[PATCH 5/7] xfsdocs: reverse-mapping btree documentation

["Darrick J. Wong" <darrick.wong@oracle.com>]

Add chapters on the operation of the reverse mapping btree and future
things we could do with rmap data.

v2: Add magic number to the table.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 .../allocation_groups.asciidoc                     |   31 +-
 design/XFS_Filesystem_Structure/docinfo.xml        |   17 +
 .../journaling_log.asciidoc                        |  122 ++++++++
 design/XFS_Filesystem_Structure/magic.asciidoc     |    3 
 .../reconstruction.asciidoc                        |   53 +++
 design/XFS_Filesystem_Structure/rmapbt.asciidoc    |  305 ++++++++++++++++++++
 .../xfs_filesystem_structure.asciidoc              |    4 
 7 files changed, 526 insertions(+), 9 deletions(-)
 create mode 100644 design/XFS_Filesystem_Structure/reconstruction.asciidoc
 create mode 100644 design/XFS_Filesystem_Structure/rmapbt.asciidoc


diff --git a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
index 55bbc50..9fcf975 100644
--- a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
+++ b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
@@ -12,6 +12,7 @@ Each AG has the following characteristics:
          * A super block describing overall filesystem info
          * Free space management
          * Inode allocation and tracking
+         * Reverse block-mapping index (optional)
 
 Having multiple AGs allows XFS to handle most operations in parallel without
 degrading performance as the number of concurrent accesses increases.
@@ -379,6 +380,12 @@ it doesn't understand the flag.
 Free inode B+tree.  Each allocation group contains a B+tree to track inode chunks
 containing free inodes.  This is a performance optimization to reduce the time
 required to allocate inodes.
+
+| +XFS_SB_FEAT_RO_COMPAT_RMAPBT+ |
+Reverse mapping B+tree.  Each allocation group contains a B+tree containing
+records mapping AG blocks to their owners.  See the section about
+xref:Reconstruction[reconstruction] for more details.
+
 |=====
 
 *sb_features_incompat*::
@@ -529,9 +536,7 @@ struct xfs_agf {
      __be32              agf_seqno;
      __be32              agf_length;
      __be32              agf_roots[XFS_BTNUM_AGF];
-     __be32              agf_spare0;
      __be32              agf_levels[XFS_BTNUM_AGF];
-     __be32              agf_spare1;
      __be32              agf_flfirst;
      __be32              agf_fllast;
      __be32              agf_flcount;
@@ -541,7 +546,9 @@ struct xfs_agf {
 
      /* version 5 filesystem fields start here */
      uuid_t              agf_uuid;
-     __be64              agf_spare64[16];
+     __be32              agf_rmap_blocks;
+     __be32              __pad;
+     __be64              agf_spare64[15];
 
      /* unlogged fields, written during buffer writeback. */
      __be64              agf_lsn;
@@ -550,9 +557,10 @@ struct xfs_agf {
 };
 ----
 
-The rest of the bytes in the sector are zeroed. +XFS_BTNUM_AGF+ is set to 2:
-index 0 for the free space B+tree indexed by block number; and index 1 for the
-free space B+tree indexed by extent size.
+The rest of the bytes in the sector are zeroed. +XFS_BTNUM_AGF+ is set to 3:
+index 0 for the free space B+tree indexed by block number; index 1 for the free
+space B+tree indexed by extent size; and index 2 for the reverse-mapping
+B+tree.
 
 *agf_magicnum*::
 Specifies the magic number for the AGF sector: ``XAGF'' (0x58414746).
@@ -570,11 +578,13 @@ this could be less than the +sb_agblocks+ value. It is this value that should
 be used to determine the size of the AG.
 
 *agf_roots*::
-Specifies the block number for the root of the two free space B+trees.
+Specifies the block number for the root of the two free space B+trees and the
+reverse-mapping B+tree, if enabled.
 
 *agf_levels*::
-Specifies the level or depth of the two free space B+trees. For a fresh AG, this
-will be one, and the ``roots'' will point to a single leaf of level 0.
+Specifies the level or depth of the two free space B+trees and the
+reverse-mapping B+tree, if enabled.  For a fresh AG, this value will be one,
+and the ``roots'' will point to a single leaf of level 0.
 
