fs: fix fsync() error reporting

There are two convenient ways to report errors to userspace

1) retun error to original syscall for example write(2)
2) mark mapping with error flag and return it on later fsync(2)

Second one is broken if (mapping->nrpages == 0) This is real-life
situation because after error pages are likey to be truncated or
invalidated.

We have to return an error regardless to number of pages in the mapping.

#Original testcase: git@github.com:dmonakhov/xfstests.git
MOUNT_OPTIONS="-b1024"
./check shared/305
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -188,6 +188,17 @@ static int sleep_on_page_killable(void *word)
 	return fatal_signal_pending(current) ? -EINTR : 0;
 }
 
+static int filemap_check_errors(struct address_space *mapping)
+{
+	int ret = 0;
+	/* Check for outstanding write errors */
+	if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
+		ret = -ENOSPC;
+	if (test_and_clear_bit(AS_EIO, &mapping->flags))
+		ret = -EIO;
+	return ret;
+}
+
 /**
  * __filemap_fdatawrite_range - start writeback on mapping dirty pages in range
  * @mapping:	address space structure to write
@@ -269,10 +280,10 @@ int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
 	pgoff_t end = end_byte >> PAGE_CACHE_SHIFT;
 	struct pagevec pvec;
 	int nr_pages;
-	int ret = 0;
+	int ret2, ret = 0;
 
 	if (end_byte < start_byte)
-		return 0;
+		goto out;
 
 	pagevec_init(&pvec, 0);
 	while ((index <= end) &&
@@ -295,12 +306,10 @@ int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-
-	/* Check for outstanding write errors */
-	if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
-		ret = -ENOSPC;
-	if (test_and_clear_bit(AS_EIO, &mapping->flags))
-		ret = -EIO;
+out:
+	ret2 = filemap_check_errors(mapping);
+	if (!ret)
+		ret = ret2;
 
 	return ret;
 }
@@ -341,6 +350,8 @@ int filemap_write_and_wait(struct address_space *mapping)
 			if (!err)
 				err = err2;
 		}
+	} else {
+		err = filemap_check_errors(mapping);
 	}
 	return err;
 }
@@ -372,6 +383,8 @@ int filemap_write_and_wait_range(struct address_space *mapping,
 			if (!err)
 				err = err2;
 		}
+	} else {
+		err = filemap_check_errors(mapping);
 	}
 	return err;
 }