mm: hwpoison: adjust for new thp refcounting

Some mm-related BUG_ON()s could trigger from hwpoison code due to recent
changes in thp refcounting rule.  This patch fixes them up.

In the new refcounting, we no longer use tail->_mapcount to keep tail's
refcount, and thereby we can simplify get/put_hwpoison_page().

And another change is that tail's refcount is not transferred to the raw
page during thp split (more precisely, in new rule we don't take
refcount on tail page any more.) So when we need thp split, we have to
transfer the refcount properly to the 4kB soft-offlined page before
migration.

thp split code goes into core code only when precheck
(total_mapcount(head) == page_count(head) - 1) passes to avoid useless
split, where we assume that one refcount is held by the caller of thp
split and the others are taken via mapping.  To meet this assumption,
this patch moves thp split part in soft_offline_page() after
get_any_page().

[[email protected]: remove unneeded #define, per Kirill]
Signed-off-by: Naoya Horiguchi <[email protected]>
Acked-by: Kirill A.  Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Naoya Horiguchi

include/linux/mm.h

@@ -2217,7 +2217,7 @@ extern int memory_failure(unsigned long pfn, int trapno, int flags);
 extern void memory_failure_queue(unsigned long pfn, int trapno, int flags);
 extern int unpoison_memory(unsigned long pfn);
 extern int get_hwpoison_page(struct page *page);
-extern void put_hwpoison_page(struct page *page);
+#define put_hwpoison_page(page)	put_page(page)
 extern int sysctl_memory_failure_early_kill;
 extern int sysctl_memory_failure_recovery;
 extern void shake_page(struct page *p, int access);

mm/memory-failure.c

@@ -882,15 +882,7 @@ int get_hwpoison_page(struct page *page)
 {
 	struct page *head = compound_head(page);
 
-	if (PageHuge(head))
-		return get_page_unless_zero(head);
-
-	/*
-	 * Thp tail page has special refcounting rule (refcount of tail pages
-	 * is stored in ->_mapcount,) so we can't call get_page_unless_zero()
-	 * directly for tail pages.
-	 */
-	if (PageTransHuge(head)) {
+	if (!PageHuge(head) && PageTransHuge(head)) {
 		/*
 		 * Non anonymous thp exists only in allocation/free time. We
 		 * can't handle such a case correctly, so let's give it up.
@@ -902,41 +894,12 @@ int get_hwpoison_page(struct page *page)
 				page_to_pfn(page));
 			return 0;
 		}
-
-		if (get_page_unless_zero(head)) {
-			if (PageTail(page))
-				get_page(page);
-			return 1;
-		} else {
-			return 0;
-		}
 	}
 
-	return get_page_unless_zero(page);
+	return get_page_unless_zero(head);
 }
 EXPORT_SYMBOL_GPL(get_hwpoison_page);
 
-/**
- * put_hwpoison_page() - Put refcount for memory error handling:
- * @page:	raw error page (hit by memory error)
- */
-void put_hwpoison_page(struct page *page)
-{
-	struct page *head = compound_head(page);
-
-	if (PageHuge(head)) {
-		put_page(head);
-		return;
-	}
-
-	if (PageTransHuge(head))
-		if (page != head)
-			put_page(head);
-
-	put_page(page);
-}
-EXPORT_SYMBOL_GPL(put_hwpoison_page);
-
 /*
  * Do all that is necessary to remove user space mappings. Unmap
  * the pages and send SIGBUS to the processes if the data was dirty.
@@ -1162,6 +1125,8 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
 			return -EBUSY;
 		}
 		unlock_page(hpage);
+		get_hwpoison_page(p);
+		put_hwpoison_page(hpage);
 		VM_BUG_ON_PAGE(!page_count(p), p);
 		hpage = compound_head(p);
 	}
@@ -1753,24 +1718,28 @@ int soft_offline_page(struct page *page, int flags)
 			put_hwpoison_page(page);
 		return -EBUSY;
 	}
-	if (!PageHuge(page) && PageTransHuge(hpage)) {
-		lock_page(page);
-		ret = split_huge_page(hpage);
-		unlock_page(page);
-		if (unlikely(ret)) {
-			pr_info("soft offline: %#lx: failed to split THP\n",
-				pfn);
-			if (flags & MF_COUNT_INCREASED)
-				put_hwpoison_page(page);
-			return -EBUSY;
-		}
-	}
 
 	get_online_mems();
-
 	ret = get_any_page(page, pfn, flags);
 	put_online_mems();
+
 	if (ret > 0) { /* for in-use pages */
+		if (!PageHuge(page) && PageTransHuge(hpage)) {
+			lock_page(hpage);
+			ret = split_huge_page(hpage);
+			unlock_page(hpage);
+			if (unlikely(ret || PageTransCompound(page) ||
+					!PageAnon(page))) {
+				pr_info("soft offline: %#lx: failed to split THP\n",
+					pfn);
+				if (flags & MF_COUNT_INCREASED)
+					put_hwpoison_page(hpage);
+				return -EBUSY;
+			}
+			get_hwpoison_page(page);
+			put_hwpoison_page(hpage);
+		}
+
 		if (PageHuge(page))
 			ret = soft_offline_huge_page(page, flags);
 		else