 *agf_flfirst*::
 Specifies the index of the first ``free list'' block. Free lists are covered in
@@ -600,6 +610,9 @@ used if the +XFS_SB_VERSION2_LAZYSBCOUNTBIT+ bit is set in +sb_features2+.
 The UUID of this block, which must match either +sb_uuid+ or +sb_meta_uuid+
 depending on which features are set.
 
+*agf_rmap_blocks*::
+The size of the reverse mapping B+tree in this allocation group, in blocks.
+
 *agf_spare64*::
 Empty space in the logged part of the AGF sector, for use for future features.
 
diff --git a/design/XFS_Filesystem_Structure/docinfo.xml b/design/XFS_Filesystem_Structure/docinfo.xml
index cc5596d..44f944a 100644
--- a/design/XFS_Filesystem_Structure/docinfo.xml
+++ b/design/XFS_Filesystem_Structure/docinfo.xml
@@ -122,4 +122,21 @@
 			</simplelist>
 		</revdescription>
 	</revision>
+	<revision>
+		<revnumber>3.141</revnumber>
+		<date>June 2016</date>
+		<author>
+			<firstname>Darrick</firstname>
+			<surname>Wong</surname>
+			<email></email>
+		</author>
+		<revdescription>
+			<simplelist>
+				<member>Document the reverse-mapping btree.</member>
+				<member>Move the b+tree info to a separate chapter.</member>
+				<member>Discuss overlapping interval b+trees.</member>
+				<member>Discuss new log items for atomic updates.</member>
+			</simplelist>
+		</revdescription>
+	</revision>
 </revhistory>
diff --git a/design/XFS_Filesystem_Structure/journaling_log.asciidoc b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
index 67d209f..78ce436 100644
--- a/design/XFS_Filesystem_Structure/journaling_log.asciidoc
+++ b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
@@ -209,6 +209,8 @@ magic number to distinguish themselves.  Buffer data items only appear after
 | +XFS_LI_DQUOT+		| 0x123d        | xref:Quota_Update_Log_Item[Update Quota]
 | +XFS_LI_QUOTAOFF+		| 0x123e        | xref:Quota_Off_Log_Item[Quota Off]
 | +XFS_LI_ICREATE+		| 0x123f        | xref:Inode_Create_Log_Item[Inode Creation]
+| +XFS_LI_RUI+			| 0x1240        | xref:RUI_Log_Item[Reverse Mapping Update Intent]
+| +XFS_LI_RUD+			| 0x1241        | xref:RUD_Log_Item[Reverse Mapping Update Done]
 |=====
 
 [[Log_Transaction_Headers]]
@@ -386,6 +388,126 @@ Variable-length array of extents to be freed.  The array length is given by
 +xfs_extent_32_t+; this can be determined from the log item size (+oh_len+) and
 the number of extents (+efd_nextents+).
 
