mm, memory_hotplug: be more verbose for memory offline failures

There is only very limited information printed when the memory offlining
fails:

[ 1984.506184] rac1 kernel: memory offlining [mem 0x82600000000-0x8267fffffff] failed due to signal backoff

This tells us that the failure is triggered by the userspace intervention
but it doesn't tell us much more about the underlying reason.  It might be
that the page migration failes repeatedly and the userspace timeout
expires and send a signal or it might be some of the earlier steps
(isolation, memory notifier) takes too long.

If the migration failes then it would be really helpful to see which page
that and its state.  The same applies to the isolation phase.  If we fail
to isolate a page from the allocator then knowing the state of the page
would be helpful as well.

Dump the page state that fails to get isolated or migrated.  This will
tell us more about the failure and what to focus on during debugging.

[akpm@linux-foundation.org: add missing printk arg]
[mhocko@suse.com: tweak dump_page() `reason' text]
  Link: http://lkml.kernel.org/r/20181116083020.20260-6-mhocko@kernel.org
Link: http://lkml.kernel.org/r/20181107101830.17405-6-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Oscar Salvador <OSalvador@suse.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -1388,10 +1388,8 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 						    page_is_file_cache(page));
 
 		} else {
-#ifdef CONFIG_DEBUG_VM
-			pr_alert("failed to isolate pfn %lx\n", pfn);
+			pr_warn("failed to isolate pfn %lx\n", pfn);
 			dump_page(page, "isolation failed");
-#endif
 			put_page(page);
 			/* Because we don't have big zone->lock. we should
 			   check this again here. */
@@ -1411,9 +1409,15 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 		/* Allocate a new page from the nearest neighbor node */
 		ret = migrate_pages(&source, new_node_page, NULL, 0,
 					MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
-		if (ret)
+		if (ret) {
+			list_for_each_entry(page, &source, lru) {
+				pr_warn("migrating pfn %lx failed ret:%d ",
+				       page_to_pfn(page), ret);
+				dump_page(page, "migration failure");
+			}
 			putback_movable_pages(&source);
 		}
+	}
 out:
 	return ret;
 }

mm/page_alloc.c

@@ -7869,6 +7869,7 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 	return false;
 unmovable:
 	WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
+	dump_page(pfn_to_page(pfn+iter), "unmovable page");
 	return true;
 }