fs/erofs: add lz4 decompression support

Support EROFS lz4 compressed files.

Signed-off-by: Huang Jianan <jnhuang95@gmail.com>

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index f4b2d51a23949db19be2f21d536cf415b68c88e6..ee4e777c5c82127372976ebbca48c27bf7ea68ff 100644 (file)
--- a/fs/erofs/Kconfig
+++ b/fs/erofs/Kconfig
@@ -10,3 +10,12 @@ config FS_EROFS
          improves storage density, keeps relatively higher compression
          ratios, which is more useful to achieve high performance for
          embedded devices with limited memory.
+
+config FS_EROFS_ZIP
+       bool "EROFS Data Compression Support"
+       depends on FS_EROFS
+       select LZ4
+       default y
+       help
+         Enable fixed-sized output compression for EROFS.
+         If you don't want to enable compression feature, say N.

diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index 7398ab7a365af95d1f3c29a7df38193b6d326767..58af6a68e4131e2fbfbff337986737d8c3ec7968 100644 (file)
--- a/fs/erofs/Makefile
+++ b/fs/erofs/Makefile
@@ -4,4 +4,6 @@
 obj-$(CONFIG_$(SPL_)FS_EROFS) = fs.o \
                                super.o \
                                namei.o \
-                               data.o
+                               data.o \
+                               decompress.o \
+                               zmap.o

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 699975c1be41912ce27394f8b970b5619206d31c..761896054c83e3e131b500270348608d14c3ae83 100644 (file)
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0+
 #include "internal.h"
+#include "decompress.h"
 
 static int erofs_map_blocks_flatmode(struct erofs_inode *inode,
                                     struct erofs_map_blocks *map,
@@ -205,6 +206,93 @@ static int erofs_read_raw_data(struct erofs_inode *inode, char *buffer,
        return 0;
 }
 
+static int z_erofs_read_data(struct erofs_inode *inode, char *buffer,
+                            erofs_off_t size, erofs_off_t offset)
+{
+       erofs_off_t end, length, skip;
+       struct erofs_map_blocks map = {
+               .index = UINT_MAX,
+       };
+       struct erofs_map_dev mdev;
+       bool partial;
+       unsigned int bufsize = 0;
+       char *raw = NULL;
+       int ret = 0;
+
+       end = offset + size;
+       while (end > offset) {
+               map.m_la = end - 1;
+
+               ret = z_erofs_map_blocks_iter(inode, &map, 0);
+               if (ret)
+                       break;
+
+               /* no device id here, thus it will always succeed */
+               mdev = (struct erofs_map_dev) {
+                       .m_pa = map.m_pa,
+               };
+               ret = erofs_map_dev(&sbi, &mdev);
+               if (ret) {
+                       DBG_BUGON(1);
+                       break;
+               }
+
+               /*
+                * trim to the needed size if the returned extent is quite
+                * larger than requested, and set up partial flag as well.
