Move files around [ci-skip]

15 files changed, 1536 insertions, 0 deletions

diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go b/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go
new file mode 100644
index 0000000..20772ba
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go
@@ -0,0 +1,113 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"errors"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/diskio"
+)
+
+type kmpPattern[K ~int64 | ~int, V comparable] interface {
+	PatLen() K
+	// Get the value at 'pos' in the sequence.  Positions start at
+	// 0 and increment naturally.  It is invalid to call Get(pos)
+	// with a pos that is >= Len().  If there is a gap/wildcard at
+	// pos, ok is false.
+	PatGet(pos K) (v V, ok bool)
+}
+
+func kmpSelfEq[K ~int64 | ~int, V comparable](s kmpPattern[K, V], aI K, bI K) bool {
+	aV, aOK := s.PatGet(aI)
+	bV, bOK := s.PatGet(bI)
+	if !aOK || !bOK {
+		return true
+	}
+	return aV == bV
+}
+
+// buildKMPTable takes the string 'substr', and returns a table such
+// that 'table[matchLen-1]' is the largest value 'val' for which 'val < matchLen' and
+// 'substr[:val] == substr[matchLen-val:matchLen]'.
+func buildKMPTable[K ~int64 | ~int, V comparable](substr kmpPattern[K, V]) []K {
+	substrLen := substr.PatLen()
+
+	table := make([]K, substrLen)
+	for j := K(0); j < substrLen; j++ {
+		if j == 0 {
+			// First entry must always be 0 (in order to
+			// satisfy 'val < matchLen').
+			continue
+		}
+		val := table[j-1]
+		// not a match; go back
+		for val > 0 && !kmpSelfEq(substr, j, val) {
+			val = table[val-1]
+		}
+		// is a match; go forward
+		if kmpSelfEq(substr, val, j) {
+			val++
+		}
+		table[j] = val
+	}
+	return table
+}
+
+func kmpEq[K ~int64 | ~int, V comparable](aV V, bS kmpPattern[K, V], bI K) bool {
+	bV, ok := bS.PatGet(bI)
+	if !ok {
+		return true
+	}
+	return aV == bV
+}
+
+// indexAll returns the starting-position of all possibly-overlapping
+// occurrences of 'substr' in the 'str' sequence.
+//
+// Will hop around in 'substr', but will only get the natural sequence
+// [0...) in order from 'str'.
+//
+// Will panic if the length of 'substr' is 0.
+//
+// The 'substr' may include wildcard characters by returning
+// ErrWildcard for a position.
+//
+// Uses the Knuth-Morris-Pratt algorithm.
+func indexAll[K ~int64 | ~int, V comparable](str diskio.Sequence[K, V], substr kmpPattern[K, V]) []K {
+	table := buildKMPTable(substr)
+	substrLen := K(len(table))
+	if substrLen == 0 {
+		panic(errors.New("rebuildmappings.IndexAll: empty substring"))
+	}
+
+	var matches []K
+	var curMatchBeg K
+	var curMatchLen K
+
+	strLen := str.SeqLen()
+	for pos := K(0); pos < strLen; pos++ {
+		chr := str.SeqGet(pos)
+
+		// Consider 'chr'
+		for curMatchLen > 0 && !kmpEq(chr, substr, curMatchLen) { // shorten the match
+			overlap := table[curMatchLen-1]
+			curMatchBeg += curMatchLen - overlap
+			curMatchLen = overlap
+		}
+		if kmpEq(chr, substr, curMatchLen) { // lengthen the match
+			if curMatchLen == 0 {
+				curMatchBeg = pos
+			}
+			curMatchLen++
+			if curMatchLen == substrLen {
+				matches = append(matches, curMatchBeg)
+				overlap := table[curMatchLen-1]
+				curMatchBeg += curMatchLen - overlap
+				curMatchLen = overlap
+			}
+		}
+	}
+	return matches
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/kmp_test.go b/cmd/btrfs-rec/inspect/rebuildmappings/kmp_test.go
new file mode 100644
index 0000000..acec9b8
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/kmp_test.go
@@ -0,0 +1,126 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/diskio"
+)
+
+type bytePattern[K ~int64 | ~int] []byte
+
+var _ kmpPattern[int, byte] = bytePattern[int]{}
+
+// PatLen implements kmpPattern.
+func (s bytePattern[K]) PatLen() K {
+	return K(len(s))
+}
+
+// PatGet implements kmpPattern.
+func (s bytePattern[K]) PatGet(i K) (byte, bool) {
+	chr := s[int(i)]
+	if chr == '.' {
+		return 0, false
+	}
+	return chr, true
+}
+
+func TestBuildKMPTable(t *testing.T) {
+	t.Parallel()
+	substr := bytePattern[int64]([]byte("ababaa"))
+	table := buildKMPTable[int64, byte](substr)
+	require.Equal(t,
+		[]int64{0, 0, 1, 2, 3, 1},
+		table)
+	for j, val := range table {
+		matchLen := j + 1
+		assert.Equalf(t, substr[:val], substr[matchLen-int(val):matchLen],
+			"for table[%d]=%d", j, val)
+	}
+}
+
+func FuzzBuildKMPTable(f *testing.F) {
+	f.Add([]byte("ababaa"))
+	f.Fuzz(func(t *testing.T, substr []byte) {
+		table := buildKMPTable[int64, byte](bytePattern[int64](substr))
+		require.Equal(t, len(substr), len(table), "length")
+		for j, val := range table {
+			matchLen := j + 1
+			assert.Equalf(t, substr[:val], substr[matchLen-int(val):matchLen],
+				"for table[%d]=%d", j, val)
+		}
+	})
+}
+
+func NaiveIndexAll(str, substr []byte) []int64 {
+	var matches []int64
+	for i := range str {
+		if bytes.HasPrefix(str[i:], substr) {
+			matches = append(matches, int64(i))
+		}
+	}
+	return matches
+}
+
+func FuzzIndexAll(f *testing.F) {
+	f.Fuzz(func(t *testing.T, str, substr []byte) {
+		if len(substr) == 0 {
+			t.Skip()
+		}
+		t.Logf("str   =%q", str)
+		t.Logf("substr=%q", substr)
+		exp := NaiveIndexAll(str, substr)
+		act := indexAll[int64, byte](
+			diskio.SliceSequence[int64, byte](str),
+			bytePattern[int64](substr))
+		assert.Equal(t, exp, act)
+	})
+}
+
+func TestKMPWildcard(t *testing.T) {
+	t.Parallel()
+	type testcase struct {
+		InStr      string
+		InSubstr   string
+		ExpMatches []int64
+	}
+	testcases := map[string]testcase{
+		"trivial-bar": {
+			InStr:      "foo_bar",
+			InSubstr:   "foo.ba.",
+			ExpMatches: []int64{0},
+		},
+		"trival-baz": {
+			InStr:      "foo-baz",
+			InSubstr:   "foo.ba.",
+			ExpMatches: []int64{0},
+		},
+		"suffix": {
+			InStr:      "foobarbaz",
+			InSubstr:   "...baz",
+			ExpMatches: []int64{3},
+		},
+		"overlap": {
+			InStr:      "foobarbar",
+			InSubstr:   "...bar",
+			ExpMatches: []int64{0, 3},
+		},
+	}
+	for tcName, tc := range testcases {
+		tc := tc
+		t.Run(tcName, func(t *testing.T) {
+			t.Parallel()