+[[RUI_Log_Item]]
+=== Reverse Mapping Updates Intent
+
+The next two operation types work together to handle deferred reverse mapping
+updates.  Naturally, the mappings to be updated can be expressed in terms of
+mapping extents:
+
+[source, c]
+----
+struct xfs_map_extent {
+     __uint64_t                me_owner;
+     __uint64_t                me_startblock;
+     __uint64_t                me_startoff;
+     __uint32_t                me_len;
+     __uint32_t                me_flags;
+};
+----
+
+*me_owner*::
+Owner of this reverse mapping.  See the values in the section about
+xref:Reverse_Mapping_Btree[reverse mapping] for more information.
+
+*me_startblock*::
+Filesystem block of this mapping.
+
+*me_startoff*::
+Logical block offset of this mapping.
+
+*me_len*::
+The length of this mapping.
+
+*me_flags*::
+The lower byte of this field is a type code indicating what sort of
+reverse mapping operation we want.  The upper three bytes are flag bits.
+
+.Reverse mapping update log intent types
+[options="header"]
+|=====
+| Value				| Description
+| +XFS_RMAP_EXTENT_MAP+		| Add a reverse mapping for file data.
+| +XFS_RMAP_EXTENT_MAP_SHARED+	| Add a reverse mapping for file data for a file with shared blocks.
+| +XFS_RMAP_EXTENT_UNMAP+	| Remove a reverse mapping for file data.
+| +XFS_RMAP_EXTENT_UNMAP_SHARED+	| Remove a reverse mapping for file data for a file with shared blocks.
+| +XFS_RMAP_EXTENT_CONVERT+	| Convert a reverse mapping for file data between unwritten and normal.
+| +XFS_RMAP_EXTENT_CONVERT_SHARED+	| Convert a reverse mapping for file data between unwritten and normal for a file with shared blocks.
+| +XFS_RMAP_EXTENT_ALLOC+	| Add a reverse mapping for non-file data.
+| +XFS_RMAP_EXTENT_FREE+	| Remove a reverse mapping for non-file data.
+|=====
+
+.Reverse mapping update log intent flags
+[options="header"]
+|=====
+| Value				| Description
+| +XFS_RMAP_EXTENT_ATTR_FORK+	| Extent is for the attribute fork.
+| +XFS_RMAP_EXTENT_BMBT_BLOCK+	| Extent is for a block mapping btree block.
+| +XFS_RMAP_EXTENT_UNWRITTEN+	| Extent is unwritten.
+|=====
+
+The ``rmap update intent'' operation comes first; it tells the log that XFS
+wants to update some reverse mappings.  This record is crucial for correct log
+recovery because it enables us to spread a complex metadata update across
+multiple transactions while ensuring that a crash midway through the complex
+update will be replayed fully during log recovery.
+
+[source, c]
+----
+struct xfs_rui_log_format {
+     __uint16_t                rui_type;
+     __uint16_t                rui_size;
+     __uint32_t                rui_nextents;
+     __uint64_t                rui_id;	
+     struct xfs_map_extent     rui_extents[1];
+};
+----
+
+*rui_type*::
+The signature of an RUI operation, 0x1240.  This value is in host-endian order,
+not big-endian like the rest of XFS.
+
+*rui_size*::
+Size of this log item.  Should be 1.
+
+*rui_nextents*::
+Number of reverse mappings.
+
+*rui_id*::
+A 64-bit number that binds the corresponding RUD log item to this RUI log item.
+
+*rui_extents*::
+Variable-length array of reverse mappings to update.
+
+[[RUD_Log_Item]]
+=== Completion of Reverse Mapping Updates
+
+The ``reverse mapping update done'' operation complements the ``reverse mapping
+update intent'' operation.  This second operation indicates that the update
+actually happened, so that log recovery needn't replay the update.  The RUD and
+the actual updates are typically found in a new transaction following the
+transaction in which the RUI was logged.
+
+[source, c]
+----
+struct xfs_rud_log_format {
+      __uint16_t               rud_type;
+      __uint16_t               rud_size;
+      __uint32_t               __pad;
+      __uint64_t               rud_rui_id;
+};
+----
+
+*rud_type*::
+The signature of an RUD operation, 0x1241.  This value is in host-endian order,
+not big-endian like the rest of XFS.
+
+*rud_size*::
+Size of this log item.  Should be 1.
+
+*rud_rui_id*::
+A 64-bit number that binds the corresponding RUI log item to this RUD log item.
+
 [[Inode_Log_Item]]
 === Inode Updates
 
diff --git a/design/XFS_Filesystem_Structure/magic.asciidoc b/design/XFS_Filesystem_Structure/magic.asciidoc
index 301cfa0..10fd15f 100644
--- a/design/XFS_Filesystem_Structure/magic.asciidoc
+++ b/design/XFS_Filesystem_Structure/magic.asciidoc
@@ -44,6 +44,7 @@ relevant chapters.  Magic numbers tend to have consistent locations:
 | +XFS_ATTR_LEAF_MAGIC+		| 0xfbee	|     	| xref:Leaf_Attributes[Leaf Attribute]
 | +XFS_ATTR3_LEAF_MAGIC+	| 0x3bee	|     	| xref:Leaf_Attributes[Leaf Attribute], v5 only
 | +XFS_ATTR3_RMT_MAGIC+		| 0x5841524d	| XARM	| xref:Remote_Values[Remote Attribute Value], v5 only
+| +XFS_RMAP_CRC_MAGIC+		| 0x524d4233	| RMB3	| xref:Reverse_Mapping_Btree[Reverse Mapping B+tree], v5 only
 |=====
 