+                */
+               if (end < map.m_la + map.m_llen) {
+                       length = end - map.m_la;
+                       partial = true;
+               } else {
+                       DBG_BUGON(end != map.m_la + map.m_llen);
+                       length = map.m_llen;
+                       partial = !(map.m_flags & EROFS_MAP_FULL_MAPPED);
+               }
+
+               if (map.m_la < offset) {
+                       skip = offset - map.m_la;
+                       end = offset;
+               } else {
+                       skip = 0;
+                       end = map.m_la;
+               }
+
+               if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+                       memset(buffer + end - offset, 0, length);
+                       end = map.m_la;
+                       continue;
+               }
+
+               if (map.m_plen > bufsize) {
+                       bufsize = map.m_plen;
+                       raw = realloc(raw, bufsize);
+                       if (!raw) {
+                               ret = -ENOMEM;
+                               break;
+                       }
+               }
+               ret = erofs_dev_read(mdev.m_deviceid, raw, mdev.m_pa, map.m_plen);
+               if (ret < 0)
+                       break;
+
+               ret = z_erofs_decompress(&(struct z_erofs_decompress_req) {
+                                       .in = raw,
+                                       .out = buffer + end - offset,
+                                       .decodedskip = skip,
+                                       .inputsize = map.m_plen,
+                                       .decodedlength = length,
+                                       .alg = map.m_algorithmformat,
+                                       .partial_decoding = partial
+                                        });
+               if (ret < 0)
+                       break;
+       }
+       if (raw)
+               free(raw);
+       return ret < 0 ? ret : 0;
+}
+
 int erofs_pread(struct erofs_inode *inode, char *buf,
                erofs_off_t count, erofs_off_t offset)
 {
@@ -215,7 +303,7 @@ int erofs_pread(struct erofs_inode *inode, char *buf,
                return erofs_read_raw_data(inode, buf, count, offset);
        case EROFS_INODE_FLAT_COMPRESSION_LEGACY:
        case EROFS_INODE_FLAT_COMPRESSION:
-               return -EOPNOTSUPP;
+               return z_erofs_read_data(inode, buf, count, offset);
        default:
                break;
        }

diff --git a/fs/erofs/decompress.c b/fs/erofs/decompress.c
new file mode 100644 (file)
index 0000000..2be3b84
--- /dev/null
+++ b/fs/erofs/decompress.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0+
+#include "decompress.h"
+
+#if IS_ENABLED(CONFIG_LZ4)
+#include <u-boot/lz4.h>
+static int z_erofs_decompress_lz4(struct z_erofs_decompress_req *rq)
+{
+       int ret = 0;
+       char *dest = rq->out;
+       char *src = rq->in;
+       char *buff = NULL;
+       bool support_0padding = false;
+       unsigned int inputmargin = 0;
+
+       if (erofs_sb_has_lz4_0padding()) {
+               support_0padding = true;
+
+               while (!src[inputmargin & ~PAGE_MASK])
+                       if (!(++inputmargin & ~PAGE_MASK))
+                               break;
+
+               if (inputmargin >= rq->inputsize)
+                       return -EIO;
+       }
+
+       if (rq->decodedskip) {
+               buff = malloc(rq->decodedlength);
+               if (!buff)
+                       return -ENOMEM;
+               dest = buff;
+       }
+
+       if (rq->partial_decoding || !support_0padding)
+               ret = LZ4_decompress_safe_partial(src + inputmargin, dest,
+                                                 rq->inputsize - inputmargin,
+                                                 rq->decodedlength, rq->decodedlength);
+       else
+               ret = LZ4_decompress_safe(src + inputmargin, dest,
+                                         rq->inputsize - inputmargin,
+                                         rq->decodedlength);
+
+       if (ret != (int)rq->decodedlength) {
