memory-failure: fix an error of mce_bad_pages statistics

When doing

    $ echo paddr > /sys/devices/system/memory/soft_offline_page

to offline a *free* page, the value of mce_bad_pages will be added, and
the page is set HWPoison flag, but it is still managed by page buddy
alocator.

   $ cat /proc/meminfo | grep HardwareCorrupted

shows the value.

If we offline the same page, the value of mce_bad_pages will be added
*again*, this means the value is incorrect now.  Assume the page is
still free during this short time.

  soft_offline_page()
    get_any_page()
      "else if (is_free_buddy_page(p))" branch return 0
        "goto done";
           "atomic_long_add(1, &mce_bad_pages);"

This patch:

Move poisoned page check at the beginning of the function in order to
fix the error.
Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Tested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory-failure.c

@@ -1419,18 +1419,17 @@ static int soft_offline_huge_page(struct page *page, int flags)
 	unsigned long pfn = page_to_pfn(page);
 	struct page *hpage = compound_head(page);
 
+	if (PageHWPoison(hpage)) {
+		pr_info("soft offline: %#lx hugepage already poisoned\n", pfn);
+		return -EBUSY;
+	}
+
 	ret = get_any_page(page, pfn, flags);
 	if (ret < 0)
 		return ret;
 	if (ret == 0)
 		goto done;
 
-	if (PageHWPoison(hpage)) {
-		put_page(hpage);
-		pr_info("soft offline: %#lx hugepage already poisoned\n", pfn);
-		return -EBUSY;
-	}
-
 	/* Keep page count to indicate a given hugepage is isolated. */
 	ret = migrate_huge_page(hpage, new_page, MPOL_MF_MOVE_ALL, false,
 				MIGRATE_SYNC);
@@ -1441,12 +1440,11 @@ static int soft_offline_huge_page(struct page *page, int flags)
 		return ret;
 	}
 done:
-	if (!PageHWPoison(hpage))
-		atomic_long_add(1 << compound_trans_order(hpage),
-				&mce_bad_pages);
+	/* keep elevated page count for bad page */
+	atomic_long_add(1 << compound_trans_order(hpage), &mce_bad_pages);
 	set_page_hwpoison_huge_page(hpage);
 	dequeue_hwpoisoned_huge_page(hpage);
-	/* keep elevated page count for bad page */
+
 	return ret;
 }
 
@@ -1488,6 +1486,11 @@ int soft_offline_page(struct page *page, int flags)
 		}
 	}
 
+	if (PageHWPoison(page)) {
+		pr_info("soft offline: %#lx page already poisoned\n", pfn);
+		return -EBUSY;
+	}
+
 	ret = get_any_page(page, pfn, flags);
 	if (ret < 0)
 		return ret;
@@ -1519,19 +1522,11 @@ int soft_offline_page(struct page *page, int flags)
 		return -EIO;
 	}
 
-	lock_page(page);
-	wait_on_page_writeback(page);
-
 	/*
 	 * Synchronized using the page lock with memory_failure()
 	 */
-	if (PageHWPoison(page)) {
-		unlock_page(page);
-		put_page(page);
-		pr_info("soft offline: %#lx page already poisoned\n", pfn);
-		return -EBUSY;
-	}
-
+	lock_page(page);
+	wait_on_page_writeback(page);
 	/*
 	 * Try to invalidate first. This should work for
 	 * non dirty unmapped page cache pages.
@@ -1583,8 +1578,9 @@ int soft_offline_page(struct page *page, int flags)
 		return ret;
 
 done:
+	/* keep elevated page count for bad page */
 	atomic_long_add(1, &mce_bad_pages);
 	SetPageHWPoison(page);
-	/* keep elevated page count for bad page */
+
 	return ret;
 }