xfs: convert big array and blob array to use memfd backendrepair-part-one_2019-02-04

There are several problems with the initial implementations of the big
array and the blob array data structures.  First, using linked lists
imposes a two-pointer overhead on every record stored.  For blobs this
isn't serious, but for fixed-size records this increases memory
requirements by 40-60%.  Second, we're using kernel memory to store the
intermediate records.  Kernel memory cannot be paged out, which means we
run the risk of OOMing the machine when we run out of physical memory.

Therefore, replace the linked lists in both structures with memfd files.
Random access becomes much easier, memory overhead drops to a negligible
amount, and because memfd pages can be swapped, we have considerably
more flexibility for memory use.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/fs/xfs/scrub/xfile.h b/fs/xfs/scrub/xfile.h
new file mode 100644
index 000000000000..fa6793b2344f
--- /dev/null
+++ b/fs/xfs/scrub/xfile.h
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2018 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ */
+#ifndef __XFS_SCRUB_XFILE_H__
+#define __XFS_SCRUB_XFILE_H__
+
+struct file *xfile_create(const char *description);
+void xfile_destroy(struct file *filp);
+
+/* read or write? */
+#define XFILE_IO_READ		(0)
+#define XFILE_IO_WRITE		(1)
+#define XFILE_IO_MASK		(1 << 0)
+int xfile_io(struct file *filp, unsigned int cmd_flags, loff_t *pos,
+		void *ptr, size_t count);
+
+void xfile_discard(struct file *filp, loff_t start, loff_t end);
+
+#endif /* __XFS_SCRUB_XFILE_H__ */