 The magic numbers for log items are at offset zero in each log item, but items
@@ -61,6 +62,8 @@ are not aligned to blocks.
 | +XFS_LI_DQUOT+		| 0x123d        |       | xref:Quota_Update_Log_Item[Update Quota Log Item]
 | +XFS_LI_QUOTAOFF+		| 0x123e        |       | xref:Quota_Off_Log_Item[Quota Off Log Item]
 | +XFS_LI_ICREATE+		| 0x123f        |       | xref:Inode_Create_Log_Item[Inode Creation Log Item]
+| +XFS_LI_RUI+			| 0x1240        |       | xref:RUI_Log_Item[Reverse Mapping Update Intent]
+| +XFS_LI_RUD+			| 0x1241        |       | xref:RUD_Log_Item[Reverse Mapping Update Done]
 |=====
 
 = Theoretical Limits
diff --git a/design/XFS_Filesystem_Structure/reconstruction.asciidoc b/design/XFS_Filesystem_Structure/reconstruction.asciidoc
new file mode 100644
index 0000000..f172e0f
--- /dev/null
+++ b/design/XFS_Filesystem_Structure/reconstruction.asciidoc
@@ -0,0 +1,53 @@
+[[Reconstruction]]
+= Metadata Reconstruction
+
+[NOTE]
+This is a theoretical discussion of how reconstruction could work; none of this
+is implemented as of 2015.
+
+A simple UNIX filesystem can be thought of in terms of a directed acyclic graph.
+To a first approximation, there exists a root directory node, which points to
+other nodes.  Those other nodes can themselves be directories or they can be
+files.  Each file, in turn, points to data blocks.
+
+XFS adds a few more details to this picture:
+
+* The real root(s) of an XFS filesystem are the allocation group headers
+(superblock, AGF, AGI, AGFL).
+* Each allocation group’s headers point to various per-AG B+trees (free space,
+inode, free inodes, free list, etc.)
+* The free space B+trees point to unused extents;
+* The inode B+trees point to blocks containing inode chunks;
+* All superblocks point to the root directory and the log;
+* Hardlinks mean that multiple directories can point to a single file node;
+* File data block pointers are indexed by file offset;
+* Files and directories can have a second collection of pointers to data blocks
+which contain extended attributes;
+* Large directories require multiple data blocks to store all the subpointers;
+* Still larger directories use high-offset data blocks to store a B+tree of
+hashes to directory entries;
+* Large extended attribute forks similarly use high-offset data blocks to store
+a B+tree of hashes to attribute keys; and
+* Symbolic links can point to data blocks.
+
+The beauty of this massive graph structure is that under normal circumstances,
+everything known to the filesystem is discoverable (access controls
+notwithstanding) from the root.  The major weakness of this structure of course
+is that breaking a edge in the graph can render entire subtrees inaccessible.
++xfs_repair+ “recovers” from broken directories by scanning for unlinked inodes
+and connecting them to +/lost+found+, but this isn’t sufficiently general to
+recover from breaks in other parts of the graph structure.  Wouldn’t it be
+useful to have back pointers as a secondary data structure?  The current repair
+strategy is to reconstruct whatever can be rebuilt, but to scrap anything that
+doesn't check out.
+
+The xref:Reverse_Mapping_Btree[reverse-mapping B+tree] fills in part of the
+puzzle.  Since it contains copies of every entry in each inode’s data and
+attribute forks, we can fix a corrupted block map with these records.
+Furthermore, if the inode B+trees become corrupt, it is possible to visit all
+inode chunks using the reverse-mapping data.  Should XFS ever gain the ability
+to store parent directory information in each inode, it also becomes possible
+to resurrect damaged directory trees, which should reduce the complaints about
+inodes ending up in +/lost+found+.  Everything else in the per-AG primary
+metadata can already be reconstructed via +xfs_repair+.  Hopefully,
+reconstruction will not turn out to be a fool's errand.