+			matches := indexAll[int64, byte](
+				diskio.StringSequence[int64](tc.InStr),
+				bytePattern[int64](tc.InSubstr))
+			assert.Equal(t, tc.ExpMatches, matches)
+		})
+	}
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process.go b/cmd/btrfs-rec/inspect/rebuildmappings/process.go
new file mode 100644
index 0000000..cdf5e5a
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process.go
@@ -0,0 +1,222 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/datawire/dlib/dlog"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfsprogs/btrfsinspect"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/maps"
+	"git.lukeshu.com/btrfs-progs-ng/lib/textui"
+)
+
+func getNodeSize(fs *btrfs.FS) (btrfsvol.AddrDelta, error) {
+	sb, err := fs.Superblock()
+	if err != nil {
+		return 0, err
+	}
+	return btrfsvol.AddrDelta(sb.NodeSize), nil
+}
+
+func RebuildMappings(ctx context.Context, fs *btrfs.FS, scanResults btrfsinspect.ScanDevicesResult) error {
+	nodeSize, err := getNodeSize(fs)
+	if err != nil {
+		return err
+	}
+
+	var numChunks, numDevExts, numBlockGroups, numNodes int
+	devIDs := maps.SortedKeys(scanResults)
+	devices := fs.LV.PhysicalVolumes()
+	for _, devID := range devIDs {
+		if _, ok := devices[devID]; !ok {
+			return fmt.Errorf("device ID %v mentioned in scan results is not part of the filesystem", devID)
+		}
+		devResults := scanResults[devID]
+		numChunks += len(devResults.FoundChunks)
+		numDevExts += len(devResults.FoundDevExtents)
+		numBlockGroups += len(devResults.FoundBlockGroups)
+		for _, paddrs := range devResults.FoundNodes {
+			numNodes += len(paddrs)
+		}
+	}
+	dlog.Infof(ctx, "plan: 1/6 process %d chunks", numChunks)
+	dlog.Infof(ctx, "plan: 2/6 process %d device extents", numDevExts)
+	dlog.Infof(ctx, "plan: 3/6 process %d nodes", numNodes)
+	dlog.Infof(ctx, "plan: 4/6 process %d block groups", numBlockGroups)
+	dlog.Infof(ctx, "plan: 5/6 search for block groups in checksum map (exact)")
+	dlog.Infof(ctx, "plan: 6/6 search for block groups in checksum map (fuzzy)")
+
+	_ctx := ctx
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "1/6")
+	dlog.Infof(_ctx, "1/6: Processing %d chunks...", numChunks)
+	for _, devID := range devIDs {
+		devResults := scanResults[devID]
+		for _, chunk := range devResults.FoundChunks {
+			for _, mapping := range chunk.Chunk.Mappings(chunk.Key) {
+				if err := fs.LV.AddMapping(mapping); err != nil {
+					dlog.Errorf(ctx, "error: adding chunk: %v", err)
+				}
+			}
+		}
+	}
+	dlog.Info(_ctx, "... done processing chunks")
+
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "2/6")
+	dlog.Infof(_ctx, "2/6: Processing %d device extents...", numDevExts)
+	for _, devID := range devIDs {
+		devResults := scanResults[devID]
+		for _, ext := range devResults.FoundDevExtents {
+			if err := fs.LV.AddMapping(ext.DevExt.Mapping(ext.Key)); err != nil {
+				dlog.Errorf(ctx, "error: adding devext: %v", err)
+			}
+		}
+	}
+	dlog.Info(_ctx, "... done processing device extents")
+
+	// Do the nodes "last" to avoid bloating the mappings table
+	// too much.  (Because nodes are numerous and small, while the
+	// others are few and large; so it is likely that many of the
+	// nodes will be subsumed by other things.)
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "3/6")
+	dlog.Infof(_ctx, "3/6: Processing %d nodes...", numNodes)
+	for _, devID := range devIDs {
+		devResults := scanResults[devID]
+		// Sort them so that progress numbers are predictable.
+		for _, laddr := range maps.SortedKeys(devResults.FoundNodes) {
+			for _, paddr := range devResults.FoundNodes[laddr] {
+				if err := fs.LV.AddMapping(btrfsvol.Mapping{
+					LAddr: laddr,
+					PAddr: btrfsvol.QualifiedPhysicalAddr{
+						Dev:  devID,
+						Addr: paddr,
+					},
+					Size:       nodeSize,
+					SizeLocked: false,
+				}); err != nil {
+					dlog.Errorf(ctx, "error: adding node ident: %v", err)
+				}
+			}
+		}
+	}
+	dlog.Info(_ctx, "... done processing nodes")
+
+	// Use block groups to add missing flags (and as a hint to
+	// combine node entries).
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "4/6")
+	dlog.Infof(_ctx, "4/6: Processing %d block groups...", numBlockGroups)
+	// First dedup them, because they change for allocations and
+	// CoW means that they'll bounce around a lot, so you likely
+	// have oodles of duplicates?
+	bgs, err := DedupBlockGroups(scanResults)
+	if err != nil {
+		return err
+	}
+	dlog.Infof(ctx, "... de-duplicated to %d block groups", len(bgs))
+	for _, bgLAddr := range maps.SortedKeys(bgs) {
+		bg := bgs[bgLAddr]
+		otherLAddr, otherPAddr := fs.LV.ResolveAny(bg.LAddr, bg.Size)
+		if otherLAddr < 0 || otherPAddr.Addr < 0 {
+			dlog.Errorf(ctx, "error: could not pair blockgroup laddr=%v (size=%v flags=%v) with a mapping",
+				bg.LAddr, bg.Size, bg.Flags)
+			continue
+		}
+
+		offsetWithinChunk := otherLAddr.Sub(bg.LAddr)
+		mapping := btrfsvol.Mapping{
+			LAddr:      bg.LAddr,
+			PAddr:      otherPAddr.Add(-offsetWithinChunk),
+			Size:       bg.Size,
+			SizeLocked: true,
+			Flags: containers.Optional[btrfsvol.BlockGroupFlags]{
+				OK:  true,
+				Val: bg.Flags,
+			},
+		}
+		if err := fs.LV.AddMapping(mapping); err != nil {
+			dlog.Errorf(ctx, "error: adding flags from blockgroup: %v", err)
+			continue
+		}
+		delete(bgs, bgLAddr)
+	}
+	dlog.Info(_ctx, "... done processing block groups")
+
+	// More than once, I've been tempted to get rid of this exact-search step and just have the fuzzy-search step.
+	// After all, looking at the timestamps in the log, it's faster per blockgroup!  For some background, the big-O
+	// for each (per blockgroup) looks like:
+	//
+	//  - exact-search: O(bgSize+physicalBlocks)
+	//  - fuzzy-search: O(bgSize*physicalBlocks) worst-case; O(bgSize*log(physicalBlocks)) expected
+	//
+	// The fuzzy-search is only fast because the exact-search is so good at getting `physicalBlocks` down.
+	// Empirically: if I remove the exact-search step, then the fuzzy-match step is more than an order of magnitude
+	// slower.
