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, 3 deletions

diff --git a/fs/btrfs/compression.h b/fs/btrfs/compression.h
index c359f20920d0..399be0b435bf 100644
--- a/fs/btrfs/compression.h
+++ b/fs/btrfs/compression.h
@@ -86,9 +86,8 @@ int btrfs_compress_pages(unsigned int type_level, struct address_space *mapping,
 			 unsigned long *total_out);
 int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
 		     unsigned long start_byte, size_t srclen, size_t destlen);
-int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
-			      unsigned long total_out, u64 disk_start,
-			      struct bio *bio);
+int btrfs_decompress_buf2page(const char *buf, u32 buf_len,
+			      struct compressed_bio *cb, u32 decompressed);
 
 blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
 				  unsigned int len, u64 disk_start,