erofs: remove the fast path of per-CPU buffer decompression

As Xiang mentioned, such path has no real impact to our current
decompression strategy, remove it directly. Also, update the return
value of z_erofs_lz4_decompress() to 0 if success to keep consistent
with LZMA which will return 0 as well for that case.

Link: https://lore.kernel.org/r/20211014065744.1787-1-zbestahu@gmail.comReviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Yue Hu <huyue2@yulong.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/decompressor.c

@@ -242,6 +242,8 @@ static int z_erofs_lz4_decompress(struct z_erofs_decompress_req *rq, u8 *out)
 		if (ret >= 0)
 			memset(out + ret, 0, rq->outputsize - ret);
 		ret = -EIO;
+	} else {
+		ret = 0;
 	}
 
 	if (maptype == 0) {
@@ -268,33 +270,6 @@ static struct z_erofs_decompressor decompressors[] = {
 	},
 };
 
-static void copy_from_pcpubuf(struct page **out, const char *dst,
-			      unsigned short pageofs_out,
-			      unsigned int outputsize)
-{
-	const char *end = dst + outputsize;
-	const unsigned int righthalf = PAGE_SIZE - pageofs_out;
-	const char *cur = dst - pageofs_out;
-
-	while (cur < end) {
-		struct page *const page = *out++;
-
-		if (page) {
-			char *buf = kmap_atomic(page);
-
-			if (cur >= dst) {
-				memcpy(buf, cur, min_t(uint, PAGE_SIZE,
-						       end - cur));
-			} else {
-				memcpy(buf + pageofs_out, cur + pageofs_out,
-				       min_t(uint, righthalf, end - cur));
-			}
-			kunmap_atomic(buf);
-		}
-		cur += PAGE_SIZE;
-	}
-}
-
 static int z_erofs_decompress_generic(struct z_erofs_decompress_req *rq,
 				      struct list_head *pagepool)
 {
@@ -305,34 +280,12 @@ static int z_erofs_decompress_generic(struct z_erofs_decompress_req *rq,
 	void *dst;
 	int ret;
 
-	/* two optimized fast paths only for non bigpcluster cases yet */
-	if (rq->inputsize <= PAGE_SIZE) {
-		if (nrpages_out == 1 && !rq->inplace_io) {
-			DBG_BUGON(!*rq->out);
-			dst = kmap_atomic(*rq->out);
-			dst_maptype = 0;
-			goto dstmap_out;
-		}
-
-		/*
-		 * For the case of small output size (especially much less
-		 * than PAGE_SIZE), memcpy the decompressed data rather than
-		 * compressed data is preferred.
-		 */
-		if (rq->outputsize <= PAGE_SIZE * 7 / 8) {
-			dst = erofs_get_pcpubuf(1);
-			if (IS_ERR(dst))
-				return PTR_ERR(dst);
-
-			rq->inplace_io = false;
-			ret = alg->decompress(rq, dst);
-			if (!ret)
-				copy_from_pcpubuf(rq->out, dst, rq->pageofs_out,
-						  rq->outputsize);
-
-			erofs_put_pcpubuf(dst);
-			return ret;
-		}
+	/* one optimized fast path only for non bigpcluster cases yet */
+	if (rq->inputsize <= PAGE_SIZE && nrpages_out == 1 && !rq->inplace_io) {
+		DBG_BUGON(!*rq->out);
+		dst = kmap_atomic(*rq->out);
+		dst_maptype = 0;
+		goto dstmap_out;
 	}
 
 	/* general decoding path which can be used for all cases */