diff --git a/design/XFS_Filesystem_Structure/rmapbt.asciidoc b/design/XFS_Filesystem_Structure/rmapbt.asciidoc
new file mode 100644
index 0000000..a8a210b
--- /dev/null
+++ b/design/XFS_Filesystem_Structure/rmapbt.asciidoc
@@ -0,0 +1,305 @@
+[[Reverse_Mapping_Btree]]
+== Reverse-Mapping B+tree
+
+[NOTE]
+This data structure is under construction!  Details may change.
+
+If the feature is enabled, each allocation group has its own reverse
+block-mapping B+tree, which grows in the free space like the free space
+B+trees.  As mentioned in the chapter about
+xref:Reconstruction[reconstruction], this data structure is another piece of
+the puzzle necessary to reconstruct the data or attribute fork of a file from
+reverse-mapping records; we can also use it to double-check allocations to
+ensure that we are not accidentally cross-linking blocks, which can cause
+severe damage to the filesystem.
+
+This B+tree is only present if the +XFS_SB_FEAT_RO_COMPAT_RMAPBT+
+feature is enabled.  The feature requires a version 5 filesystem.
+
+Each record in the reverse-mapping B+tree has the following structure:
+
+[source, c]
+----
+struct xfs_rmap_rec {
+     __be32                     rm_startblock;
+     __be32                     rm_blockcount;
+     __be64                     rm_owner;
+     __be64                     rm_fork:1;
+     __be64                     rm_bmbt:1;
+     __be64                     rm_unwritten:1;
+     __be64                     rm_unused:7;
+     __be64                     rm_offset:54;
+};
+----
+
+*rm_startblock*::
+AG block number of this record.
+
+*rm_blockcount*::
+The length of this extent.
+
+*rm_owner*::
+A 64-bit number describing the owner of this extent.  This is typically the
+absolute inode number, but can also correspond to one of the following:
+
+.Special owner values
+[options="header"]
+|=====
+| Value				| Description
+| +XFS_RMAP_OWN_NULL+           | No owner.  This should never appear on disk.
+| +XFS_RMAP_OWN_UNKNOWN+        | Unknown owner; for EFI recovery.  This should never appear on disk.
+| +XFS_RMAP_OWN_FS+             | Allocation group headers
+| +XFS_RMAP_OWN_LOG+            | XFS log blocks
+| +XFS_RMAP_OWN_AG+             | Per-allocation group B+tree blocks.  This means free space B+tree blocks, blocks on the freelist, and reverse-mapping B+tree blocks.
+| +XFS_RMAP_OWN_INOBT+          | Per-allocation group inode B+tree blocks.  This includes free inode B+tree blocks.
+| +XFS_RMAP_OWN_INODES+         | Inode chunks
+|=====
+
+*rm_fork*::
+If +rm_owner+ describes an inode, this can be 1 if this record is for an
+attribute fork.
+
+*rm_bmbt*::
+If +rm_owner+ describes an inode, this can be 1 to signify that this record is
+for a block map B+tree block.  In this case, +rm_offset+ has no meaning.
+
+*rm_unwritten*::
+A flag indicating that the extent is unwritten.  This corresponds to the flag in
+the xref:Data_Extents[extent record] format which means +XFS_EXT_UNWRITTEN+.
+
+*rm_offset*::
+The 54-bit logical file block offset, if +rm_owner+ describes an inode.
+Meaningless otherwise.
+
+[NOTE]
+The single-bit flag values +rm_unwritten+, +rm_fork+, and +rm_bmbt+ are packed
+into the larger fields in the C structure definition.
+
+The key has the following structure:
+
+[source, c]
+----
+struct xfs_rmap_key {
+     __be32                     rm_startblock;
+     __be64                     rm_owner;
+     __be64                     rm_fork:1;
+     __be64                     rm_bmbt:1;
+     __be64                     rm_reserved:1;
+     __be64                     rm_unused:7;
+     __be64                     rm_offset:54;
+};
+----
+
+For the reverse-mapping B+tree on a filesystem that supports sharing of file
+data blocks, the key definition is larger than the usual AG block number.  On a
+classic XFS filesystem, each block has only one owner, which means that
