btrfs: send: use fallocate for hole punching with send stream v2

Currently holes are sent as writes full of zeroes, which results in
unnecessarily using disk space at the receiving end and increasing the
stream size.

In some cases we avoid sending writes of zeroes, like during a full
send operation where we just skip writes for holes.

But for some cases we fill previous holes with writes of zeroes too, like
in this scenario:

1) We have a file with a hole in the range [2M, 3M), we snapshot the
   subvolume and do a full send. The range [2M, 3M) stays as a hole at
   the receiver since we skip sending write commands full of zeroes;

2) We punch a hole for the range [3M, 4M) in our file, so that now it
   has a 2M hole in the range [2M, 4M), and snapshot the subvolume.
   Now if we do an incremental send, we will send write commands full
   of zeroes for the range [2M, 4M), removing the hole for [2M, 3M) at
   the receiver.

We could improve cases such as this last one by doing additional
comparisons of file extent items (or their absence) between the parent
and send snapshots, but that's a lot of code to add plus additional CPU
and IO costs.

Since the send stream v2 already has a fallocate command and btrfs-progs
implements a callback to execute fallocate since the send stream v2
support was added to it, update the kernel to use fallocate for punching
holes for V2+ streams.

Test coverage is provided by btrfs/284 which is a version of btrfs/007
that exercises send stream v2 instead of v1, using fsstress with random
operations and fssum to verify file contents.

Link: kdave/btrfs-progs#1001
Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/send.c

@@ -4,6 +4,7 @@
  */
 
 #include <linux/bsearch.h>
+#include <linux/falloc.h>
 #include <linux/fs.h>
 #include <linux/file.h>
 #include <linux/sort.h>
@@ -5405,13 +5406,45 @@ static int send_update_extent(struct send_ctx *sctx,
 	return ret;
 }
 
+static int send_fallocate(struct send_ctx *sctx, u32 mode, u64 offset, u64 len)
+{
+	struct fs_path *path;
+	int ret;
+
+	path = get_cur_inode_path(sctx);
+	if (IS_ERR(path))
+		return PTR_ERR(path);
+
+	ret = begin_cmd(sctx, BTRFS_SEND_C_FALLOCATE);
+	if (ret < 0)
+		return ret;
+
+	TLV_PUT_PATH(sctx, BTRFS_SEND_A_PATH, path);
+	TLV_PUT_U32(sctx, BTRFS_SEND_A_FALLOCATE_MODE, mode);
+	TLV_PUT_U64(sctx, BTRFS_SEND_A_FILE_OFFSET, offset);
+	TLV_PUT_U64(sctx, BTRFS_SEND_A_SIZE, len);
+
+	ret = send_cmd(sctx);
+
+tlv_put_failure:
+	return ret;
+}
+
 static int send_hole(struct send_ctx *sctx, u64 end)
 {
 	struct fs_path *p = NULL;
 	u64 read_size = max_send_read_size(sctx);
 	u64 offset = sctx->cur_inode_last_extent;
 	int ret = 0;
 
+	/*
+	 * Starting with send stream v2 we have fallocate and can use it to
+	 * punch holes instead of sending writes full of zeroes.
+	 */
+	if (proto_cmd_ok(sctx, BTRFS_SEND_C_FALLOCATE))
+		return send_fallocate(sctx, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+				      offset, end - offset);
+
 	/*
 	 * A hole that starts at EOF or beyond it. Since we do not yet support
 	 * fallocate (for extent preallocation and hole punching), sending a