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "5/6")
+	dlog.Infof(_ctx, "5/6: Searching for %d block groups in checksum map (exact)...", len(bgs))
+	physicalSums := ExtractPhysicalSums(scanResults)
+	logicalSums := ExtractLogicalSums(ctx, scanResults)
+	if err := matchBlockGroupSums(ctx, fs, bgs, physicalSums, logicalSums); err != nil {
+		return err
+	}
+	dlog.Info(ctx, "... done searching for exact block groups")
+
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "6/6")
+	dlog.Infof(_ctx, "6/6: Searching for %d block groups in checksum map (fuzzy)...", len(bgs))
+	if err := fuzzyMatchBlockGroupSums(ctx, fs, bgs, physicalSums, logicalSums); err != nil {
+		return err
+	}
+	dlog.Info(_ctx, "... done searching for fuzzy block groups")
+
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.step", "report")
+	dlog.Info(_ctx, "report:")
+
+	unmappedPhysicalRegions := ListUnmappedPhysicalRegions(fs)
+	var unmappedPhysical btrfsvol.AddrDelta
+	var numUnmappedPhysical int
+	for _, devRegions := range unmappedPhysicalRegions {
+		numUnmappedPhysical += len(devRegions)
+		for _, region := range devRegions {
+			unmappedPhysical += region.End.Sub(region.Beg)
+		}
+	}
+	dlog.Infof(ctx, "... %d of unmapped physical space (across %d regions)", textui.IEC(unmappedPhysical, "B"), numUnmappedPhysical)
+
+	unmappedLogicalRegions := ListUnmappedLogicalRegions(fs, logicalSums)
+	var unmappedLogical btrfsvol.AddrDelta
+	for _, region := range unmappedLogicalRegions {
+		unmappedLogical += region.Size()
+	}
+	dlog.Infof(ctx, "... %d of unmapped summed logical space (across %d regions)", textui.IEC(unmappedLogical, "B"), len(unmappedLogicalRegions))
+
+	var unmappedBlockGroups btrfsvol.AddrDelta
+	for _, bg := range bgs {
+		unmappedBlockGroups += bg.Size
+	}
+	dlog.Infof(ctx, "... %d of unmapped block groups (across %d groups)", textui.IEC(unmappedBlockGroups, "B"), len(bgs))
+
+	dlog.Info(_ctx, "detailed report:")
+	for _, devID := range maps.SortedKeys(unmappedPhysicalRegions) {
+		for _, region := range unmappedPhysicalRegions[devID] {
+			dlog.Infof(ctx, "... unmapped physical region: dev=%v beg=%v end=%v (size=%v)",
+				devID, region.Beg, region.End, region.End.Sub(region.Beg))
+		}
+	}
+	for _, region := range unmappedLogicalRegions {
+		dlog.Infof(ctx, "... umapped summed logical region:  beg=%v end=%v (size=%v)",
+			region.Addr, region.Addr.Add(region.Size()), region.Size())
+	}
+	for _, laddr := range maps.SortedKeys(bgs) {
+		bg := bgs[laddr]
+		dlog.Infof(ctx, "... umapped block group:            beg=%v end=%v (size=%v) flags=%v",
+			bg.LAddr, bg.LAddr.Add(bg.Size), bg.Size, bg.Flags)
+	}
+
+	return nil
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process_blockgroups.go b/cmd/btrfs-rec/inspect/rebuildmappings/process_blockgroups.go
new file mode 100644
index 0000000..0e2d5a0
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process_blockgroups.go
@@ -0,0 +1,58 @@
+// Copyright (C) 2022  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"fmt"
+	"sort"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfsprogs/btrfsinspect"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/maps"
+)
+
+type BlockGroup struct {
+	LAddr btrfsvol.LogicalAddr
+	Size  btrfsvol.AddrDelta
+	Flags btrfsvol.BlockGroupFlags
+}
+
+func DedupBlockGroups(scanResults btrfsinspect.ScanDevicesResult) (map[btrfsvol.LogicalAddr]BlockGroup, error) {
+	// Dedup
+	bgsSet := make(containers.Set[BlockGroup])
+	for _, devResults := range scanResults {
+		for _, bg := range devResults.FoundBlockGroups {
+			bgsSet.Insert(BlockGroup{
+				LAddr: btrfsvol.LogicalAddr(bg.Key.ObjectID),
+				Size:  btrfsvol.AddrDelta(bg.Key.Offset),
+				Flags: bg.BG.Flags,
+			})
+		}
+	}
+
+	// Sort
+	bgsOrdered := maps.Keys(bgsSet)
+	sort.Slice(bgsOrdered, func(i, j int) bool {
+		return bgsOrdered[i].LAddr < bgsOrdered[j].LAddr
+	})
+
+	// Sanity check
+	var pos btrfsvol.LogicalAddr
+	for _, bg := range bgsOrdered {
+		if bg.LAddr < pos || bg.Size < 0 {
+			return nil, fmt.Errorf("found block groups are inconsistent")
+		}
+		pos = bg.LAddr.Add(bg.Size)
+	}
+
+	// Return.  We return a map instead of a slice in order to
+	// facilitate easy deletes.
+	bgsMap := make(map[btrfsvol.LogicalAddr]BlockGroup, len(bgsSet))
+	for bg := range bgsSet {
+		bgsMap[bg.LAddr] = bg
+	}
+	return bgsMap, nil
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_exact.go b/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_exact.go
new file mode 100644
index 0000000..a3e724e
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_exact.go
@@ -0,0 +1,78 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+
+	"github.com/datawire/dlib/dlog"
+	"golang.org/x/text/number"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/maps"
+)
+
+func matchBlockGroupSums(ctx context.Context,
+	fs *btrfs.FS,
+	blockgroups map[btrfsvol.LogicalAddr]BlockGroup,
+	physicalSums map[btrfsvol.DeviceID]btrfssum.SumRun[btrfsvol.PhysicalAddr],
+	logicalSums SumRunWithGaps[btrfsvol.LogicalAddr],
+) error {
+	regions := ListUnmappedPhysicalRegions(fs)
+	numBlockgroups := len(blockgroups)
+	for i, bgLAddr := range maps.SortedKeys(blockgroups) {
+		blockgroup := blockgroups[bgLAddr]
+		bgRun := SumsForLogicalRegion(logicalSums, blockgroup.LAddr, blockgroup.Size)
+		if len(bgRun.Runs) == 0 {
+			dlog.Errorf(ctx, "(%v/%v) blockgroup[laddr=%v] can't be matched because it has 0 runs",
+				i+1, numBlockgroups, bgLAddr)
+			continue
+		}
+
+		var matches []btrfsvol.QualifiedPhysicalAddr
+		if err := WalkUnmappedPhysicalRegions(ctx, physicalSums, regions, func(devID btrfsvol.DeviceID, region btrfssum.SumRun[btrfsvol.PhysicalAddr]) error {
+			rawMatches := indexAll[int, btrfssum.ShortSum](region, bgRun)
+			for _, match := range rawMatches {
+				matches = append(matches, btrfsvol.QualifiedPhysicalAddr{
+					Dev:  devID,
+					Addr: region.Addr + (btrfsvol.PhysicalAddr(match) * btrfssum.BlockSize),
+				})
+			}
+			return nil
+		}); err != nil {
+			return err
+		}
+
+		lvl := dlog.LogLevelError
+		if len(matches) == 1 {
+			lvl = dlog.LogLevelInfo
+		}
+		dlog.Logf(ctx, lvl, "(%v/%v) blockgroup[laddr=%v] has %v matches based on %v coverage from %v runs",
+			i+1, numBlockgroups, bgLAddr, len(matches), number.Percent(bgRun.PctFull()), len(bgRun.Runs))
+		if len(matches) != 1 {
+			continue
+		}
+
+		mapping := btrfsvol.Mapping{
+			LAddr:      blockgroup.LAddr,
+			PAddr:      matches[0],
+			Size:       blockgroup.Size,
+			SizeLocked: true,
+			Flags: containers.Optional[btrfsvol.BlockGroupFlags]{
+				OK:  true,
+				Val: blockgroup.Flags,
+			},
+		}
+		if err := fs.LV.AddMapping(mapping); err != nil {
+			dlog.Errorf(ctx, "error: %v", err)
+			continue
+		}
+		delete(blockgroups, bgLAddr)
+	}
+	return nil
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_fuzzy.go b/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_fuzzy.go
new file mode 100644
index 0000000..4724c12
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process_matchsums_fuzzy.go
@@ -0,0 +1,159 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+	"sort"
+
+	"github.com/datawire/dlib/dlog"
+	"golang.org/x/text/number"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/maps"
+	"git.lukeshu.com/btrfs-progs-ng/lib/textui"
+)
+
+var minFuzzyPct = textui.Tunable(0.5)
+
+type fuzzyRecord struct {
+	PAddr btrfsvol.QualifiedPhysicalAddr
+	N     int
+}
+
+func (a fuzzyRecord) Compare(b fuzzyRecord) int {
+	switch {
+	case a.N < b.N:
+		return -1
+	case a.N > b.N:
+		return 1
+	default:
+		return 0
+	}
+}
+
+func fuzzyMatchBlockGroupSums(ctx context.Context,
+	fs *btrfs.FS,
+	blockgroups map[btrfsvol.LogicalAddr]BlockGroup,
+	physicalSums map[btrfsvol.DeviceID]btrfssum.SumRun[btrfsvol.PhysicalAddr],
+	logicalSums SumRunWithGaps[btrfsvol.LogicalAddr],
+) error {
+	_ctx := ctx
+
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.substep", "indexing")
+	dlog.Info(ctx, "Indexing physical regions...") // O(m)
+	regions := ListUnmappedPhysicalRegions(fs)
+	physicalIndex := make(map[btrfssum.ShortSum][]btrfsvol.QualifiedPhysicalAddr)
+	if err := WalkUnmappedPhysicalRegions(ctx, physicalSums, regions, func(devID btrfsvol.DeviceID, region btrfssum.SumRun[btrfsvol.PhysicalAddr]) error {
+		return region.Walk(ctx, func(paddr btrfsvol.PhysicalAddr, sum btrfssum.ShortSum) error {
+			physicalIndex[sum] = append(physicalIndex[sum], btrfsvol.QualifiedPhysicalAddr{
+				Dev:  devID,
+				Addr: paddr,
+			})
+			return nil
+		})
+	}); err != nil {
+		return err
+	}
+	dlog.Info(ctx, "... done indexing")
+
+	ctx = dlog.WithField(_ctx, "btrfsinspect.rebuild-mappings.substep", "searching")
+	dlog.Info(ctx, "Searching...")
