HWPOISON: Define a new error_remove_page address space op for async truncation

Truncating metadata pages is not safe right now before
we haven't audited all file systems.

To enable truncation only for data address space define
a new address_space callback error_remove_page.

This is used for memory_failure.c memory error handling.

This can be then set to truncate_inode_page()

This patch just defines the new operation and adds documentation.

Callers and users come in followon patches.

Signed-off-by: Andi Kleen <ak@linux.intel.com>

Documentation/filesystems/vfs.txt

@@ -536,6 +536,7 @@ struct address_space_operations {
 	/* migrate the contents of a page to the specified target */
 	int (*migratepage) (struct page *, struct page *);
 	int (*launder_page) (struct page *);
+	int (*error_remove_page) (struct mapping *mapping, struct page *page);
 };
 
   writepage: called by the VM to write a dirty page to backing store.
@@ -694,6 +695,12 @@ struct address_space_operations {
   	prevent redirtying the page, it is kept locked during the whole
 	operation.
 
+  error_remove_page: normally set to generic_error_remove_page if truncation
+	is ok for this address space. Used for memory failure handling.
+	Setting this implies you deal with pages going away under you,
+	unless you have them locked or reference counts increased.
+
+
 The File Object
 ===============