btrfs: rework btrfs_decompress_buf2page()

There are several bugs inside the function btrfs_decompress_buf2page()

- @start_byte doesn't take bvec.bv_offset into consideration
  Thus it can't handle case where the target range is not page aligned.

- Too many helper variables
  There are tons of helper variables, @buf_offset, @current_buf_start,
  @start_byte, @prev_start_byte, @working_bytes, @bytes.
  This hurts anyone who wants to read the function.

- No obvious main cursor for the iteartion
  A new problem caused by previous problem.

- Comments for parameter list makes no sense
  Like @buf_start is the offset to @buf, or offset inside the full
  decompressed extent? (Spoiler alert, the later case)
  And @total_out acts more like @buf_start + @size_of_buf.

  The worst is @disk_start.
  The real meaning of it is the file offset of the full decompressed
  extent.

This patch will rework the whole function by:

- Add a proper comment with ASCII art to explain the parameter list

- Rework parameter list
  The old @buf_start is renamed to @decompressed, to show how many bytes
  are already decompressed inside the full decompressed extent.
  The old @total_out is replaced by @buf_len, which is the decompressed
  data size.
  For old @disk_start and @bio, just pass @compressed_bio in.

- Use single main cursor
  The main cursor will be @cur_file_offset, to show what's the current
  file offset.
  Other helper variables will be declared inside the main loop, and only
  minimal amount of helper variables:
  * offset_inside_decompressed_buf:	The only real helper
  * copy_start_file_offset:		File offset we start memcpy
  * bvec_file_offset:			File offset of current bvec

Even with all these extensive comments, the final function is still
smaller than the original function, which is definitely a win.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
index 200ba08bfae6..56dce9f00988 100644
--- a/fs/btrfs/zstd.c
+++ b/fs/btrfs/zstd.c
@@ -540,8 +540,6 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 {
 	struct workspace *workspace = list_entry(ws, struct workspace, list);
 	struct page **pages_in = cb->compressed_pages;
-	u64 disk_start = cb->start;
-	struct bio *orig_bio = cb->orig_bio;
 	size_t srclen = cb->compressed_len;
 	ZSTD_DStream *stream;
 	int ret = 0;
@@ -582,7 +580,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 		workspace->out_buf.pos = 0;
 
 		ret = btrfs_decompress_buf2page(workspace->out_buf.dst,
-				buf_start, total_out, disk_start, orig_bio);
+				total_out - buf_start, cb, buf_start);
 		if (ret == 0)
 			break;
 
@@ -607,7 +605,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 		}
 	}
 	ret = 0;
-	zero_fill_bio(orig_bio);
+	zero_fill_bio(cb->orig_bio);
 done:
 	return ret;
 }