[PATCH 1/2] btrfs-progs: receive: fallback to buffered copy if clone failed

[Qu Wenruo <wqu@suse.com>]

[BUG]
There are two every 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 has 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 <wqu@suse.com>
---
 Documentation/btrfs-receive.asciidoc | 13 ++++++
 cmds/receive.c                       | 60 ++++++++++++++++++++++++++--
 2 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/Documentation/btrfs-receive.asciidoc b/Documentation/btrfs-receive.asciidoc
index e4c4d2c0bf3d..3b88643abe5f 100644
--- a/Documentation/btrfs-receive.asciidoc
+++ b/Documentation/btrfs-receive.asciidoc
@@ -65,6 +65,19 @@ dump the stream metadata, one line per operation
 +
 Does not require the 'path' parameter. The filesystem remains unchanged.
 
+--clone-fallback::
+allow failed clone operation to fallback to buffer copy from source file.
++
+Clone operations have various requirement which can be affected by things like
+mount options (source file has no NODATASUM flag while current fs is mounted
+with NODATASUM), sectorsizes (the stream is from 4K sectorsize fs while the fs
+is in 64K page size).
++
+This option allows users to let receive to handle such failed clone with
+buffered copy from the source, at the cost of less clone operations and even
+some unexposed send bugs. Thus this behavior must be explicitly specified by
+the user.
+
 -q|--quiet::
 (deprecated) alias for global '-q' option
 
diff --git a/cmds/receive.c b/cmds/receive.c
index 48c774cea587..60691b9b61ae 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:
@@ -1238,11 +1287,13 @@ 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 +1337,9 @@ 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