[PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism of raw error pages

[Naoya Horiguchi <nao.horiguchi@gmail.com>]

From: Naoya Horiguchi <naoya.horiguchi@nec.com>

When handling memory error on a hugetlb page, the error handler tries to
dissolve and turn it into 4kB pages.  If it's successfully dissolved,
PageHWPoison flag is moved to the raw error page, so that's all right.
However, dissolve sometimes fails, then the error page is left as
hwpoisoned hugepage. It's useful if we can retry to dissolve it to save
healthy pages, but that's not possible now because the information about
where the raw error pages is lost.

Use the private field of a few tail pages to keep that information.  The
code path of shrinking hugepage pool uses this info to try delayed dissolve.
In order to remember multiple errors in a hugepage, a singly-linked list
originated from SUBPAGE_INDEX_HWPOISON-th tail page is constructed.  Only
simple operations (adding an entry or clearing all) are required and the
list is assumed not to be very long, so this simple data structure should
be enough.

If we failed to save raw error info, the hwpoison hugepage has errors on
unknown subpage, then this new saving mechanism does not work any more,
so disable saving new raw error info and freeing hwpoison hugepages.

Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
---
v1 -> v2:
- support hwpoison hugepage with multiple errors,
- moved the new interface functions to mm/memory-failure.c,
- define additional subpage index SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
- stop freeing/dissolving hwpoison hugepages with unreliable raw error info,
- drop hugetlb_clear_page_hwpoison() in dissolve_free_huge_page() because
  that's done in update_and_free_page(),
- move setting/clearing PG_hwpoison flag to the new interfaces,
- checking already hwpoisoned or not on a subpage basis.

ChangeLog since previous post on 4/27:
- fixed typo in patch description (by Miaohe)
- fixed config value in #ifdef statement (by Miaohe)
- added sentences about "multiple hwpoison pages" scenario in patch
  description

Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
---
 include/linux/hugetlb.h | 13 ++++++
 mm/hugetlb.c            | 39 +++++++++--------
 mm/memory-failure.c     | 95 ++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 125 insertions(+), 22 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index e4cff27d1198..ac13c2022517 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -42,6 +42,10 @@ enum {
 	SUBPAGE_INDEX_CGROUP,		/* reuse page->private */
 	SUBPAGE_INDEX_CGROUP_RSVD,	/* reuse page->private */
 	__MAX_CGROUP_SUBPAGE_INDEX = SUBPAGE_INDEX_CGROUP_RSVD,
+#endif
+#ifdef CONFIG_MEMORY_FAILURE
+	SUBPAGE_INDEX_HWPOISON,
+	SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
 #endif
 	__NR_USED_SUBPAGE,
 };
@@ -798,6 +802,15 @@ extern int dissolve_free_huge_page(struct page *page);
 extern int dissolve_free_huge_pages(unsigned long start_pfn,
 				    unsigned long end_pfn);
 
+#ifdef CONFIG_MEMORY_FAILURE
+extern int hugetlb_clear_page_hwpoison(struct page *hpage);
+#else
+static inline int hugetlb_clear_page_hwpoison(struct page *hpage)
+{
+	return 0;
+}
+#endif
+
 #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
 #ifndef arch_hugetlb_migration_supported
 static inline bool arch_hugetlb_migration_supported(struct hstate *h)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index b7ae5f73f3b2..19ef427aa1e8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1541,17 +1541,15 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return;
 
-	if (hugetlb_vmemmap_alloc(h, page)) {
-		spin_lock_irq(&hugetlb_lock);
-		/*
-		 * If we cannot allocate vmemmap pages, just refuse to free the
-		 * page and put the page back on the hugetlb free list and treat
-		 * as a surplus page.
-		 */
-		add_hugetlb_page(h, page, true);
-		spin_unlock_irq(&hugetlb_lock);
-		return;
-	}
+	if (hugetlb_vmemmap_alloc(h, page))
+		goto fail;
+
+	/*
+	 * Move PageHWPoison flag from head page to the raw error pages,
+	 * which makes any healthy subpages reusable.
+	 */
+	if (unlikely(PageHWPoison(page) && hugetlb_clear_page_hwpoison(page)))
+		goto fail;
 
 	for (i = 0; i < pages_per_huge_page(h);
 	     i++, subpage = mem_map_next(subpage, page, i)) {
@@ -1572,6 +1570,16 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
 	} else {
 		__free_pages(page, huge_page_order(h));
 	}
+	return;
+fail:
+	spin_lock_irq(&hugetlb_lock);
+	/*
+	 * If we cannot allocate vmemmap pages or cannot identify raw hwpoison
+	 * subpages reliably, just refuse to free the page and put the page
+	 * back on the hugetlb free list and treat as a surplus page.
+	 */
+	add_hugetlb_page(h, page, true);
+	spin_unlock_irq(&hugetlb_lock);
 }
 
