[BUG]
There are two very basic send streams:
(a/m/ctime and uuid omitted)

  Stream 1: (Parent subvolume)
  subvol   ./parent_subv           transid=8
  chown    ./parent_subv/          gid=0 uid=0
  chmod    ./parent_subv/          mode=755
  utimes   ./parent_subv/
  mkfile   ./parent_subv/o257-7-0
  rename   ./parent_subv/o257-7-0  dest=./parent_subv/source
  utimes   ./parent_subv/
  write    ./parent_subv/source    offset=0 len=16384
  chown    ./parent_subv/source    gid=0 uid=0
  chmod    ./parent_subv/source    mode=600
  utimes   ./parent_subv/source

  Stream 2: (snapshot and clone)
  snapshot ./dest_subv             transid=14 parent_transid=10
  utimes   ./dest_subv/
  mkfile   ./dest_subv/o258-14-0
  rename   ./dest_subv/o258-14-0   dest=./dest_subv/reflink
  utimes   ./dest_subv/
  clone    ./dest_subv/reflink     offset=0 len=16384 from=./dest_subv/source clone_offset=0
  chown    ./dest_subv/reflink     gid=0 uid=0
  chmod    ./dest_subv/reflink     mode=600
  utimes   ./dest_subv/reflink

But if we receive the first stream with default mount, then remount to
nodatasum, and try to receive the second stream, it will fail:

 # mount /mnt/btrfs
 # btrfs receive -f ~/parent_stream /mnt/btrfs/
 At subvol parent_subv
 # mount -o remount,nodatasum /mnt/btrfs
 # btrfs receive -f ~/clone_stream /mnt/btrfs/
 At snapshot dest_subv
 ERROR: failed to clone extents to reflink: Invalid argument
 # echo $?
 1

[CAUSE]
Btrfs doesn't allow clone source and destination files have different
NODATASUM flags.
This is to prevent a data extent to be owned by both NODATASUM inode and
regular DATASUM inode.

For above receive operations, the clone destination is inheriting the
NODATASUM flag from mount option, while the clone source has no
NODATASUM flag, thus preventing us from doing the clone.

[FIX]
Btrfs send/receive doesn't require the underlying inode has the same
flags (thus we can send from compressed extent and receive on a
non-compressed filesystem).

So here we add a new command line option, '--clone-fallback', to allow
btrfs-receive to fall back to buffered write to copy data from the
source file.

Since such behavior can result much less clone operations, which may not
be what the end users really want, and can hide bugs in send stream.
Thus this behavior must be explicitly specified by the new option.

And we will output a warning message each time such fallback is
triggered if the user wants extra debug output.

Signed-off-by: Qu Wenruo <redacted>
---
 Documentation/btrfs-receive.asciidoc | 12 ++++++
 cmds/receive.c                       | 62 ++++++++++++++++++++++++++--
 2 files changed, 71 insertions(+), 3 deletions(-)

diff --git a/Documentation/btrfs-receive.asciidoc b/Documentation/btrfs-receive.asciidoc
index e4c4d2c0bf3d..9c934a399a9c 100644
--- a/Documentation/btrfs-receive.asciidoc
+++ b/Documentation/btrfs-receive.asciidoc

@@ -65,6 +65,18 @@ dump the stream metadata, one line per operation
 +
 Does not require the 'path' parameter. The filesystem remains unchanged.

+--clone-fallback::
+when clone opeartions fail, attempt to directly copy the data instead.