+               ret = -EIO;
+               goto out;
+       }
+
+       if (rq->decodedskip)
+               memcpy(rq->out, dest + rq->decodedskip,
+                      rq->decodedlength - rq->decodedskip);
+
+out:
+       if (buff)
+               free(buff);
+
+       return ret;
+}
+#endif
+
+int z_erofs_decompress(struct z_erofs_decompress_req *rq)
+{
+       if (rq->alg == Z_EROFS_COMPRESSION_SHIFTED) {
+               if (rq->inputsize != EROFS_BLKSIZ)
+                       return -EFSCORRUPTED;
+
+               DBG_BUGON(rq->decodedlength > EROFS_BLKSIZ);
+               DBG_BUGON(rq->decodedlength < rq->decodedskip);
+
+               memcpy(rq->out, rq->in + rq->decodedskip,
+                      rq->decodedlength - rq->decodedskip);
+               return 0;
+       }
+
+#if IS_ENABLED(CONFIG_LZ4)
+       if (rq->alg == Z_EROFS_COMPRESSION_LZ4)
+               return z_erofs_decompress_lz4(rq);
+#endif
+       return -EOPNOTSUPP;
+}

diff --git a/fs/erofs/decompress.h b/fs/erofs/decompress.h
new file mode 100644 (file)
index 0000000..81d5fb8
--- /dev/null
+++ b/fs/erofs/decompress.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+#ifndef __EROFS_DECOMPRESS_H
+#define __EROFS_DECOMPRESS_H
+
+#include "internal.h"
+
+struct z_erofs_decompress_req {
+       char *in, *out;
+
+       /*
+        * initial decompressed bytes that need to be skipped
+        * when finally copying to output buffer
+        */
+       unsigned int decodedskip;
+       unsigned int inputsize, decodedlength;
+
+       /* indicate the algorithm will be used for decompression */
+       unsigned int alg;
+       bool partial_decoding;
+};
+
+int z_erofs_decompress(struct z_erofs_decompress_req *rq);
+
+#endif

diff --git a/fs/erofs/namei.c b/fs/erofs/namei.c
index f1195a09ea652e0f7534cc1afa801208df894b62..d1d4757c5071bb6022bd26ce4f4f5ba8bcb1db21 100644 (file)
--- a/fs/erofs/namei.c
+++ b/fs/erofs/namei.c
@@ -114,7 +114,7 @@ int erofs_read_inode_from_disk(struct erofs_inode *vi)
                vi->u.chunkbits = LOG_BLOCK_SIZE +
                        (vi->u.chunkformat & EROFS_CHUNK_FORMAT_BLKBITS_MASK);
        } else if (erofs_inode_is_data_compressed(vi->datalayout))
-               return -EOPNOTSUPP;
+               z_erofs_fill_inode(vi);
        return 0;
 bogusimode:
        erofs_err("bogus i_mode (%o) @ nid %llu", vi->i_mode, vi->nid | 0ULL);

diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
new file mode 100644 (file)
index 0000000..be2599a
--- /dev/null
+++ b/fs/erofs/zmap.c
@@ -0,0 +1,601 @@
+// SPDX-License-Identifier: GPL-2.0+
+#include "internal.h"
+
+int z_erofs_fill_inode(struct erofs_inode *vi)
+{
+       if (!erofs_sb_has_big_pcluster() &&
+           vi->datalayout == EROFS_INODE_FLAT_COMPRESSION_LEGACY) {
+               vi->z_advise = 0;
+               vi->z_algorithmtype[0] = 0;
+               vi->z_algorithmtype[1] = 0;
+               vi->z_logical_clusterbits = LOG_BLOCK_SIZE;
+
+               vi->flags |= EROFS_I_Z_INITED;
+       }
+       return 0;
+}
+
+static int z_erofs_fill_inode_lazy(struct erofs_inode *vi)
+{
+       int ret;
+       erofs_off_t pos;
+       struct z_erofs_map_header *h;
+       char buf[sizeof(struct z_erofs_map_header)];
+
+       if (vi->flags & EROFS_I_Z_INITED)
+               return 0;
+
+       DBG_BUGON(!erofs_sb_has_big_pcluster() &&
+                 vi->datalayout == EROFS_INODE_FLAT_COMPRESSION_LEGACY);
+       pos = round_up(iloc(vi->nid) + vi->inode_isize + vi->xattr_isize, 8);
+
+       ret = erofs_dev_read(0, buf, pos, sizeof(buf));
+       if (ret < 0)
+               return -EIO;
+
+       h = (struct z_erofs_map_header *)buf;
+       vi->z_advise = le16_to_cpu(h->h_advise);
+       vi->z_algorithmtype[0] = h->h_algorithmtype & 15;
+       vi->z_algorithmtype[1] = h->h_algorithmtype >> 4;
+
+       if (vi->z_algorithmtype[0] >= Z_EROFS_COMPRESSION_MAX) {
+               erofs_err("unknown compression format %u for nid %llu",
+                         vi->z_algorithmtype[0], (unsigned long long)vi->nid);
+               return -EOPNOTSUPP;
+       }
+
+       vi->z_logical_clusterbits = LOG_BLOCK_SIZE + (h->h_clusterbits & 7);
