xfs: support in-memory buffer cache targets

Allow the buffer cache to target in-memory files by making it possible
to have a buftarg that maps pages from private shmem files.  As the
prevous patch alludes, the in-memory buftarg contains its own cache,
points to a shmem file, and does not point to a block_device.

The next few patches will make it possible to construct an xfs_btree in
pageable memory by using this buftarg.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 30 insertions, 0 deletions

diff --git a/fs/xfs/xfs_buf_mem.h b/fs/xfs/xfs_buf_mem.h
new file mode 100644
index 000000000000..945f4b610998
--- /dev/null
+++ b/fs/xfs/xfs_buf_mem.h
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2023-2024 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#ifndef __XFS_BUF_MEM_H__
+#define __XFS_BUF_MEM_H__
+
+#define XMBUF_BLOCKSIZE			(PAGE_SIZE)
+#define XMBUF_BLOCKSHIFT		(PAGE_SHIFT)
+
+#ifdef CONFIG_XFS_MEMORY_BUFS
+static inline bool xfs_buftarg_is_mem(const struct xfs_buftarg *btp)
+{
+	return btp->bt_bdev == NULL;
+}
+
+int xmbuf_alloc(struct xfs_mount *mp, const char *descr,
+		struct xfs_buftarg **btpp);
+void xmbuf_free(struct xfs_buftarg *btp);
+
+int xmbuf_map_page(struct xfs_buf *bp);
+void xmbuf_unmap_page(struct xfs_buf *bp);
+#else
+# define xfs_buftarg_is_mem(...)	(false)
+# define xmbuf_map_page(...)		(-ENOMEM)
+# define xmbuf_unmap_page(...)		((void)0)
+#endif /* CONFIG_XFS_MEMORY_BUFS */
+
+#endif /* __XFS_BUF_MEM_H__ */