Re: [PATCH v5 1/2] erofs: add 'fsoffset' mount option for file-backed & bdev-based mounts

[Hongbo Li]

On 2025/5/13 21:56, Hongbo Li wrote:

On 2025/5/13 19:34, Sheng Yong wrote:

From: Sheng Yong <shengyo...@xiaomi.com>

When attempting to use an archive file, such as APEX on android,
as a file-backed mount source, it fails because EROFS image within
the archive file does not start at offset 0. As a result, a loop
device is still needed to attach the image file at an appropriate
offset first. Similarly, if an EROFS image within a block device
does not start at offset 0, it cannot be mounted directly either.

To address this issue, this patch adds a new mount option `fsoffset=x'
to accept a start offset for both file-backed and bdev-based mounts.
The offset should be aligned to block size. EROFS will add this offset
before performing read requests.

Signed-off-by: Sheng Yong <shengyo...@xiaomi.com>
Signed-off-by: Wang Shuai <wangshua...@xiaomi.com>
---
  Documentation/filesystems/erofs.rst |  1 +
  fs/erofs/data.c                     |  8 ++++++--
  fs/erofs/fileio.c                   |  3 ++-
  fs/erofs/internal.h                 |  2 ++
  fs/erofs/super.c                    | 12 +++++++++++-
  fs/erofs/zdata.c                    |  3 ++-
  6 files changed, 24 insertions(+), 5 deletions(-)
---
v5: * fix fsoffset on multiple device by adding off when creating io
       request, erofs_map_device selects the target device an only
       primary device has an off
     * remove unnecessary checks of fsoffset value
     * try to combine off and dax_part_off, but it is not easy to do
       that, because dax_part_off is not needed when reading metadata

v4: * change mount option `offset=x' to `fsoffset=x'
https://lore.kernel.org/linux-erofs/c5110e03-90ea-40be-b05f-bc920332a...@linux.alibaba.com

v3: * rename `offs' to `off'
     * parse offset using fsparam_u64 and validate it in fill_super
     * update bi_sector inline

https://lore.kernel.org/linux-erofs/98585dd8-d0b6-4000-b46d-a08c64eae...@linux.alibaba.com

v2: * add a new mount option `offset=X' for start offset, and offset
        should be aligned to PAGE_SIZE
     * add start offset for both file-backed and bdev-based mounts

https://lore.kernel.org/linux-erofs/0725c2ec-528c-42a8-9557-4713e7e35...@linux.alibaba.com

v1: https://lore.kernel.org/all/20250324022849.2715578-1-shengyo...@xiaomi.com/

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst

index c293f8e37468..0fa4c7826203 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst

@@ -128,6 +128,7 @@ device=%s              Specify a path to an extra device to be used together.   fsid=%s                Specify a filesystem image ID for Fscache back-end.   domain_id=%s           Specify a domain ID in fscache mode so that different images                          with the same blobs under a given domain ID can share storage. +fsoffset=%s            Specify image offset for file-backed or bdev-based mounts.

Hi, Yong

fsoffset should be formatted with %lu ?

Thanks,
Hongbo

  =================== =========================================================

  Sysfs Entries
diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 2409d2ab0c28..599a44d5d782 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -27,9 +27,12 @@ void erofs_put_metabuf(struct erofs_buf *buf)

  void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)

  {
-    pgoff_t index = offset >> PAGE_SHIFT;
+    pgoff_t index;
      struct folio *folio = NULL;
+    offset += buf->off;
+    index = offset >> PAGE_SHIFT;
+
      if (buf->page) {
          folio = page_folio(buf->page);
          if (folio_file_page(folio, index) != buf->page)

@@ -54,6 +57,7 @@ void erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb)

      struct erofs_sb_info *sbi = EROFS_SB(sb);
      buf->file = NULL;
+    buf->off = sbi->dif0.off;
      if (erofs_is_fileio_mode(sbi)) {
          buf->file = sbi->dif0.file;    /* some fs like FUSE needs it */
          buf->mapping = buf->file->f_mapping;

