mm: soft-offline: close the race against page allocation A process can be killed with SIGBUS(BUS_MCEERR_AR) when it tries to allocate a page that was just freed on the way of soft-offline. This is undesirable because soft-offline (which is about corrected error) is less aggressive than hard-offline (which is about uncorrected error), and we can make soft-offline fail and keep using the page for good reason like "system is busy." Two main changes of this patch are: - setting migrate type of the target page to MIGRATE_ISOLATE. As done in free_unref_page_commit(), this makes kernel bypass pcplist when freeing the page. So we can assume that the page is in freelist just after put_page() returns, - setting PG_hwpoison on free page under zone->lock which protects freelists, so this allows us to avoid setting PG_hwpoison on a page that is decided to be allocated soon. [[email protected]: tweak set_hwpoison_free_buddy_page() comment] Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Naoya Horiguchi <[email protected]> Reported-by: Xishi Qiu <[email protected]> Tested-by: Mike Kravetz <[email protected]> Cc: Michal Hocko <[email protected]> Cc: <[email protected]> Cc: Mike Kravetz <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 49dc32c..192d0bb 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c

@@ -57,6 +57,7 @@
 #include <linux/mm_inline.h>
 #include <linux/kfifo.h>
 #include <linux/ratelimit.h>
+#include <linux/page-isolation.h>
 #include "internal.h"
 #include "ras/ras_event.h"
 
@@ -1697,6 +1698,7 @@ static int __soft_offline_page(struct page *page, int flags)
 static int soft_offline_in_use_page(struct page *page, int flags)
 {
 	int ret;
+	int mt;
 	struct page *hpage = compound_head(page);
 
 	if (!PageHuge(page) && PageTransHuge(hpage)) {
@@ -1715,23 +1717,37 @@ static int soft_offline_in_use_page(struct page *page, int flags)
 		put_hwpoison_page(hpage);
 	}
 
+	/*
+	 * Setting MIGRATE_ISOLATE here ensures that the page will be linked
+	 * to free list immediately (not via pcplist) when released after
+	 * successful page migration. Otherwise we can't guarantee that the
+	 * page is really free after put_page() returns, so
+	 * set_hwpoison_free_buddy_page() highly likely fails.
+	 */
+	mt = get_pageblock_migratetype(page);
+	set_pageblock_migratetype(page, MIGRATE_ISOLATE);
 	if (PageHuge(page))
 		ret = soft_offline_huge_page(page, flags);
 	else
 		ret = __soft_offline_page(page, flags);
-
+	set_pageblock_migratetype(page, mt);
 	return ret;
 }
 
-static void soft_offline_free_page(struct page *page)
+static int soft_offline_free_page(struct page *page)
 {
 	int rc = 0;
 	struct page *head = compound_head(page);
 
 	if (PageHuge(head))
 		rc = dissolve_free_huge_page(page);
-	if (!rc && !TestSetPageHWPoison(page))
-		num_poisoned_pages_inc();
+	if (!rc) {
+		if (set_hwpoison_free_buddy_page(page))
+			num_poisoned_pages_inc();
+		else
+			rc = -EBUSY;
+	}
+	return rc;
 }
 
 /**
@@ -1775,7 +1791,7 @@ int soft_offline_page(struct page *page, int flags)
 	if (ret > 0)
 		ret = soft_offline_in_use_page(page, flags);
 	else if (ret == 0)
-		soft_offline_free_page(page);
+		ret = soft_offline_free_page(page);
 
 	return ret;
 }

diff --git a/mm/migrate.c b/mm/migrate.c
index 91a9945..d6a2e89 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -1212,7 +1212,7 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
 			 * intentionally. Although it's rather weird,
 			 * it's how HWPoison flag works at the moment.
 			 */
-			if (!test_set_page_hwpoison(page))
+			if (set_hwpoison_free_buddy_page(page))
 				num_poisoned_pages_inc();
 		}
 	} else {

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c677c15..e75865d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -8096,3 +8096,33 @@ bool is_free_buddy_page(struct page *page)
 
 	return order < MAX_ORDER;
 }
+
+#ifdef CONFIG_MEMORY_FAILURE
+/*
+ * Set PG_hwpoison flag if a given page is confirmed to be a free page.  This
+ * test is performed under the zone lock to prevent a race against page
+ * allocation.
+ */
+bool set_hwpoison_free_buddy_page(struct page *page)
+{
+	struct zone *zone = page_zone(page);
+	unsigned long pfn = page_to_pfn(page);
+	unsigned long flags;
+	unsigned int order;
+	bool hwpoisoned = false;
+
+	spin_lock_irqsave(&zone->lock, flags);
+	for (order = 0; order < MAX_ORDER; order++) {
+		struct page *page_head = page - (pfn & ((1 << order) - 1));
+
+		if (PageBuddy(page_head) && page_order(page_head) >= order) {
+			if (!TestSetPageHWPoison(page))
+				hwpoisoned = true;
+			break;
+		}
+	}
+	spin_unlock_irqrestore(&zone->lock, flags);
+
+	return hwpoisoned;
+}
+#endif