+       if (vi->datalayout == EROFS_INODE_FLAT_COMPRESSION &&
+           !(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_1) ^
+           !(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_2)) {
+               erofs_err("big pcluster head1/2 of compact indexes should be consistent for nid %llu",
+                         vi->nid * 1ULL);
+               return -EFSCORRUPTED;
+       }
+       vi->flags |= EROFS_I_Z_INITED;
+       return 0;
+}
+
+struct z_erofs_maprecorder {
+       struct erofs_inode *inode;
+       struct erofs_map_blocks *map;
+       void *kaddr;
+
+       unsigned long lcn;
+       /* compression extent information gathered */
+       u8  type, headtype;
+       u16 clusterofs;
+       u16 delta[2];
+       erofs_blk_t pblk, compressedlcs;
+};
+
+static int z_erofs_reload_indexes(struct z_erofs_maprecorder *m,
+                                 erofs_blk_t eblk)
+{
+       int ret;
+       struct erofs_map_blocks *const map = m->map;
+       char *mpage = map->mpage;
+
+       if (map->index == eblk)
+               return 0;
+
+       ret = erofs_blk_read(mpage, eblk, 1);
+       if (ret < 0)
+               return -EIO;
+
+       map->index = eblk;
+
+       return 0;
+}
+
+static int legacy_load_cluster_from_disk(struct z_erofs_maprecorder *m,
+                                        unsigned long lcn)
+{
+       struct erofs_inode *const vi = m->inode;
+       const erofs_off_t ibase = iloc(vi->nid);
+       const erofs_off_t pos =
+               Z_EROFS_VLE_LEGACY_INDEX_ALIGN(ibase + vi->inode_isize +
+                                              vi->xattr_isize) +
+               lcn * sizeof(struct z_erofs_vle_decompressed_index);
+       struct z_erofs_vle_decompressed_index *di;
+       unsigned int advise, type;
+       int err;
+
+       err = z_erofs_reload_indexes(m, erofs_blknr(pos));
+       if (err)
+               return err;
+
+       m->lcn = lcn;
+       di = m->kaddr + erofs_blkoff(pos);
+
+       advise = le16_to_cpu(di->di_advise);
+       type = (advise >> Z_EROFS_VLE_DI_CLUSTER_TYPE_BIT) &
+               ((1 << Z_EROFS_VLE_DI_CLUSTER_TYPE_BITS) - 1);
+       switch (type) {
+       case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
+               m->clusterofs = 1 << vi->z_logical_clusterbits;
+               m->delta[0] = le16_to_cpu(di->di_u.delta[0]);
+               if (m->delta[0] & Z_EROFS_VLE_DI_D0_CBLKCNT) {
+                       if (!(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_1)) {
+                               DBG_BUGON(1);
+                               return -EFSCORRUPTED;
+                       }
+                       m->compressedlcs = m->delta[0] &
+                               ~Z_EROFS_VLE_DI_D0_CBLKCNT;
+                       m->delta[0] = 1;
+               }
+               m->delta[1] = le16_to_cpu(di->di_u.delta[1]);
+               break;
+       case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
+       case Z_EROFS_VLE_CLUSTER_TYPE_HEAD:
+               m->clusterofs = le16_to_cpu(di->di_clusterofs);
+               m->pblk = le32_to_cpu(di->di_u.blkaddr);
+               break;
+       default:
+               DBG_BUGON(1);
+               return -EOPNOTSUPP;
+       }
+       m->type = type;
+       return 0;
+}
+
+static unsigned int decode_compactedbits(unsigned int lobits,
+                                        unsigned int lomask,
+                                        u8 *in, unsigned int pos, u8 *type)
+{
+       const unsigned int v = get_unaligned_le32(in + pos / 8) >> (pos & 7);
+       const unsigned int lo = v & lomask;
+
+       *type = (v >> lobits) & 3;
+       return lo;
+}
+
+static int get_compacted_la_distance(unsigned int lclusterbits,
+                                    unsigned int encodebits,
+                                    unsigned int vcnt, u8 *in, int i)
+{
+       const unsigned int lomask = (1 << lclusterbits) - 1;
+       unsigned int lo, d1 = 0;
+       u8 type;
+
+       DBG_BUGON(i >= vcnt);
+
+       do {
+               lo = decode_compactedbits(lclusterbits, lomask,
+                                         in, encodebits * i, &type);
+
+               if (type != Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD)
