavarty <pbadari@us.ibm.com>
+Description:
+                The /sys/devices/system/memory contains a snapshot of the
+                internal state of the kernel memory blocks. Files could be
+                added or removed dynamically to represent hot-add/remove
+                operations.
+
+Users:          hotplug memory add/remove tools
+                https://w3.opensource.ibm.com/projects/powerpc-utils/
+
+What:           /sys/devices/system/memory/memoryX/removable
+Date:           June 2008
+Contact:        Badari Pulavarty <pbadari@us.ibm.com>
+Description:
+                The file /sys/devices/system/memory/memoryX/removable
+                indicates whether this memory block is removable or not.
+                This is useful for a user-level agent to determine
+                identify removable sections of the memory before attempting
+                potentially expensive hot-remove memory operation
+
+Users:          hotplug memory remove tools
+                https://w3.opensource.ibm.com/projects/powerpc-utils/
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 4d4e0e7b6e92..855ed1a9f97b 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -101,6 +101,21 @@ static ssize_t show_mem_phys_index(struct sys_device *dev,
 }
 
 /*
+ * Show whether the section of memory is likely to be hot-removable
+ */
+static ssize_t show_mem_removable(struct sys_device *dev, char *buf)
+{
+        unsigned long start_pfn;
+        int ret;
+        struct memory_block *mem =
+                container_of(dev, struct memory_block, sysdev);
+
+        start_pfn = section_nr_to_pfn(mem->phys_index);
+        ret = is_mem_section_removable(start_pfn, PAGES_PER_SECTION);
+        return sprintf(buf, "%d\n", ret);
+}
+
+/*
  * online, offline, going offline, etc.
  */
 static ssize_t show_mem_state(struct sys_device *dev,
@@ -262,6 +277,7 @@ static ssize_t show_phys_device(struct sys_device *dev,
 static SYSDEV_ATTR(phys_index, 0444, show_mem_phys_index, NULL);
 static SYSDEV_ATTR(state, 0644, show_mem_state, store_mem_state);
 static SYSDEV_ATTR(phys_device, 0444, show_phys_device, NULL);
+static SYSDEV_ATTR(removable, 0444, show_mem_removable, NULL);
 
 #define mem_create_simple_file(mem, attr_name)  \
         sysdev_create_file(&mem->sysdev, &attr_##attr_name)
@@ -350,6 +366,8 @@ static int add_memory_block(unsigned long node_id, struct mem_section *section,
                 ret = mem_create_simple_file(mem, state);
         if (!ret)
                 ret = mem_create_simple_file(mem, phys_device);
+        if (!ret)
+                ret = mem_create_simple_file(mem, removable);
 
         return ret;
 }
@@ -394,6 +412,7 @@ int remove_memory_block(unsigned long node_id, struct mem_section *section,
         mem_remove_simple_file(mem, phys_index);
         mem_remove_simple_file(mem, state);
         mem_remove_simple_file(mem, phys_device);
+        mem_remove_simple_file(mem, removable);
         unregister_memory(mem, section);
 
         return 0;
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 3628e5088f64..763ba81fc0f0 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -199,6 +199,18 @@ extern int walk_memory_resource(unsigned long start_pfn,
                         unsigned long nr_pages, void *arg,
                         int (*func)(unsigned long, unsigned long, void *));
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+
+extern int is_mem_section_removable(unsigned long pfn, unsigned long nr_pages);
+
+#else
+static inline int is_mem_section_removable(unsigned long pfn,
+                                        unsigned long nr_pages)
+{
+        return 0;
+}
+#endif /* CONFIG_MEMORY_HOTREMOVE */
+
 extern int add_memory(int nid, u64 start, u64 size);
 extern int arch_add_memory(int nid, u64 start, u64 size);
 extern int remove_memory(u64 start, u64 size);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 93aba78dc8b6..89fee2dcb039 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -523,6 +523,66 @@ EXPORT_SYMBOL_GPL(add_memory);
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
 /*
+ * A free page on the buddy free lists (not the per-cpu lists) has PageBuddy
+ * set and the size of the free page is given by page_order(). Using this,
+ * the function determines if the pageblock contains only free pages.
+ * Due to buddy contraints, a free page at least the size of a pageblock will
+ * be located at the start of the pageblock
+ */
+static inline int pageblock_free(struct page *page)
+{
+        return PageBuddy(page) && page_order(page) >= pageblock_order;
+}
+
+/* Return the start of the next active pageblock after a given page */
+static struct page *next_active_pageblock(struct page *page)
+{
+        int pageblocks_stride;
+
+        /* Ensure the starting page is pageblock-aligned */
+        BUG_ON(page_to_pfn(page) & (pageblock_nr_pages - 1));
+
+        /* Move forward by at least 1 * pageblock_nr_pages */
+        pageblocks_stride = 1;
+
+        /* If the entire pageblock is free, move to the end of free page */
+        if (pageblock_free(page))
+                pageblocks_stride += page_order(page) - pageblock_order;
+
+        return page + (pageblocks_stride * pageblock_nr_pages);
+}
+
+/* Checks if this range of memory is likely to be hot-removable. */
+int is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
+{
+        int type;
+        struct page *page = pfn_to_page(start_pfn);
+        struct page *end_page = page + nr_pages;
+
+        /* Check the starting page of each pageblock within the range */
+        for (; page < end_page; page = next_active_pageblock(page)) {
+                type = get_pageblock_migratetype(page);
+
+                /*
+                 * A pageblock containing MOVABLE or free pages is considered
+                 * removable
+                 */
+                if (type != MIGRATE_MOVABLE && !pageblock_free(page))
+                        return 0;
+
+                /*
+                 * A pageblock starting with a PageReserved page is not
+                 * considered removable.
+                 */
+                if (PageReserved(page))
+                        return 0;
+        }
+
+        /* All pageblocks in the memory block are likely to be hot-removable */
+        return 1;
+}
+
+/*
  * Confirm all pages in a range [start, end) is belongs to the same zone.
  */
 static int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)