On Mon, Jul 05, 2021 at 10:00:58AM +0800, Qu Wenruo wrote:

+int btrfs_decompress_buf2page(const char *buf, u32 buf_len,
+			      struct compressed_bio *cb, u32 decompressed)
 {
-	unsigned long buf_offset;
-	unsigned long current_buf_start;
-	unsigned long start_byte;
-	unsigned long prev_start_byte;
-	unsigned long working_bytes = total_out - buf_start;
-	unsigned long bytes;
-	struct bio_vec bvec = bio_iter_iovec(bio, bio->bi_iter);
-
-	/*
-	 * start byte is the first byte of the page we're currently
-	 * copying into relative to the start of the compressed data.
-	 */
-	start_byte = page_offset(bvec.bv_page) - disk_start;
-
-	/* we haven't yet hit data corresponding to this page */
-	if (total_out <= start_byte)
-		return 1;
+	struct bio *orig_bio = cb->orig_bio;
+	u32 cur_offset;	/* Offset inside the full decompressed extent */
 
-	/*
-	 * the start of the data we care about is offset into
-	 * the middle of our working buffer
-	 */
-	if (total_out > start_byte && buf_start < start_byte) {
-		buf_offset = start_byte - buf_start;
-		working_bytes -= buf_offset;
-	} else {
-		buf_offset = 0;
-	}
-	current_buf_start = buf_start;
+	cur_offset = decompressed;
+	/* The main loop to do the copy */
+	while (cur_offset < decompressed + buf_len) {
+		struct bio_vec bvec = bio_iter_iovec(orig_bio,
+						     orig_bio->bi_iter);
+		size_t copy_len;
+		u32 copy_start;
+		u32 bvec_offset; /* Offset inside the full decompressed extent */
 
-	/* copy bytes from the working buffer into the pages */
-	while (working_bytes > 0) {
-		bytes = min_t(unsigned long, bvec.bv_len,
-				PAGE_SIZE - (buf_offset % PAGE_SIZE));
-		bytes = min(bytes, working_bytes);
+		bvec = bio_iter_iovec(orig_bio, orig_bio->bi_iter);

Looks like a duplicate initialization, the same is in the delcaration,
I'll keep the first one as it's a simple struct init.

 
-		memcpy_to_page(bvec.bv_page, bvec.bv_offset, buf + buf_offset,
-			       bytes);
-		flush_dcache_page(bvec.bv_page);
+		/*
+		 * cb->start may underflow, but minusing that value can still
+		 * give us correct offset inside the full decompressed extent.
+		 */
+		bvec_offset = page_offset(bvec.bv_page) + bvec.bv_offset
+			      - cb->start;