+                       return d1;
+               ++d1;
+       } while (++i < vcnt);
+
+       /* vcnt - 1 (Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD) item */
+       if (!(lo & Z_EROFS_VLE_DI_D0_CBLKCNT))
+               d1 += lo - 1;
+       return d1;
+}
+
+static int unpack_compacted_index(struct z_erofs_maprecorder *m,
+                                 unsigned int amortizedshift,
+                                 unsigned int eofs, bool lookahead)
+{
+       struct erofs_inode *const vi = m->inode;
+       const unsigned int lclusterbits = vi->z_logical_clusterbits;
+       const unsigned int lomask = (1 << lclusterbits) - 1;
+       unsigned int vcnt, base, lo, encodebits, nblk;
+       int i;
+       u8 *in, type;
+       bool big_pcluster;
+
+       if (1 << amortizedshift == 4)
+               vcnt = 2;
+       else if (1 << amortizedshift == 2 && lclusterbits == 12)
+               vcnt = 16;
+       else
+               return -EOPNOTSUPP;
+
+       big_pcluster = vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_1;
+       encodebits = ((vcnt << amortizedshift) - sizeof(__le32)) * 8 / vcnt;
+       base = round_down(eofs, vcnt << amortizedshift);
+       in = m->kaddr + base;
+
+       i = (eofs - base) >> amortizedshift;
+
+       lo = decode_compactedbits(lclusterbits, lomask,
+                                 in, encodebits * i, &type);
+       m->type = type;
+       if (type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD) {
+               m->clusterofs = 1 << lclusterbits;
+
+               /* figure out lookahead_distance: delta[1] if needed */
+               if (lookahead)
+                       m->delta[1] = get_compacted_la_distance(lclusterbits,
+                                                               encodebits,
+                                                               vcnt, in, i);
+               if (lo & Z_EROFS_VLE_DI_D0_CBLKCNT) {
+                       if (!big_pcluster) {
+                               DBG_BUGON(1);
+                               return -EFSCORRUPTED;
+                       }
+                       m->compressedlcs = lo & ~Z_EROFS_VLE_DI_D0_CBLKCNT;
+                       m->delta[0] = 1;
+                       return 0;
+               } else if (i + 1 != (int)vcnt) {
+                       m->delta[0] = lo;
+                       return 0;
+               }
+               /*
+                * since the last lcluster in the pack is special,
+                * of which lo saves delta[1] rather than delta[0].
+                * Hence, get delta[0] by the previous lcluster indirectly.
+                */
+               lo = decode_compactedbits(lclusterbits, lomask,
+                                         in, encodebits * (i - 1), &type);
+               if (type != Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD)
+                       lo = 0;
+               else if (lo & Z_EROFS_VLE_DI_D0_CBLKCNT)
+                       lo = 1;
+               m->delta[0] = lo + 1;
+               return 0;
+       }
+       m->clusterofs = lo;
+       m->delta[0] = 0;
+       /* figout out blkaddr (pblk) for HEAD lclusters */
+       if (!big_pcluster) {
+               nblk = 1;
+               while (i > 0) {
+                       --i;
+                       lo = decode_compactedbits(lclusterbits, lomask,
+                                                 in, encodebits * i, &type);
+                       if (type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD)
+                               i -= lo;
+
+                       if (i >= 0)
+                               ++nblk;
+               }
+       } else {
+               nblk = 0;
+               while (i > 0) {
+                       --i;
+                       lo = decode_compactedbits(lclusterbits, lomask,
+                                                 in, encodebits * i, &type);
+                       if (type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD) {
+                               if (lo & Z_EROFS_VLE_DI_D0_CBLKCNT) {
+                                       --i;