++rm_startblock+ is sufficient to uniquely identify each record.  However,
+shared block support (reflink) on XFS breaks that assumption; now filesystem
+blocks can be linked to any logical block offset of any file inode.  Therefore,
+the key must include the owner and offset information to preserve the 1 to 1
+relation between key and record.
+
+* As the reference counting is AG relative, all the block numbers are only
+32-bits.
+* The +bb_magic+ value is "RMB3" (0x524d4233).
+* The +xfs_btree_sblock_t+ header is used for intermediate B+tree node as well
+as the leaves.
+* Each pointer is associated with two keys.  The first of these is the "low
+key", which is the key of the smallest record accessible through the pointer.
+This low key has the same meaning as the key in all other btrees.  The second
+key is the high key, which is the maximum of the largest key that can be used
+to access a given record underneath the pointer.  Recall that each record
+in the reverse mapping b+tree describes an interval of physical blocks mapped
+to an interval of logical file block offsets; therefore, it makes sense that
+a range of keys can be used to find to a record.
+
+=== xfs_db rmapbt Example
+
+This example shows a reverse-mapping B+tree from a freshly populated root
+filesystem:
+
+----
+xfs_db> agf 0
+xfs_db> addr rmaproot
+xfs_db> p
+magic = 0x524d4233
+level = 1
+numrecs = 43
+leftsib = null
+rightsib = null
+bno = 56
+lsn = 0x3000004c8
+uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
+owner = 0
+crc = 0x7cf8be6f (correct)
+keys[1-43] = [startblock,owner,offset]
+keys[1-43] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,
+	     offset_hi,attrfork_hi,bmbtblock_hi]
+        1:[0,-3,0,0,0,351,4418,66,0,0]
+        2:[417,285,0,0,0,827,4419,2,0,0]
+        3:[829,499,0,0,0,2352,573,55,0,0]
+        4:[1292,710,0,0,0,32168,262923,47,0,0]
+        5:[32215,-5,0,0,0,34655,2365,3411,0,0]
+        6:[34083,1161,0,0,0,34895,265220,1,0,1]
+        7:[34896,256191,0,0,0,36522,-9,0,0,0]
+        ...
+        41:[50998,326734,0,0,0,51430,-5,0,0,0]
+        42:[51431,327010,0,0,0,51600,325722,11,0,0]
+        43:[51611,327112,0,0,0,94063,23522,28375272,0,0]
+ptrs[1-43] = 1:5 2:6 3:8 4:9 5:10 6:11 7:418 ... 41:46377 42:48784 43:49522
+----
+
+We arbitrarily pick pointer 17 to traverse downwards:
+
+----
+xfs_db> addr ptrs[17]
+xfs_db> p
+magic = 0x524d4233
+level = 0
+numrecs = 168
+leftsib = 36284
+rightsib = 37617
+bno = 294760
+lsn = 0x200002761
+uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
+owner = 0
+crc = 0x2dad3fbe (correct)
+recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+        1:[40326,1,259615,0,0,0,0] 2:[40327,1,-5,0,0,0,0]
+        3:[40328,2,259618,0,0,0,0] 4:[40330,1,259619,0,0,0,0]
+        ...
+        127:[40540,1,324266,0,0,0,0] 128:[40541,1,324266,8388608,0,0,0]
+        129:[40542,2,324266,1,0,0,0] 130:[40544,32,-7,0,0,0,0]
+----
+
+Several interesting things pop out here.  The first record shows that inode
+259,615 has mapped AG block 40,326 at offset 0.  We confirm this by looking at
+the block map for that inode:
+
+----
+xfs_db> inode 259615
+xfs_db> bmap
+data offset 0 startblock 40326 (0/40326) count 1 flag 0
+----
+
+Next, notice records 127 and 128, which describe neighboring AG blocks that are
+mapped to non-contiguous logical blocks in inode 324,266.  Given the logical
+offset of 8,388,608 we surmise that this is a leaf directory, but let us
+confirm:
+
+----
+xfs_db> inode 324266
+xfs_db> p core.mode
+core.mode = 040755
+xfs_db> bmap
+data offset 0 startblock 40540 (0/40540) count 1 flag 0
+data offset 1 startblock 40542 (0/40542) count 2 flag 0
+data offset 3 startblock 40576 (0/40576) count 1 flag 0
+data offset 8388608 startblock 40541 (0/40541) count 1 flag 0
+xfs_db> p core.mode
+core.mode = 0100644
+xfs_db> dblock 0
