[PATCH, RFC] check for frozen filesystems in the mmap path

[Eric Sandeen <sandeen@redhat.com>]

Stephen Tweedie mentioned to me a concern that while a filesystem
is frozen, data could be dirtied for it via mmap, thereby using
up enough memory that the unfreeze process may be stuck trying
to allocate memory by writing back mmap-dirty data to the frozen
fs.

Christoph suggested maybe a check_frozen in the mmap path to
prevent this; does the sort of thing below look sane?

signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -1944,6 +1944,7 @@ static int do_wp_page(struct mm_struct *
 		 * read-only shared pages can get COWed by
 		 * get_user_pages(.write=1, .force=1).
 		 */
+		vfs_check_frozen(old_page->mapping->host->i_sb, SB_FREEZE_WRITE);
 		if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
 			struct vm_fault vmf;
 			int tmp;
@@ -2660,6 +2661,7 @@ static int __do_fault(struct mm_struct *
 			 * address space wants to know that the page is about
 			 * to become writable
 			 */
+			vfs_check_frozen(vmf.page->mapping->host->i_sb, SB_FREEZE_WRITE);
 			if (vma->vm_ops->page_mkwrite) {
 				int tmp;