+	numBlockgroups := len(blockgroups)
+	for i, bgLAddr := range maps.SortedKeys(blockgroups) {
+		blockgroup := blockgroups[bgLAddr]
+		bgRun := SumsForLogicalRegion(logicalSums, blockgroup.LAddr, blockgroup.Size)
+
+		d := bgRun.PatLen()
+		matches := make(map[btrfsvol.QualifiedPhysicalAddr]int)
+		if err := bgRun.Walk(ctx, func(laddr btrfsvol.LogicalAddr, sum btrfssum.ShortSum) error { // O(n*…
+			off := laddr.Sub(bgRun.Addr)
+			for _, paddr := range physicalIndex[sum] { // …log(m)) expected but …m) worst
+				key := btrfsvol.QualifiedPhysicalAddr{
+					Dev:  paddr.Dev,
+					Addr: paddr.Addr.Add(-off),
+				}
+				matches[key]++
+			}
+			return nil
+		}); err != nil {
+			return err
+		}
+
+		best := lowestN[fuzzyRecord]{N: 2}
+		for paddr, n := range matches { // O(m)
+			best.Insert(fuzzyRecord{
+				PAddr: paddr,
+				N:     d - n,
+			})
+		}
+
+		var apply bool
+		var matchesStr string
+		switch len(best.Dat) {
+		case 0: // can happen if there are no sums in the run
+			matchesStr = ""
+		case 1: // not sure how this can happen, but whatev
+			pct := float64(d-best.Dat[0].N) / float64(d)
+			matchesStr = textui.Sprintf("%v", number.Percent(pct))
+			apply = pct > minFuzzyPct
+		case 2:
+			pct := float64(d-best.Dat[0].N) / float64(d)
+			pct2 := float64(d-best.Dat[1].N) / float64(d)
+			matchesStr = textui.Sprintf("best=%v secondbest=%v", number.Percent(pct), number.Percent(pct2))
+			apply = pct > minFuzzyPct && pct2 < minFuzzyPct
+		}
+		lvl := dlog.LogLevelError
+		if apply {
+			lvl = dlog.LogLevelInfo
+		}
+		dlog.Logf(ctx, lvl, "(%v/%v) blockgroup[laddr=%v] matches=[%s]; bestpossible=%v (based on %v runs)",
+			i+1, numBlockgroups, bgLAddr, matchesStr, number.Percent(bgRun.PctFull()), len(bgRun.Runs))
+		if !apply {
+			continue
+		}
+
+		mapping := btrfsvol.Mapping{
+			LAddr:      blockgroup.LAddr,
+			PAddr:      best.Dat[0].PAddr,
+			Size:       blockgroup.Size,
+			SizeLocked: true,
+			Flags: containers.Optional[btrfsvol.BlockGroupFlags]{
+				OK:  true,
+				Val: blockgroup.Flags,
+			},
+		}
+		if err := fs.LV.AddMapping(mapping); err != nil {
+			dlog.Errorf(ctx, "error: %v", err)
+			continue
+		}
+		delete(blockgroups, bgLAddr)
+	}
+	dlog.Info(ctx, "done searching")
+
+	return nil
+}
+
+type lowestN[T containers.Ordered[T]] struct {
+	N   int
+	Dat []T
+}
+
+func (l *lowestN[T]) Insert(v T) {
+	switch {
+	case len(l.Dat) < l.N:
+		l.Dat = append(l.Dat, v)
+	case v.Compare(l.Dat[0]) < 0:
+		l.Dat[0] = v
+	default:
+		return
+	}
+	sort.Slice(l.Dat, func(i, j int) bool {
+		return l.Dat[i].Compare(l.Dat[j]) < 0
+	})
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_logical.go b/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_logical.go
new file mode 100644
index 0000000..7c02d05
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_logical.go
@@ -0,0 +1,249 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+	"sort"
+	"strings"
+
+	"github.com/datawire/dlib/dlog"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsitem"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfsprogs/btrfsinspect"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/slices"
+)
+
+func ExtractLogicalSums(ctx context.Context, scanResults btrfsinspect.ScanDevicesResult) SumRunWithGaps[btrfsvol.LogicalAddr] {
+	var records []btrfsinspect.SysExtentCSum
+	for _, devResults := range scanResults {
+		records = append(records, devResults.FoundExtentCSums...)
+	}
+	// Sort lower-generation items earlier; then sort by addr.
+	sort.Slice(records, func(i, j int) bool {
+		a, b := records[i], records[j]
+		switch {
+		case a.Generation < b.Generation:
+			return true
+		case a.Generation > b.Generation:
+			return false
+		default:
+			return a.Sums.Addr < b.Sums.Addr
+		}
+	})
+	if len(records) == 0 {
+		return SumRunWithGaps[btrfsvol.LogicalAddr]{}
+	}
+	sumSize := records[0].Sums.ChecksumSize
+
+	// Now build them in to a coherent address space.
+	//
+	// We can't just reverse-sort by generation to avoid mutations, because given
+	//
+	//	gen1 AAAAAAA
+	//      gen2    BBBBBBBB
+	//      gen3          CCCCCCC
+	//
+	// "AAAAAAA" shouldn't be present, and if we just discard "BBBBBBBB"
+	// because it conflicts with "CCCCCCC", then we would erroneously
+	// include "AAAAAAA".
+	addrspace := new(containers.RBTree[btrfsinspect.SysExtentCSum])
+	for _, newRecord := range records {
+		for {
+			conflict := addrspace.Search(func(oldRecord btrfsinspect.SysExtentCSum) int {
+				switch {
+				case newRecord.Sums.Addr.Add(newRecord.Sums.Size()) <= oldRecord.Sums.Addr:
+					// 'newRecord' is wholly to the left of 'oldRecord'.
+					return -1
+				case oldRecord.Sums.Addr.Add(oldRecord.Sums.Size()) <= newRecord.Sums.Addr:
+					// 'oldRecord' is wholly to the left of 'newRecord'.
