mm: migrate: provide buffer_migrate_page_norefs()

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>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 26a8607..1cda664 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3269,8 +3269,12 @@ extern int generic_check_addressable(unsigned, u64);
 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 *);

diff --git a/mm/migrate.c b/mm/migrate.c
index 8392140..8dd5760 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -743,13 +743,9 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
        return true;
 }
 
-/*
- * 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;
@@ -767,6 +763,33 @@ int buffer_migrate_page(struct address_space *mapping,
        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;
@@ -803,7 +826,30 @@ 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
 
 /*