+xfs_db> p dhdr.hdr.magic
+dhdr.hdr.magic = 0x58444433
+xfs_db> dblock 8388608
+xfs_db> p lhdr.info.hdr.magic
+lhdr.info.hdr.magic = 0x3df1
+----
+
+Indeed, this inode 324,266 appears to be a leaf directory, as it has regular
+directory data blocks at low offsets, and a single leaf block.
+
+Notice further the two reverse-mapping records with negative owners.  An owner
+of -7 corresponds to +XFS_RMAP_OWN_INODES+, which is an inode chunk, and an
+owner code of -5 corresponds to +XFS_RMAP_OWN_AG+, which covers free space
+B+trees and free space.  Let's see if block 40,544 is part of an inode chunk:
+
+----
+xfs_db> blockget
+xfs_db> fsblock 40544
+xfs_db> blockuse
+block 40544 (0/40544) type inode
+xfs_db> stack
+1:
+        byte offset 166068224, length 4096
+        buffer block 324352 (fsbno 40544), 8 bbs
+        inode 324266, dir inode 324266, type data
+xfs_db> type inode
+xfs_db> p
+core.magic = 0x494e
+----
+
+Our suspicions are confirmed.  Let's also see if 40,327 is part of a free space
+tree:
+
+----
+xfs_db> fsblock 40327
+xfs_db> blockuse
+block 40327 (0/40327) type btrmap
+xfs_db> type rmapbt
+xfs_db> p
+magic = 0x524d4233
+----
+
+As you can see, the reverse block-mapping B+tree is an important secondary
+metadata structure, which can be used to reconstruct damaged primary metadata.
+Now let's look at an extend rmap btree:
+
+----
+xfs_db> agf 0
+xfs_db> addr rmaproot
+xfs_db> p
+magic = 0x34524d42
+level = 1
+numrecs = 5
+leftsib = null
+rightsib = null
+bno = 6368
+lsn = 0x100000d1b
+uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
+owner = 0
+crc = 0x8d4ace05 (correct)
+keys[1-5] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,offset_hi,attrfork_hi,bmbtblock_hi]
+1:[0,-3,0,0,0,705,132,681,0,0]
+2:[24,5761,0,0,0,548,5761,524,0,0]
+3:[24,5929,0,0,0,380,5929,356,0,0]
+4:[24,6097,0,0,0,212,6097,188,0,0]
+5:[24,6277,0,0,0,807,-7,0,0,0]
+ptrs[1-5] = 1:5 2:771 3:9 4:10 5:11
+----
+
+The second pointer stores both the low key [24,5761,0,0,0] and the high key
+[548,5761,524,0,0], which means that we can expect block 771 to contain records
+starting at physical block 24, inode 5761, offset zero; and that one of the
+records can be used to find a reverse mapping for physical block 548, inode
+5761, and offset 524:
+
+----
+xfs_db> addr ptrs[2]
+xfs_db> p
+magic = 0x34524d42
+level = 0
+numrecs = 168
+leftsib = 5
+rightsib = 9
+bno = 6168
+lsn = 0x100000d1b
+uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
+owner = 0
+crc = 0xd58eff0e (correct)
+recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
+1:[24,525,5761,0,0,0,0]
+2:[24,524,5762,0,0,0,0]
+3:[24,523,5763,0,0,0,0]
+...
+166:[24,360,5926,0,0,0,0]
+167:[24,359,5927,0,0,0,0]
+168:[24,358,5928,0,0,0,0]
+----
+
+Observe that the first record in the block starts at physical block 24, inode
+5761, offset zero, just as we expected.  Note that this first record is also
+indexed by the highest key as provided in the node block; physical block 548,
+inode 5761, offset 524 is the very last block mapped by this record.  Furthermore,
+note that record 168, despite being the last record in this block, has a lower
+maximum key (physical block 382, inode 5928, offset 23) than the first record.
diff --git a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
index 62502b3..1b8658d 100644
--- a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
+++ b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
@@ -48,6 +48,8 @@ include::overview.asciidoc[]
 
 include::metadata_integrity.asciidoc[]
 
+include::reconstruction.asciidoc[]
+
 include::common_types.asciidoc[]
 
 include::magic.asciidoc[]
@@ -66,6 +68,8 @@ include::btrees.asciidoc[]
 
 include::allocation_groups.asciidoc[]
 
+include::rmapbt.asciidoc[]
+
 include::journaling_log.asciidoc[]
 
 include::internal_inodes.asciidoc[]

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