+					return 1
+				default:
+					// There is some overlap.
+					return 0
+				}
+			})
+			if conflict == nil {
+				// We can insert it
+				addrspace.Insert(newRecord)
+				break
+			}
+			oldRecord := conflict.Value
+			if oldRecord == newRecord {
+				// Duplicates are to be expected.
+				break
+			}
+			if oldRecord.Generation < newRecord.Generation {
+				// Newer generation wins.
+				addrspace.Delete(conflict)
+				// loop around to check for more conflicts
+				continue
+			}
+			if oldRecord.Generation > newRecord.Generation {
+				// We sorted them!  This shouldn't happen.
+				panic("should not happen")
+			}
+			// Since sums are stored multiple times (RAID?), but may
+			// be split between items differently between copies, we
+			// should take the union (after verifying that they
+			// agree on the overlapping part).
+			overlapBeg := slices.Max(
+				oldRecord.Sums.Addr,
+				newRecord.Sums.Addr)
+			overlapEnd := slices.Min(
+				oldRecord.Sums.Addr.Add(oldRecord.Sums.Size()),
+				newRecord.Sums.Addr.Add(newRecord.Sums.Size()))
+
+			oldOverlapBeg := int(overlapBeg.Sub(oldRecord.Sums.Addr)/btrfssum.BlockSize) * sumSize
+			oldOverlapEnd := int(overlapEnd.Sub(oldRecord.Sums.Addr)/btrfssum.BlockSize) * sumSize
+			oldOverlap := oldRecord.Sums.Sums[oldOverlapBeg:oldOverlapEnd]
+
+			newOverlapBeg := int(overlapBeg.Sub(newRecord.Sums.Addr)/btrfssum.BlockSize) * sumSize
+			newOverlapEnd := int(overlapEnd.Sub(newRecord.Sums.Addr)/btrfssum.BlockSize) * sumSize
+			newOverlap := newRecord.Sums.Sums[newOverlapBeg:newOverlapEnd]
+
+			if oldOverlap != newOverlap {
+				dlog.Errorf(ctx, "mismatch: {gen:%v, addr:%v, size:%v} conflicts with {gen:%v, addr:%v, size:%v}",
+					oldRecord.Generation, oldRecord.Sums.Addr, oldRecord.Sums.Size(),
+					newRecord.Generation, newRecord.Sums.Addr, newRecord.Sums.Size())
+				break
+			}
+			// OK, we match, so take the union.
+			var prefix, suffix btrfssum.ShortSum
+			switch {
+			case oldRecord.Sums.Addr < overlapBeg:
+				prefix = oldRecord.Sums.Sums[:oldOverlapBeg]
+			case newRecord.Sums.Addr < overlapBeg:
+				prefix = newRecord.Sums.Sums[:newOverlapBeg]
+			}
+			switch {
+			case oldRecord.Sums.Addr.Add(oldRecord.Sums.Size()) > overlapEnd:
+				suffix = oldRecord.Sums.Sums[oldOverlapEnd:]
+			case newRecord.Sums.Addr.Add(newRecord.Sums.Size()) > overlapEnd:
+				suffix = newRecord.Sums.Sums[newOverlapEnd:]
+			}
+			unionRecord := btrfsinspect.SysExtentCSum{
+				Generation: oldRecord.Generation,
+				Sums: btrfsitem.ExtentCSum{
+					SumRun: btrfssum.SumRun[btrfsvol.LogicalAddr]{
+						ChecksumSize: oldRecord.Sums.ChecksumSize,
+						Addr:         slices.Min(oldRecord.Sums.Addr, newRecord.Sums.Addr),
+						Sums:         prefix + oldOverlap + suffix,
+					},
+				},
+			}
+			addrspace.Delete(conflict)
+			newRecord = unionRecord
+			// loop around to check for more conflicts
+		}
+	}
+
+	// Now flatten that RBTree in to a SumRunWithGaps.
+	var flattened SumRunWithGaps[btrfsvol.LogicalAddr]
+	var curAddr btrfsvol.LogicalAddr
+	var curSums strings.Builder
+	addrspace.Range(func(node *containers.RBNode[btrfsinspect.SysExtentCSum]) bool {
+		curEnd := curAddr + (btrfsvol.LogicalAddr(curSums.Len()/sumSize) * btrfssum.BlockSize)
+		if node.Value.Sums.Addr != curEnd {
+			if curSums.Len() > 0 {
+				flattened.Runs = append(flattened.Runs, btrfssum.SumRun[btrfsvol.LogicalAddr]{
+					ChecksumSize: sumSize,
+					Addr:         curAddr,
+					Sums:         btrfssum.ShortSum(curSums.String()),
+				})
+			}
+			curAddr = node.Value.Sums.Addr
+			curSums.Reset()
+		}
+		curSums.WriteString(string(node.Value.Sums.Sums))
+		return true
+	})
+	if curSums.Len() > 0 {
+		flattened.Runs = append(flattened.Runs, btrfssum.SumRun[btrfsvol.LogicalAddr]{
+			ChecksumSize: sumSize,
+			Addr:         curAddr,
+			Sums:         btrfssum.ShortSum(curSums.String()),
+		})
+	}
+	flattened.Addr = flattened.Runs[0].Addr
+	last := flattened.Runs[len(flattened.Runs)-1]
+	end := last.Addr.Add(last.Size())
+	flattened.Size = end.Sub(flattened.Addr)
+
+	return flattened
+}
+
+func ListUnmappedLogicalRegions(fs *btrfs.FS, logicalSums SumRunWithGaps[btrfsvol.LogicalAddr]) []btrfssum.SumRun[btrfsvol.LogicalAddr] {
+	// There are a lot of ways this algorithm could be made
+	// faster.