 /*
@@ -2115,15 +2123,6 @@ int dissolve_free_huge_page(struct page *page)
 		 */
 		rc = hugetlb_vmemmap_alloc(h, head);
 		if (!rc) {
-			/*
-			 * Move PageHWPoison flag from head page to the raw
-			 * error page, which makes any subpages rather than
-			 * the error page reusable.
-			 */
-			if (PageHWPoison(head) && page != head) {
-				SetPageHWPoison(page);
-				ClearPageHWPoison(head);
-			}
 			update_and_free_page(h, head, false);
 		} else {
 			spin_lock_irq(&hugetlb_lock);
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index fb3feb1f363e..af571fa6f2af 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1499,6 +1499,97 @@ static int try_to_split_thp_page(struct page *page, const char *msg)
 }
 
 #ifdef CONFIG_HUGETLB_PAGE
+/*
+ * Struct raw_hwp_page represents information about "raw error page",
+ * constructing singly linked list originated from ->private field of
+ * SUBPAGE_INDEX_HWPOISON-th tail page.
+ */
+struct raw_hwp_page {
+	struct llist_node node;
+	struct page *page;
+};
+
+static inline struct llist_head *raw_hwp_list_head(struct page *hpage)
+{
+	return (struct llist_head *)&page_private(hpage + SUBPAGE_INDEX_HWPOISON);
+}
+
+static inline int raw_hwp_unreliable(struct page *hpage)
+{
+	return page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE);
+}
+
+static inline void set_raw_hwp_unreliable(struct page *hpage)
+{
+	set_page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE, 1);
+}
+
+/*
+ * about race consideration
+ */
+static inline int hugetlb_set_page_hwpoison(struct page *hpage,
+					struct page *page)
+{
+	struct llist_head *head;
+	struct raw_hwp_page *raw_hwp;
+	struct llist_node *t, *tnode;
+	int ret;
+
+	/*
+	 * Once the hwpoison hugepage has lost reliable raw error info,
+	 * there is little mean to keep additional error info precisely,
+	 * so skip to add additional raw error info.
+	 */
+	if (raw_hwp_unreliable(hpage))
+		return -EHWPOISON;
+	head = raw_hwp_list_head(hpage);
+	llist_for_each_safe(tnode, t, head->first) {
+		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
+
+		if (p->page == page)
+			return -EHWPOISON;
+	}
+
+	ret = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
+	/* the first error event will be counted in action_result(). */
+	if (ret)
+		num_poisoned_pages_inc();
+
+	raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_KERNEL);
+	if (raw_hwp) {
+		raw_hwp->page = page;
+		llist_add(&raw_hwp->node, head);
+	} else {
+		/*
+		 * Failed to save raw error info.  We no longer trace all
+		 * hwpoisoned subpages, and we need refuse to free/dissolve
+		 * this hwpoisoned hugepage.
+		 */
+		set_raw_hwp_unreliable(hpage);
+		return ret;
+	}
+	return ret;
+}
+
+inline int hugetlb_clear_page_hwpoison(struct page *hpage)
+{
+	struct llist_head *head;
+	struct llist_node *t, *tnode;
+
+	if (raw_hwp_unreliable(hpage))
+		return -EBUSY;
+	ClearPageHWPoison(hpage);
+	head = raw_hwp_list_head(hpage);
+	llist_for_each_safe(tnode, t, head->first) {
+		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
+
+		SetPageHWPoison(p->page);
+		kfree(p);
+	}
+	llist_del_all(head);
+	return 0;
+}
+
 /*
  * Called from hugetlb code with hugetlb_lock held.
  *
@@ -1533,7 +1624,7 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
 		goto out;
 	}
 
-	if (TestSetPageHWPoison(head)) {
+	if (hugetlb_set_page_hwpoison(head, page)) {
 		ret = -EHWPOISON;
 		goto out;
 	}
@@ -1585,7 +1676,7 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
 	lock_page(head);
 
 	if (hwpoison_filter(p)) {
-		ClearPageHWPoison(head);
+		hugetlb_clear_page_hwpoison(head);
 		res = -EOPNOTSUPP;
 		goto out;
 	}
-- 
2.25.1