From 89cb0888ca1483ad72648844ddd1b801863a8949 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Fri, 28 Dec 2018 00:39:12 -0800
Subject: [PATCH] mm: migrate: provide buffer_migrate_page_norefs()
Git-commit: 89cb0888ca1483ad72648844ddd1b801863a8949
Patch-mainline: v4.21-rc1
References: bsc#1084216

Provide a variant of buffer_migrate_page() that also checks whether there
are no unexpected references to buffer heads.  This function will then be
safe to use for block device pages.

[akpm@linux-foundation.org: remove EXPORT_SYMBOL(buffer_migrate_page_norefs)]
Link: http://lkml.kernel.org/r/20181211172143.7358-5-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Jan Kara <jack@suse.cz>

---
 include/linux/fs.h |    4 +++
 mm/migrate.c       |   60 ++++++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 57 insertions(+), 7 deletions(-)

--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3109,8 +3109,12 @@ extern int generic_check_addressable(uns
 extern int buffer_migrate_page(struct address_space *,
 				struct page *, struct page *,
 				enum migrate_mode);
+extern int buffer_migrate_page_norefs(struct address_space *,
+				struct page *, struct page *,
+				enum migrate_mode);
 #else
 #define buffer_migrate_page NULL
+#define buffer_migrate_page_norefs NULL
 #endif
 
 extern int setattr_prepare(struct dentry *, struct iattr *);
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -733,13 +733,9 @@ int migrate_page(struct address_space *m
 EXPORT_SYMBOL(migrate_page);
 
 #ifdef CONFIG_BLOCK
-/*
- * Migration function for pages with buffers. This function can only be used
- * if the underlying filesystem guarantees that no other references to "page"
- * exist.
- */
-int buffer_migrate_page(struct address_space *mapping,
-		struct page *newpage, struct page *page, enum migrate_mode mode)
+static int __buffer_migrate_page(struct address_space *mapping,
+		struct page *newpage, struct page *page, enum migrate_mode mode,
+		bool check_refs)
 {
 	struct buffer_head *bh, *head;
 	int rc;
@@ -757,6 +753,33 @@ int buffer_migrate_page(struct address_s
 	if (!buffer_migrate_lock_buffers(head, mode))
 		return -EAGAIN;
 
+	if (check_refs) {
+		bool busy;
+		bool invalidated = false;
+
+recheck_buffers:
+		busy = false;
+		spin_lock(&mapping->private_lock);
+		bh = head;
+		do {
+			if (atomic_read(&bh->b_count)) {
+				busy = true;
+				break;
+			}
+			bh = bh->b_this_page;
+		} while (bh != head);
+		spin_unlock(&mapping->private_lock);
+		if (busy) {
+			if (invalidated) {
+				rc = -EAGAIN;
+				goto unlock_buffers;
+			}
+			invalidate_bh_lrus();
+			invalidated = true;
+			goto recheck_buffers;
+		}
+	}
+
 	rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		goto unlock_buffers;
@@ -793,7 +816,30 @@ unlock_buffers:
 
 	return rc;
 }
+
+/*
+ * Migration function for pages with buffers. This function can only be used
+ * if the underlying filesystem guarantees that no other references to "page"
+ * exist. For example attached buffer heads are accessed only under page lock.
+ */
+int buffer_migrate_page(struct address_space *mapping,
+		struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+	return __buffer_migrate_page(mapping, newpage, page, mode, false);
+}
 EXPORT_SYMBOL(buffer_migrate_page);
+
+/*
+ * Same as above except that this variant is more careful and checks that there
+ * are also no buffer head references. This function is the right one for
+ * mappings where buffer heads are directly looked up and referenced (such as
+ * block device mappings).
+ */
+int buffer_migrate_page_norefs(struct address_space *mapping,
+		struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+	return __buffer_migrate_page(mapping, newpage, page, mode, true);
+}
 #endif
 
 /*