+	var ret []btrfssum.SumRun[btrfsvol.LogicalAddr]
+	var cur struct {
+		Addr btrfsvol.LogicalAddr
+		Size btrfsvol.AddrDelta
+	}
+	for _, run := range logicalSums.Runs {
+		for addr := run.Addr; addr < run.Addr.Add(run.Size()); addr += btrfssum.BlockSize {
+			if _, maxlen := fs.LV.Resolve(addr); maxlen < btrfssum.BlockSize {
+				if cur.Size == 0 {
+					cur.Addr = addr
+					cur.Size = 0
+				}
+				cur.Size += btrfssum.BlockSize
+			} else if cur.Size > 0 {
+				begIdx := int(cur.Addr.Sub(run.Addr)/btrfssum.BlockSize) * run.ChecksumSize
+				lenIdx := (int(cur.Size) / btrfssum.BlockSize) * run.ChecksumSize
+				endIdx := begIdx + lenIdx
+				ret = append(ret, btrfssum.SumRun[btrfsvol.LogicalAddr]{
+					ChecksumSize: run.ChecksumSize,
+					Addr:         cur.Addr,
+					Sums:         run.Sums[begIdx:endIdx],
+				})
+				cur.Size = 0
+			}
+		}
+		if cur.Size > 0 {
+			begIdx := int(cur.Addr.Sub(run.Addr)/btrfssum.BlockSize) * run.ChecksumSize
+			lenIdx := (int(cur.Size) / btrfssum.BlockSize) * run.ChecksumSize
+			endIdx := begIdx + lenIdx
+			ret = append(ret, btrfssum.SumRun[btrfsvol.LogicalAddr]{
+				ChecksumSize: run.ChecksumSize,
+				Addr:         cur.Addr,
+				Sums:         run.Sums[begIdx:endIdx],
+			})
+			cur.Size = 0
+		}
+	}
+	return ret
+}
+
+func SumsForLogicalRegion(sums SumRunWithGaps[btrfsvol.LogicalAddr], beg btrfsvol.LogicalAddr, size btrfsvol.AddrDelta) SumRunWithGaps[btrfsvol.LogicalAddr] {
+	runs := SumRunWithGaps[btrfsvol.LogicalAddr]{
+		Addr: beg,
+		Size: size,
+	}
+	for laddr := beg; laddr < beg.Add(size); {
+		run, next, ok := sums.RunForAddr(laddr)
+		if !ok {
+			laddr = next
+			continue
+		}
+		off := int((laddr-run.Addr)/btrfssum.BlockSize) * run.ChecksumSize
+		deltaAddr := slices.Min[btrfsvol.AddrDelta](
+			size-laddr.Sub(beg),
+			btrfsvol.AddrDelta((len(run.Sums)-off)/run.ChecksumSize)*btrfssum.BlockSize)
+		deltaOff := int(deltaAddr/btrfssum.BlockSize) * run.ChecksumSize
+		runs.Runs = append(runs.Runs, btrfssum.SumRun[btrfsvol.LogicalAddr]{
+			ChecksumSize: run.ChecksumSize,
+			Addr:         laddr,
+			Sums:         run.Sums[off : off+deltaOff],
+		})
+		laddr = laddr.Add(deltaAddr)
+	}
+	return runs
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_physical.go b/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_physical.go
new file mode 100644
index 0000000..da22fbf
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/process_sums_physical.go
@@ -0,0 +1,89 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+	"sort"
+
+	"golang.org/x/exp/constraints"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfsprogs/btrfsinspect"
+	"git.lukeshu.com/btrfs-progs-ng/lib/maps"
+)
+
+func ExtractPhysicalSums(scanResults btrfsinspect.ScanDevicesResult) map[btrfsvol.DeviceID]btrfssum.SumRun[btrfsvol.PhysicalAddr] {
+	ret := make(map[btrfsvol.DeviceID]btrfssum.SumRun[btrfsvol.PhysicalAddr], len(scanResults))
+	for devID, devResults := range scanResults {
+		ret[devID] = devResults.Checksums
+	}
+	return ret
+}
+
+type PhysicalRegion struct {
+	Beg, End btrfsvol.PhysicalAddr
+}
+
+func ListUnmappedPhysicalRegions(fs *btrfs.FS) map[btrfsvol.DeviceID][]PhysicalRegion {
+	regions := make(map[btrfsvol.DeviceID][]PhysicalRegion)
+	pos := make(map[btrfsvol.DeviceID]btrfsvol.PhysicalAddr)
+	mappings := fs.LV.Mappings()
+	sort.Slice(mappings, func(i, j int) bool {
+		return mappings[i].PAddr.Compare(mappings[j].PAddr) < 0
+	})
+	for _, mapping := range mappings {
+		if pos[mapping.PAddr.Dev] < mapping.PAddr.Addr {
+			regions[mapping.PAddr.Dev] = append(regions[mapping.PAddr.Dev], PhysicalRegion{
+				Beg: pos[mapping.PAddr.Dev],
+				End: mapping.PAddr.Addr,
+			})
+		}
+		if pos[mapping.PAddr.Dev] < mapping.PAddr.Addr.Add(mapping.Size) {
+			pos[mapping.PAddr.Dev] = mapping.PAddr.Addr.Add(mapping.Size)
+		}
+	}
+	for devID, dev := range fs.LV.PhysicalVolumes() {
+		devSize := dev.Size()
+		if pos[devID] < devSize {
+			regions[devID] = append(regions[devID], PhysicalRegion{
+				Beg: pos[devID],
+				End: devSize,
+			})
+		}
+	}
+	return regions
+}
+
+func roundUp[T constraints.Integer](x, multiple T) T {
+	return ((x + multiple - 1) / multiple) * multiple
+}
+
+func WalkUnmappedPhysicalRegions(ctx context.Context,
+	physicalSums map[btrfsvol.DeviceID]btrfssum.SumRun[btrfsvol.PhysicalAddr],
+	gaps map[btrfsvol.DeviceID][]PhysicalRegion,
+	fn func(btrfsvol.DeviceID, btrfssum.SumRun[btrfsvol.PhysicalAddr]) error,
+) error {
+	for _, devID := range maps.SortedKeys(gaps) {
+		for _, gap := range gaps[devID] {
+			if err := ctx.Err(); err != nil {
+				return err
+			}
+			begAddr := roundUp(gap.Beg, btrfssum.BlockSize)
+			begOff := int(begAddr/btrfssum.BlockSize) * physicalSums[devID].ChecksumSize
+			endOff := int(gap.End/btrfssum.BlockSize) * physicalSums[devID].ChecksumSize
+			if err := fn(devID, btrfssum.SumRun[btrfsvol.PhysicalAddr]{
+				ChecksumSize: physicalSums[devID].ChecksumSize,
+				Addr:         begAddr,
+				Sums:         physicalSums[devID].Sums[begOff:endOff],
+			}); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/scan.go b/cmd/btrfs-rec/inspect/rebuildmappings/scan.go
new file mode 100644
index 0000000..d54be71
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/scan.go
@@ -0,0 +1,260 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package btrfsinspect
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/datawire/dlib/dgroup"
+	"github.com/datawire/dlib/dlog"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/binstruct"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsitem"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsprim"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfstree"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
+	"git.lukeshu.com/btrfs-progs-ng/lib/textui"
+)
+
+type ScanDevicesResult map[btrfsvol.DeviceID]ScanOneDeviceResult
+
+func ScanDevices(ctx context.Context, fs *btrfs.FS) (ScanDevicesResult, error) {
+	grp := dgroup.NewGroup(ctx, dgroup.GroupConfig{})
+	var mu sync.Mutex
+	result := make(map[btrfsvol.DeviceID]ScanOneDeviceResult)
+	for id, dev := range fs.LV.PhysicalVolumes() {
+		id := id
+		dev := dev
+		grp.Go(fmt.Sprintf("dev-%d", id), func(ctx context.Context) error {
+			sb, err := dev.Superblock()
+			if err != nil {
+				return err
+			}
+			devResult, err := ScanOneDevice(ctx, dev, *sb)
+			if err != nil {
+				return err
+			}
+			mu.Lock()
+			result[id] = devResult
+			mu.Unlock()
+			return nil
+		})
+	}
+	if err := grp.Wait(); err != nil {
+		return nil, err
+	}
+	return result, nil
+}
+
+type ScanOneDeviceResult struct {
+	Checksums        btrfssum.SumRun[btrfsvol.PhysicalAddr]
+	FoundNodes       map[btrfsvol.LogicalAddr][]btrfsvol.PhysicalAddr
+	FoundChunks      []btrfstree.SysChunk
+	FoundBlockGroups []SysBlockGroup
+	FoundDevExtents  []SysDevExtent
+	FoundExtentCSums []SysExtentCSum
+}
+
+type SysBlockGroup struct {
+	Key btrfsprim.Key
+	BG  btrfsitem.BlockGroup
+}
+
+type SysDevExtent struct {
+	Key    btrfsprim.Key
+	DevExt btrfsitem.DevExtent
+}
+
+type SysExtentCSum struct {
+	Generation btrfsprim.Generation
+	Sums       btrfsitem.ExtentCSum
+}
+
+// Compare implements containers.Ordered.
+func (a SysExtentCSum) Compare(b SysExtentCSum) int {
+	return containers.NativeCompare(a.Sums.Addr, b.Sums.Addr)
+}
+
+type scanStats struct {
+	textui.Portion[btrfsvol.PhysicalAddr]
+
+	NumFoundNodes       int
+	NumFoundChunks      int
+	NumFoundBlockGroups int
+	NumFoundDevExtents  int
+	NumFoundExtentCSums int
+}
+
+func (s scanStats) String() string {
+	return textui.Sprintf("scanned %v (found: %v nodes, %v chunks, %v block groups, %v dev extents, %v sum items)",
+		s.Portion,
+		s.NumFoundNodes,
+		s.NumFoundChunks,
+		s.NumFoundBlockGroups,
+		s.NumFoundDevExtents,
+		s.NumFoundExtentCSums)
+}
+
+var sbSize = btrfsvol.PhysicalAddr(binstruct.StaticSize(btrfstree.Superblock{}))
+
+// ScanOneDevice mostly mimics btrfs-progs
+// cmds/rescue-chunk-recover.c:scan_one_device().