+                                       nblk += lo & ~Z_EROFS_VLE_DI_D0_CBLKCNT;
+                                       continue;
+                               }
+                               if (lo == 1) {
+                                       DBG_BUGON(1);
+                                       /* --i; ++nblk; continue; */
+                                       return -EFSCORRUPTED;
+                               }
+                               i -= lo - 2;
+                               continue;
+                       }
+                       ++nblk;
+               }
+       }
+       in += (vcnt << amortizedshift) - sizeof(__le32);
+       m->pblk = le32_to_cpu(*(__le32 *)in) + nblk;
+       return 0;
+}
+
+static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
+                                           unsigned long lcn, bool lookahead)
+{
+       struct erofs_inode *const vi = m->inode;
+       const unsigned int lclusterbits = vi->z_logical_clusterbits;
+       const erofs_off_t ebase = round_up(iloc(vi->nid) + vi->inode_isize +
+                                          vi->xattr_isize, 8) +
+               sizeof(struct z_erofs_map_header);
+       const unsigned int totalidx = DIV_ROUND_UP(vi->i_size, EROFS_BLKSIZ);
+       unsigned int compacted_4b_initial, compacted_2b;
+       unsigned int amortizedshift;
+       erofs_off_t pos;
+       int err;
+
+       if (lclusterbits != 12)
+               return -EOPNOTSUPP;
+
+       if (lcn >= totalidx)
+               return -EINVAL;
+
+       m->lcn = lcn;
+       /* used to align to 32-byte (compacted_2b) alignment */
+       compacted_4b_initial = (32 - ebase % 32) / 4;
+       if (compacted_4b_initial == 32 / 4)
+               compacted_4b_initial = 0;
+
+       if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
+               compacted_2b = rounddown(totalidx - compacted_4b_initial, 16);
+       else
+               compacted_2b = 0;
+
+       pos = ebase;
+       if (lcn < compacted_4b_initial) {
+               amortizedshift = 2;
+               goto out;
+       }
+       pos += compacted_4b_initial * 4;
+       lcn -= compacted_4b_initial;
+
+       if (lcn < compacted_2b) {
+               amortizedshift = 1;
+               goto out;
+       }
+       pos += compacted_2b * 2;
+       lcn -= compacted_2b;
+       amortizedshift = 2;
+out:
+       pos += lcn * (1 << amortizedshift);
+       err = z_erofs_reload_indexes(m, erofs_blknr(pos));
+       if (err)
+               return err;
+       return unpack_compacted_index(m, amortizedshift, erofs_blkoff(pos),
+                                     lookahead);
+}
+
+static int z_erofs_load_cluster_from_disk(struct z_erofs_maprecorder *m,
+                                         unsigned int lcn, bool lookahead)
+{
+       const unsigned int datamode = m->inode->datalayout;
+
+       if (datamode == EROFS_INODE_FLAT_COMPRESSION_LEGACY)
+               return legacy_load_cluster_from_disk(m, lcn);
+
+       if (datamode == EROFS_INODE_FLAT_COMPRESSION)
+               return compacted_load_cluster_from_disk(m, lcn, lookahead);
+
+       return -EINVAL;
+}
+
+static int z_erofs_extent_lookback(struct z_erofs_maprecorder *m,
+                                  unsigned int lookback_distance)
+{
+       struct erofs_inode *const vi = m->inode;
+       struct erofs_map_blocks *const map = m->map;
+       const unsigned int lclusterbits = vi->z_logical_clusterbits;
+       unsigned long lcn = m->lcn;
+       int err;
+
+       if (lcn < lookback_distance) {
+               erofs_err("bogus lookback distance @ nid %llu",
+                         (unsigned long long)vi->nid);
+               DBG_BUGON(1);
+               return -EFSCORRUPTED;
+       }
+
+       /* load extent head logical cluster if needed */
+       lcn -= lookback_distance;
+       err = z_erofs_load_cluster_from_disk(m, lcn, false);
+       if (err)
+               return err;
+
+       switch (m->type) {
+       case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
+               if (!m->delta[0]) {
+                       erofs_err("invalid lookback distance 0 @ nid %llu",
+                                 (unsigned long long)vi->nid);