@@ -299,7 +303,7 @@ static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,

          iomap->private = buf.base;
      } else {
          iomap->type = IOMAP_MAPPED;
-        iomap->addr = mdev.m_pa;
+        iomap->addr = mdev.m_dif->off + mdev.m_pa;
          if (flags & IOMAP_DAX)
              iomap->addr += mdev.m_dif->dax_part_off;
      }
diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index 60c7cc4c105c..a2c7001ff789 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c

@@ -147,7 +147,8 @@ static int erofs_fileio_scan_folio(struct erofs_fileio *io, struct folio *folio)

                  if (err)
                      break;
                  io->rq = erofs_fileio_rq_alloc(&io->dev);
-                io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >> 9;
+                io->rq->bio.bi_iter.bi_sector =
+                    (io->dev.m_dif->off + io->dev.m_pa) >> 9;
                  attached = 0;
              }
              if (!bio_add_folio(&io->rq->bio, folio, len, cur))
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 4ac188d5d894..10656bd986bd 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -43,6 +43,7 @@ struct erofs_device_info {
      char *path;
      struct erofs_fscache *fscache;
      struct file *file;
+    loff_t off;

Use u64 is better?

      struct dax_device *dax_dev;
      u64 dax_part_off;
@@ -199,6 +200,7 @@ enum {
  struct erofs_buf {
      struct address_space *mapping;
      struct file *file;
+    loff_t off;

Same here.

      struct page *page;
      void *base;
  };
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index da6ee7c39290..512877d7d855 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c

@@ -356,7 +356,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi)

  enum {
      Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
-    Opt_device, Opt_fsid, Opt_domain_id, Opt_directio,
+    Opt_device, Opt_fsid, Opt_domain_id, Opt_directio, Opt_fsoffset,
  };
  static const struct constant_table erofs_param_cache_strategy[] = {

@@ -383,6 +383,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {

      fsparam_string("fsid",        Opt_fsid),
      fsparam_string("domain_id",    Opt_domain_id),
      fsparam_flag_no("directio",    Opt_directio),
+    fsparam_u64("fsoffset",        Opt_fsoffset),
      {}
  };

@@ -506,6 +507,9 @@ static int erofs_fc_parse_param(struct fs_context *fc,           errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);

  #endif
          break;
+    case Opt_fsoffset:
+        sbi->dif0.off = result.uint_64;
+        break;
      }
      return 0;
  }

@@ -599,6 +603,10 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)

                  &sbi->dif0.dax_part_off, NULL, NULL);
      }
+    if (sbi->dif0.off & ((1 << sbi->blkszbits) - 1))
+        return invalfc(fc, "fsoffset %lld not aligned to block size",
+                   sbi->dif0.off);
+
      err = erofs_read_superblock(sb);
      if (err)
          return err;

@@ -947,6 +955,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)

      if (sbi->domain_id)
          seq_printf(seq, ",domain_id=%s", sbi->domain_id);
  #endif
+    if (sbi->dif0.off)
+        seq_printf(seq, ",fsoffset=%lld", sbi->dif0.off);
      return 0;
  }
diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index b8e6b76c23d5..4f910d7ffb2f 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c

@@ -1707,7 +1707,8 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,

                      bio = bio_alloc(mdev.m_bdev, BIO_MAX_VECS,
                              REQ_OP_READ, GFP_NOIO);
                  bio->bi_end_io = z_erofs_endio;
-                bio->bi_iter.bi_sector = cur >> 9;
+                bio->bi_iter.bi_sector =
+                        (mdev.m_dif->off + cur) >> 9;
                  bio->bi_private = q[JQ_SUBMIT];
                  if (readahead)
                      bio->bi_opf |= REQ_RAHEAD;