xfs: support in-memory btrees

Adapt the generic btree cursor code to be able to create a btree whose
buffers come from a (presumably in-memory) buftarg with a header block
that's specific to in-memory btrees.  We'll connect this to other parts
of online scrub in the next patches.

Note that in-memory btrees always have a block size matching the system
memory page size for efficiency reasons.  There are also a few things we
need to do to finalize a btree update; that's covered in the next patch.

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

Author: Darrick J. Wong

Documentation/filesystems/xfs/xfs-online-fsck-design.rst

@@ -2270,13 +2270,12 @@ follows:
    pointing to the xfile.
 
 3. Pass the buffer cache target, buffer ops, and other information to
-   ``xfbtree_create`` to write an initial tree header and root block to the
-   xfile.
+   ``xfbtree_init`` to initialize the passed in ``struct xfbtree`` and write an
+   initial root block to the xfile.
    Each btree type should define a wrapper that passes necessary arguments to
    the creation function.
    For example, rmap btrees define ``xfs_rmapbt_mem_create`` to take care of
    all the necessary details for callers.
-   A ``struct xfbtree`` object will be returned.
 
 4. Pass the xfbtree object to the btree cursor creation function for the
    btree type.

fs/xfs/Kconfig

@@ -131,6 +131,9 @@ config XFS_LIVE_HOOKS
 config XFS_MEMORY_BUFS
 	bool
 
+config XFS_BTREE_IN_MEM
+	bool
+
 config XFS_ONLINE_SCRUB
 	bool "XFS online metadata check support"
 	default n
@@ -173,6 +176,7 @@ config XFS_ONLINE_REPAIR
 	bool "XFS online metadata repair support"
 	default n
 	depends on XFS_FS && XFS_ONLINE_SCRUB
+	select XFS_BTREE_IN_MEM
 	help
 	  If you say Y here you will be able to repair metadata on a
 	  mounted XFS filesystem.  This feature is intended to reduce

fs/xfs/Makefile

@@ -138,6 +138,7 @@ endif
 xfs-$(CONFIG_XFS_DRAIN_INTENTS)	+= xfs_drain.o
 xfs-$(CONFIG_XFS_LIVE_HOOKS)	+= xfs_hooks.o
 xfs-$(CONFIG_XFS_MEMORY_BUFS)	+= xfs_buf_mem.o
+xfs-$(CONFIG_XFS_BTREE_IN_MEM)	+= libxfs/xfs_btree_mem.o
 
 # online scrub/repair
 ifeq ($(CONFIG_XFS_ONLINE_SCRUB),y)