+                       DBG_BUGON(1);
+                       return -EFSCORRUPTED;
+               }
+               return z_erofs_extent_lookback(m, m->delta[0]);
+       case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
+       case Z_EROFS_VLE_CLUSTER_TYPE_HEAD:
+               m->headtype = m->type;
+               map->m_la = (lcn << lclusterbits) | m->clusterofs;
+               break;
+       default:
+               erofs_err("unknown type %u @ lcn %lu of nid %llu",
+                         m->type, lcn, (unsigned long long)vi->nid);
+               DBG_BUGON(1);
+               return -EOPNOTSUPP;
+       }
+       return 0;
+}
+
+static int z_erofs_get_extent_compressedlen(struct z_erofs_maprecorder *m,
+                                           unsigned int initial_lcn)
+{
+       struct erofs_inode *const vi = m->inode;
+       struct erofs_map_blocks *const map = m->map;
+       const unsigned int lclusterbits = vi->z_logical_clusterbits;
+       unsigned long lcn;
+       int err;
+
+       DBG_BUGON(m->type != Z_EROFS_VLE_CLUSTER_TYPE_PLAIN &&
+                 m->type != Z_EROFS_VLE_CLUSTER_TYPE_HEAD);
+       if (m->headtype == Z_EROFS_VLE_CLUSTER_TYPE_PLAIN ||
+           !(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_1)) {
+               map->m_plen = 1 << lclusterbits;
+               return 0;
+       }
+
+       lcn = m->lcn + 1;
+       if (m->compressedlcs)
+               goto out;
+
+       err = z_erofs_load_cluster_from_disk(m, lcn, false);
+       if (err)
+               return err;
+
+       /*
+        * If the 1st NONHEAD lcluster has already been handled initially w/o
+        * valid compressedlcs, which means at least it mustn't be CBLKCNT, or
+        * an internal implemenatation error is detected.
+        *
+        * The following code can also handle it properly anyway, but let's
+        * BUG_ON in the debugging mode only for developers to notice that.
+        */
+       DBG_BUGON(lcn == initial_lcn &&
+                 m->type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD);
+
+       switch (m->type) {
+       case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
+       case Z_EROFS_VLE_CLUSTER_TYPE_HEAD:
+               /*
+                * if the 1st NONHEAD lcluster is actually PLAIN or HEAD type
+                * rather than CBLKCNT, it's a 1 lcluster-sized pcluster.
+                */
+               m->compressedlcs = 1;
+               break;
+       case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
+               if (m->delta[0] != 1)
+                       goto err_bonus_cblkcnt;
+               if (m->compressedlcs)
+                       break;
+               /* fallthrough */
+       default:
+               erofs_err("cannot found CBLKCNT @ lcn %lu of nid %llu",
+                         lcn, vi->nid | 0ULL);
+               DBG_BUGON(1);
+               return -EFSCORRUPTED;
+       }
+out:
+       map->m_plen = m->compressedlcs << lclusterbits;
+       return 0;
+err_bonus_cblkcnt:
+       erofs_err("bogus CBLKCNT @ lcn %lu of nid %llu",
+                 lcn, vi->nid | 0ULL);
+       DBG_BUGON(1);
+       return -EFSCORRUPTED;
+}
+
+static int z_erofs_get_extent_decompressedlen(struct z_erofs_maprecorder *m)
+{
+       struct erofs_inode *const vi = m->inode;
+       struct erofs_map_blocks *map = m->map;
+       unsigned int lclusterbits = vi->z_logical_clusterbits;
+       u64 lcn = m->lcn, headlcn = map->m_la >> lclusterbits;
+       int err;
+
+       do {
+               /* handle the last EOF pcluster (no next HEAD lcluster) */
+               if ((lcn << lclusterbits) >= vi->i_size) {
+                       map->m_llen = vi->i_size - map->m_la;
+                       return 0;
+               }
+
+               err = z_erofs_load_cluster_from_disk(m, lcn, true);
+               if (err)
+                       return err;
+
+               if (m->type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD) {
+                       DBG_BUGON(!m->delta[1] &&
+                                 m->clusterofs != 1 << lclusterbits);
+               } else if (m->type == Z_EROFS_VLE_CLUSTER_TYPE_PLAIN ||