+func ScanOneDevice(ctx context.Context, dev *btrfs.Device, sb btrfstree.Superblock) (ScanOneDeviceResult, error) {
+	ctx = dlog.WithField(ctx, "btrfsinspect.scandevices.dev", dev.Name())
+
+	result := ScanOneDeviceResult{
+		FoundNodes: make(map[btrfsvol.LogicalAddr][]btrfsvol.PhysicalAddr),
+	}
+
+	devSize := dev.Size()
+	if sb.NodeSize < sb.SectorSize {
+		return result, fmt.Errorf("node_size(%v) < sector_size(%v)",
+			sb.NodeSize, sb.SectorSize)
+	}
+	if sb.SectorSize != btrfssum.BlockSize {
+		// TODO: probably handle this?
+		return result, fmt.Errorf("sector_size(%v) != btrfssum.BlockSize",
+			sb.SectorSize)
+	}
+	alg := sb.ChecksumType
+	csumSize := alg.Size()
+	numSums := int(devSize / btrfssum.BlockSize)
+	var sums strings.Builder
+	sums.Grow(numSums * csumSize)
+
+	progressWriter := textui.NewProgress[scanStats](ctx, dlog.LogLevelInfo, textui.Tunable(1*time.Second))
+	progress := func(pos btrfsvol.PhysicalAddr) {
+		progressWriter.Set(scanStats{
+			Portion: textui.Portion[btrfsvol.PhysicalAddr]{
+				N: pos,
+				D: devSize,
+			},
+			NumFoundNodes:       len(result.FoundNodes),
+			NumFoundChunks:      len(result.FoundChunks),
+			NumFoundBlockGroups: len(result.FoundBlockGroups),
+			NumFoundDevExtents:  len(result.FoundDevExtents),
+			NumFoundExtentCSums: len(result.FoundExtentCSums),
+		})
+	}
+
+	var minNextNode btrfsvol.PhysicalAddr
+	for i := 0; i < numSums; i++ {
+		if ctx.Err() != nil {
+			return result, ctx.Err()
+		}
+		pos := btrfsvol.PhysicalAddr(i * btrfssum.BlockSize)
+		progress(pos)
+
+		sum, err := btrfs.ChecksumPhysical(dev, alg, pos)
+		if err != nil {
+			return result, err
+		}
+		sums.Write(sum[:csumSize])
+
+		checkForNode := pos >= minNextNode && pos+btrfsvol.PhysicalAddr(sb.NodeSize) <= devSize
+		if checkForNode {
+			for _, sbAddr := range btrfs.SuperblockAddrs {
+				if sbAddr <= pos && pos < sbAddr+sbSize {
+					checkForNode = false
+					break
+				}
+			}
+		}
+
+		if checkForNode {
+			nodeRef, err := btrfstree.ReadNode[btrfsvol.PhysicalAddr](dev, sb, pos, btrfstree.NodeExpectations{})
+			if err != nil {
+				if !errors.Is(err, btrfstree.ErrNotANode) {
+					dlog.Errorf(ctx, "error: %v", err)
+				}
+			} else {
+				result.FoundNodes[nodeRef.Data.Head.Addr] = append(result.FoundNodes[nodeRef.Data.Head.Addr], nodeRef.Addr)
+				for i, item := range nodeRef.Data.BodyLeaf {
+					switch item.Key.ItemType {
+					case btrfsitem.CHUNK_ITEM_KEY:
+						switch itemBody := item.Body.(type) {
+						case *btrfsitem.Chunk:
+							dlog.Tracef(ctx, "node@%v: item %v: found chunk",
+								nodeRef.Addr, i)
+							result.FoundChunks = append(result.FoundChunks, btrfstree.SysChunk{
+								Key:   item.Key,
+								Chunk: *itemBody,
+							})
+						case *btrfsitem.Error:
+							dlog.Errorf(ctx, "node@%v: item %v: error: malformed CHUNK_ITEM: %v",
+								nodeRef.Addr, i, itemBody.Err)
+						default:
+							panic(fmt.Errorf("should not happen: CHUNK_ITEM has unexpected item type: %T", itemBody))
+						}
+					case btrfsitem.BLOCK_GROUP_ITEM_KEY:
+						switch itemBody := item.Body.(type) {
+						case *btrfsitem.BlockGroup:
+							dlog.Tracef(ctx, "node@%v: item %v: found block group",
+								nodeRef.Addr, i)
+							result.FoundBlockGroups = append(result.FoundBlockGroups, SysBlockGroup{
+								Key: item.Key,
+								BG:  *itemBody,
+							})
+						case *btrfsitem.Error:
+							dlog.Errorf(ctx, "node@%v: item %v: error: malformed BLOCK_GROUP_ITEM: %v",
+								nodeRef.Addr, i, itemBody.Err)
+						default:
+							panic(fmt.Errorf("should not happen: BLOCK_GROUP_ITEM has unexpected item type: %T", itemBody))
+						}
+					case btrfsitem.DEV_EXTENT_KEY:
+						switch itemBody := item.Body.(type) {
+						case *btrfsitem.DevExtent:
+							dlog.Tracef(ctx, "node@%v: item %v: found dev extent",
+								nodeRef.Addr, i)
+							result.FoundDevExtents = append(result.FoundDevExtents, SysDevExtent{
+								Key:    item.Key,
+								DevExt: *itemBody,
+							})
+						case *btrfsitem.Error:
+							dlog.Errorf(ctx, "node@%v: item %v: error: malformed DEV_EXTENT: %v",
+								nodeRef.Addr, i, itemBody.Err)
+						default:
+							panic(fmt.Errorf("should not happen: DEV_EXTENT has unexpected item type: %T", itemBody))
+						}
+					case btrfsitem.EXTENT_CSUM_KEY:
+						switch itemBody := item.Body.(type) {
+						case *btrfsitem.ExtentCSum:
+							dlog.Tracef(ctx, "node@%v: item %v: found csums",
+								nodeRef.Addr, i)
+							result.FoundExtentCSums = append(result.FoundExtentCSums, SysExtentCSum{
+								Generation: nodeRef.Data.Head.Generation,
+								Sums:       *itemBody,
+							})
+						case *btrfsitem.Error:
+							dlog.Errorf(ctx, "node@%v: item %v: error: malformed is EXTENT_CSUM: %v",
+								nodeRef.Addr, i, itemBody.Err)
+						default:
+							panic(fmt.Errorf("should not happen: EXTENT_CSUM has unexpected item type: %T", itemBody))
+						}
+					}
+				}
+				minNextNode = pos + btrfsvol.PhysicalAddr(sb.NodeSize)
+			}
+			btrfstree.FreeNodeRef(nodeRef)
+		}
+	}
+	progress(devSize)
+	progressWriter.Done()
+
+	result.Checksums = btrfssum.SumRun[btrfsvol.PhysicalAddr]{
+		ChecksumSize: csumSize,
+		Sums:         btrfssum.ShortSum(sums.String()),
+	}
+
+	return result, nil
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/sumrunwithgaps.go b/cmd/btrfs-rec/inspect/rebuildmappings/sumrunwithgaps.go
new file mode 100644
index 0000000..f79e2be
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/sumrunwithgaps.go
@@ -0,0 +1,167 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package rebuildmappings
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"math"
+
+	"git.lukeshu.com/go/lowmemjson"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/slices"
+)
+
+type SumRunWithGaps[Addr btrfsvol.IntAddr[Addr]] struct {
+	// Store the start address and size, in order to facilitate
+	// leading and trailing gaps.