+
 -q|--quiet::
 (deprecated) alias for global '-q' option

diff --git a/cmds/receive.c b/cmds/receive.c
index 48c774cea587..31746d571016 100644
--- a/cmds/receive.c
+++ b/cmds/receive.c

@@ -76,6 +76,8 @@ struct btrfs_receive
        struct subvol_uuid_search sus;

        int honor_end_cmd;
+
+       bool clone_fallback;
 };

 static int finish_subvol(struct btrfs_receive *rctx)

@@ -705,6 +707,44 @@ out:
        return ret;
 }

+static int buffered_copy(int src_fd, int dst_fd, u64 src_offset, u64 len,
+                        u64 dest_offset)
+{
+       unsigned char buf[SZ_32K];
+       u64 copied = 0;
+       int ret = 0;
+
+       while (copied < len) {
+               u32 copy_len = min_t(u32, ARRAY_SIZE(buf), len - copied);
+               u32 written = 0;
+               ssize_t read_size;
+
+               read_size = pread(src_fd, buf, copy_len, src_offset + copied);
+               if (read < 0) {
+                       ret = -errno;
+                       error("failed to read source file: %m");
+                       return ret;
+               }
+
+               /* Write the buffer to dest file */
+               while (written < read_size) {
+                       ssize_t write_size;
+
+                       write_size = pwrite(dst_fd, buf + written,
+                                       read_size - written,
+                                       dest_offset + copied + written);
+                       if (write_size < 0) {
+                               ret = -errno;
+                               error("failed to write source file: %m");
+                               return ret;
+                       }
+                       written += write_size;
+               }
+               copied += read_size;
+       }
+       return ret;
+}
+
 static int process_clone(const char *path, u64 offset, u64 len,
                         const u8 *clone_uuid, u64 clone_ctransid,
                         const char *clone_path, u64 clone_offset,

@@ -788,8 +828,17 @@ static int process_clone(const char *path, u64 offset, u64 len,
        ret = ioctl(rctx->write_fd, BTRFS_IOC_CLONE_RANGE, &clone_args);
        if (ret < 0) {
                ret = -errno;
-               error("failed to clone extents to %s: %m", path);
-               goto out;
+               if (ret != -EINVAL || !rctx->clone_fallback) {
+                       error("failed to clone extents to %s: %m", path);
+                       goto out;
+               }
+
+               if (bconf.verbose >= 2)
+                       warning(
+               "failed to clone extents to %s, fallback to buffered write",
+                               path);
+               ret = buffered_copy(clone_fd, rctx->write_fd, clone_offset,
+                                   len, offset);
        }

 out:

@@ -1197,6 +1246,8 @@ static const char * const cmd_receive_usage[] = {
        "                 this file system is mounted.",
        "--dump           dump stream metadata, one line per operation,",
        "                 does not require the MOUNT parameter",
+       "--clone-fallback when clone operations fail, attempt to directly copy"
+       "                 the data instead"
        "-v               deprecated, alias for global -v option",
        HELPINFO_INSERT_GLOBALS,
        HELPINFO_INSERT_VERBOSE,

@@ -1238,11 +1289,13 @@ static int cmd_receive(const struct cmd_struct *cmd, int argc, char **argv)
        optind = 0;
        while (1) {
                int c;
-               enum { GETOPT_VAL_DUMP = 257 };
+               enum { GETOPT_VAL_DUMP = 257, GETOPT_VAL_CLONE_FALLBACK };
                static const struct option long_opts[] = {
                        { "max-errors", required_argument, NULL, 'E' },
                        { "chroot", no_argument, NULL, 'C' },
                        { "dump", no_argument, NULL, GETOPT_VAL_DUMP },
+                       { "clone-fallback", no_argument, NULL,
+                               GETOPT_VAL_CLONE_FALLBACK},
                        { "quiet", no_argument, NULL, 'q' },
                        { NULL, 0, NULL, 0 }
                };

@@ -1286,6 +1339,9 @@ static int cmd_receive(const struct cmd_struct *cmd, int argc, char **argv)
                case GETOPT_VAL_DUMP:
                        dump = 1;
                        break;
+               case GETOPT_VAL_CLONE_FALLBACK:
+                       rctx.clone_fallback = true;
+                       break;
                default:
                        usage_unknown_option(cmd, argv);
                }
--

2.33.0