+                          m->type == Z_EROFS_VLE_CLUSTER_TYPE_HEAD) {
+                       /* go on until the next HEAD lcluster */
+                       if (lcn != headlcn)
+                               break;
+                       m->delta[1] = 1;
+               } else {
+                       erofs_err("unknown type %u @ lcn %llu of nid %llu",
+                                 m->type, lcn | 0ULL,
+                                 (unsigned long long)vi->nid);
+                       DBG_BUGON(1);
+                       return -EOPNOTSUPP;
+               }
+               lcn += m->delta[1];
+       } while (m->delta[1]);
+
+       map->m_llen = (lcn << lclusterbits) + m->clusterofs - map->m_la;
+       return 0;
+}
+
+int z_erofs_map_blocks_iter(struct erofs_inode *vi,
+                           struct erofs_map_blocks *map,
+                           int flags)
+{
+       struct z_erofs_maprecorder m = {
+               .inode = vi,
+               .map = map,
+               .kaddr = map->mpage,
+       };
+       int err = 0;
+       unsigned int lclusterbits, endoff;
+       unsigned long initial_lcn;
+       unsigned long long ofs, end;
+
+       /* when trying to read beyond EOF, leave it unmapped */
+       if (map->m_la >= vi->i_size) {
+               map->m_llen = map->m_la + 1 - vi->i_size;
+               map->m_la = vi->i_size;
+               map->m_flags = 0;
+               goto out;
+       }
+
+       err = z_erofs_fill_inode_lazy(vi);
+       if (err)
+               goto out;
+
+       lclusterbits = vi->z_logical_clusterbits;
+       ofs = map->m_la;
+       initial_lcn = ofs >> lclusterbits;
+       endoff = ofs & ((1 << lclusterbits) - 1);
+
+       err = z_erofs_load_cluster_from_disk(&m, initial_lcn, false);
+       if (err)
+               goto out;
+
+       map->m_flags = EROFS_MAP_MAPPED | EROFS_MAP_ENCODED;
+       end = (m.lcn + 1ULL) << lclusterbits;
+       switch (m.type) {
+       case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
+       case Z_EROFS_VLE_CLUSTER_TYPE_HEAD:
+               if (endoff >= m.clusterofs) {
+                       m.headtype = m.type;
+                       map->m_la = (m.lcn << lclusterbits) | m.clusterofs;
+                       break;
+               }
+               /* m.lcn should be >= 1 if endoff < m.clusterofs */
+               if (!m.lcn) {
+                       erofs_err("invalid logical cluster 0 at nid %llu",
+                                 (unsigned long long)vi->nid);
+                       err = -EFSCORRUPTED;
+                       goto out;
+               }
+               end = (m.lcn << lclusterbits) | m.clusterofs;
+               map->m_flags |= EROFS_MAP_FULL_MAPPED;
+               m.delta[0] = 1;
+               /* fallthrough */
+       case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
+               /* get the correspoinding first chunk */
+               err = z_erofs_extent_lookback(&m, m.delta[0]);
+               if (err)
+                       goto out;
+               break;
+       default:
+               erofs_err("unknown type %u @ offset %llu of nid %llu",
+                         m.type, ofs, (unsigned long long)vi->nid);
+               err = -EOPNOTSUPP;
+               goto out;
+       }
+
+       map->m_llen = end - map->m_la;
+       map->m_pa = blknr_to_addr(m.pblk);
+
+       err = z_erofs_get_extent_compressedlen(&m, initial_lcn);
+       if (err)
+               goto out;
+
+       if (m.headtype == Z_EROFS_VLE_CLUSTER_TYPE_PLAIN)
+               map->m_algorithmformat = Z_EROFS_COMPRESSION_SHIFTED;
+       else
+               map->m_algorithmformat = vi->z_algorithmtype[0];
+
+       if (flags & EROFS_GET_BLOCKS_FIEMAP) {
+               err = z_erofs_get_extent_decompressedlen(&m);
+               if (!err)
+                       map->m_flags |= EROFS_MAP_FULL_MAPPED;
+       }
+
+out:
+       erofs_dbg("m_la %" PRIu64 " m_pa %" PRIu64 " m_llen %" PRIu64 " m_plen %" PRIu64 " m_flags 0%o",
+                 map->m_la, map->m_pa,
+                 map->m_llen, map->m_plen, map->m_flags);
+
+       DBG_BUGON(err < 0 && err != -ENOMEM);
+       return err;
+}