+	Addr Addr
+	Size btrfsvol.AddrDelta
+
+	Runs []btrfssum.SumRun[Addr]
+}
+
+var (
+	_ lowmemjson.Encodable = SumRunWithGaps[btrfsvol.LogicalAddr]{}
+	_ lowmemjson.Decodable = (*SumRunWithGaps[btrfsvol.LogicalAddr])(nil)
+)
+
+// PatLen implements kmpPattern[int, ShortSum].
+func (sg SumRunWithGaps[Addr]) PatLen() int {
+	return int(sg.Size / btrfssum.BlockSize)
+}
+
+func (sg SumRunWithGaps[Addr]) PctFull() float64 {
+	total := sg.PatLen()
+	var full int
+	for _, run := range sg.Runs {
+		full += run.SeqLen()
+	}
+	return float64(full) / float64(total)
+}
+
+func (sg SumRunWithGaps[Addr]) RunForAddr(addr Addr) (btrfssum.SumRun[Addr], Addr, bool) {
+	for _, run := range sg.Runs {
+		if run.Addr > addr {
+			return btrfssum.SumRun[Addr]{}, run.Addr, false
+		}
+		if run.Addr.Add(run.Size()) <= addr {
+			continue
+		}
+		return run, 0, true
+	}
+	return btrfssum.SumRun[Addr]{}, math.MaxInt64, false
+}
+
+func (sg SumRunWithGaps[Addr]) SumForAddr(addr Addr) (btrfssum.ShortSum, bool) {
+	if addr < sg.Addr || addr >= sg.Addr.Add(sg.Size) {
+		return "", false
+	}
+	runIdx, ok := slices.Search(sg.Runs, func(run btrfssum.SumRun[Addr]) int {
+		switch {
+		case addr < run.Addr:
+			return -1
+		case addr >= run.Addr.Add(run.Size()):
+			return 1
+		default:
+			return 0
+		}
+	})
+	if !ok {
+		return "", false
+	}
+	run := sg.Runs[runIdx]
+	off := int((addr-run.Addr)/btrfssum.BlockSize) * run.ChecksumSize
+	return run.Sums[off : off+run.ChecksumSize], true
+}
+
+func (sg SumRunWithGaps[Addr]) Walk(ctx context.Context, fn func(Addr, btrfssum.ShortSum) error) error {
+	for _, run := range sg.Runs {
+		if err := run.Walk(ctx, fn); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// PatGet implements kmpPattern[int, ShortSum].
+func (sg SumRunWithGaps[Addr]) PatGet(sumIdx int) (btrfssum.ShortSum, bool) {
+	addr := sg.Addr.Add(btrfsvol.AddrDelta(sumIdx) * btrfssum.BlockSize)
+	return sg.SumForAddr(addr)
+}
+
+func (sg SumRunWithGaps[Addr]) EncodeJSON(w io.Writer) error {
+	if _, err := fmt.Fprintf(w, `{"Addr":%d,"Size":%d,"Runs":[`, sg.Addr, sg.Size); err != nil {
+		return err
+	}
+	cur := sg.Addr
+	for i, run := range sg.Runs {
+		if i > 0 {
+			if _, err := w.Write([]byte{','}); err != nil {
+				return err
+			}
+		}
+		switch {
+		case run.Addr < cur:
+			return fmt.Errorf("invalid %T: addr went backwards: %v < %v", sg, run.Addr, cur)
+		case run.Addr > cur:
+			if _, err := fmt.Fprintf(w, `{"Gap":%d},`, run.Addr.Sub(cur)); err != nil {
+				return err
+			}
+			fallthrough
+		default:
+			if err := lowmemjson.NewEncoder(w).Encode(run); err != nil {
+				return err
+			}
+			cur = run.Addr.Add(run.Size())
+		}
+	}
+	end := sg.Addr.Add(sg.Size)
+	switch {
+	case end < cur:
+		return fmt.Errorf("invalid %T: addr went backwards: %v < %v", sg, end, cur)
+	case end > cur:
+		if _, err := fmt.Fprintf(w, `,{"Gap":%d}`, end.Sub(cur)); err != nil {
+			return err
+		}
+	}
+	if _, err := w.Write([]byte("]}")); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (sg *SumRunWithGaps[Addr]) DecodeJSON(r io.RuneScanner) error {
+	*sg = SumRunWithGaps[Addr]{}
+	var name string
+	return lowmemjson.DecodeObject(r,
+		func(r io.RuneScanner) error {
+			return lowmemjson.NewDecoder(r).Decode(&name)
+		},
+		func(r io.RuneScanner) error {
+			switch name {
+			case "Addr":
+				return lowmemjson.NewDecoder(r).Decode(&sg.Addr)
+			case "Size":
+				return lowmemjson.NewDecoder(r).Decode(&sg.Size)
+			case "Runs":
+				return lowmemjson.DecodeArray(r, func(r io.RuneScanner) error {
+					var run btrfssum.SumRun[Addr]
+					if err := lowmemjson.NewDecoder(r).Decode(&run); err != nil {
+						return err
+					}
+					if run.ChecksumSize > 0 {
+						sg.Runs = append(sg.Runs, run)
+					}
+					return nil
+				})
+			default:
+				return fmt.Errorf("unknown key %q", name)
+			}
+		})
+}
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/593a5dd328ee86bac6820e3f98383aebadd2af3a22aa248546a228b08451c30d b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/593a5dd328ee86bac6820e3f98383aebadd2af3a22aa248546a228b08451c30d
new file mode 100644
index 0000000..9d14adf
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/593a5dd328ee86bac6820e3f98383aebadd2af3a22aa248546a228b08451c30d
@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("0")
+[]byte("1")
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/6bd9babbebf7eb78814e1f3425bfa6ccd6bfd42824a26e8cbd63b5117934e600 b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/6bd9babbebf7eb78814e1f3425bfa6ccd6bfd42824a26e8cbd63b5117934e600
new file mode 100644
index 0000000..269f061
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/6bd9babbebf7eb78814e1f3425bfa6ccd6bfd42824a26e8cbd63b5117934e600
@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("0")
+[]byte("0")
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/84ed65595ad05a58e293dbf423c1a816b697e2763a29d7c37aa476d6eef6fd60 b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/84ed65595ad05a58e293dbf423c1a816b697e2763a29d7c37aa476d6eef6fd60
new file mode 100644
index 0000000..b8f1562
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/84ed65595ad05a58e293dbf423c1a816b697e2763a29d7c37aa476d6eef6fd60
@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("0")
+[]byte("")
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/9be40f71bc49b1b5c3b8e08d58d0f69cc22aefb12fd9b5931e49ff0e41953738 b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/9be40f71bc49b1b5c3b8e08d58d0f69cc22aefb12fd9b5931e49ff0e41953738
new file mode 100644
index 0000000..be67506
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/9be40f71bc49b1b5c3b8e08d58d0f69cc22aefb12fd9b5931e49ff0e41953738
@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("\xde\xdb!")
+[]byte("\xde\xdb")
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/e43317ec61b0da9627d7b2fc0237d369082a6bf5dfe39e05c113b42ff6218d5d b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/e43317ec61b0da9627d7b2fc0237d369082a6bf5dfe39e05c113b42ff6218d5d
new file mode 100644
index 0000000..c3bfa37
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/testdata/fuzz/FuzzIndexAll/e43317ec61b0da9627d7b2fc0237d369082a6bf5dfe39e05c113b42ff6218d5d
@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("\x10\x10\x15")
+[]byte("\x10\x15")