vfs: Block mmapped writes while the fs is frozen

We should not allow file modification via mmap while the filesystem is
frozen. So block in block_page_mkwrite() while the filesystem is frozen.
We cannot do the blocking wait in __block_page_mkwrite() since e.g. ext4
will want to call that function with transaction started in some cases
and that would deadlock. But we can at least do the non-blocking reliable
check in __block_page_mkwrite() which is the hardest part anyway.

We have to check for frozen filesystem with the page marked dirty and under
page lock with which we then return from ->page_mkwrite(). Only that way we
cannot race with writeback done by freezing code - either we mark the page
dirty after the writeback has started, see freezing in progress and block, or
writeback will wait for our page lock which is released only when the fault is
done and then writeback will writeout and writeprotect the page again.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/buffer.c

@@ -2331,6 +2331,9 @@ EXPORT_SYMBOL(block_commit_write);
  * page lock we can determine safely if the page is beyond EOF. If it is not
  * beyond EOF, then the page is guaranteed safe against truncation until we
  * unlock the page.
+ *
+ * Direct callers of this function should call vfs_check_frozen() so that page
+ * fault does not busyloop until the fs is thawed.
  */
 int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 			 get_block_t get_block)
@@ -2362,6 +2365,18 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 
 	if (unlikely(ret < 0))
 		goto out_unlock;
+	/*
+	 * Freezing in progress? We check after the page is marked dirty and
+	 * with page lock held so if the test here fails, we are sure freezing
+	 * code will wait during syncing until the page fault is done - at that
+	 * point page will be dirty and unlocked so freezing code will write it
+	 * and writeprotect it again.
+	 */
+	set_page_dirty(page);
+	if (inode->i_sb->s_frozen != SB_UNFROZEN) {
+		ret = -EAGAIN;
+		goto out_unlock;
+	}
 	return 0;
 out_unlock:
 	unlock_page(page);
@@ -2372,8 +2387,15 @@ EXPORT_SYMBOL(__block_page_mkwrite);
 int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 		   get_block_t get_block)
 {
-	int ret = __block_page_mkwrite(vma, vmf, get_block);
+	int ret;
+	struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;
 
+	/*
+	 * This check is racy but catches the common case. The check in
+	 * __block_page_mkwrite() is reliable.
+	 */
+	vfs_check_frozen(sb, SB_FREEZE_WRITE);
+	ret = __block_page_mkwrite(vma, vmf, get_block);
 	return block_page_mkwrite_return(ret);
 }
 EXPORT_SYMBOL(block_page_mkwrite);

include/linux/buffer_head.h

@@ -230,6 +230,8 @@ static inline int block_page_mkwrite_return(int err)
 		return VM_FAULT_NOPAGE;
 	if (err == -ENOMEM)
 		return VM_FAULT_OOM;
+	if (err == -EAGAIN)
+		return VM_FAULT_RETRY;
 	/* -ENOSPC, -EDQUOT, -EIO ... */
 	return VM_FAULT_SIGBUS;
 }