erofs: tidy up z_erofs_do_read_page()

 - Fix a typo: spiltted => split;

 - Move !EROFS_MAP_MAPPED and EROFS_MAP_FRAGMENT upwards;

 - Increase `split` in advance to avoid unnecessary repeats.
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230817082813.81180-4-hsiangkao@linux.alibaba.com

fs/erofs/zdata.c

@@ -981,49 +981,34 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 	struct erofs_map_blocks *const map = &fe->map;
 	const loff_t offset = page_offset(page);
 	bool tight = true, exclusive;
-	unsigned int cur, end, len, spiltted;
+	unsigned int cur, end, len, split;
 	int err = 0;
 
-	/* register locked file pages as online pages in pack */
 	z_erofs_onlinepage_init(page);
 
-	spiltted = 0;
+	split = 0;
 	end = PAGE_SIZE;
 repeat:
-	cur = end - 1;
-
-	if (offset + cur < map->m_la ||
-	    offset + cur >= map->m_la + map->m_llen) {
+	if (offset + end - 1 < map->m_la ||
+	    offset + end - 1 >= map->m_la + map->m_llen) {
 		z_erofs_pcluster_end(fe);
-		map->m_la = offset + cur;
+		map->m_la = offset + end - 1;
 		map->m_llen = 0;
 		err = z_erofs_map_blocks_iter(inode, map, 0);
 		if (err)
 			goto out;
-	} else if (fe->pcl) {
-		goto hitted;
 	}
 
-	if ((map->m_flags & EROFS_MAP_MAPPED) &&
-	    !(map->m_flags & EROFS_MAP_FRAGMENT)) {
-		err = z_erofs_pcluster_begin(fe);
-		if (err)
-			goto out;
-	}
-hitted:
-	/*
-	 * Ensure the current partial page belongs to this submit chain rather
-	 * than other concurrent submit chains or the noio(bypass) chain since
-	 * those chains are handled asynchronously thus the page cannot be used
-	 * for inplace I/O or bvpage (should be processed in a strict order.)
-	 */
-	tight &= (fe->mode > Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE);
+	cur = offset > map->m_la ? 0 : map->m_la - offset;
+	/* bump split parts first to avoid several separate cases */
+	++split;
 
-	cur = end - min_t(erofs_off_t, offset + end - map->m_la, end);
 	if (!(map->m_flags & EROFS_MAP_MAPPED)) {
 		zero_user_segment(page, cur, end);
+		tight = false;
 		goto next_part;
 	}
+
 	if (map->m_flags & EROFS_MAP_FRAGMENT) {
 		erofs_off_t fpos = offset + cur - map->m_la;
 
@@ -1032,12 +1017,24 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 				EROFS_I(inode)->z_fragmentoff + fpos);
 		if (err)
 			goto out;
-		++spiltted;
 		tight = false;
 		goto next_part;
 	}
 
-	exclusive = (!cur && (!spiltted || tight));
+	if (!fe->pcl) {
+		err = z_erofs_pcluster_begin(fe);
+		if (err)
+			goto out;
+	}
+
+	/*
+	 * Ensure the current partial page belongs to this submit chain rather
+	 * than other concurrent submit chains or the noio(bypass) chain since
+	 * those chains are handled asynchronously thus the page cannot be used
+	 * for inplace I/O or bvpage (should be processed in a strict order.)
+	 */
+	tight &= (fe->mode > Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE);
+	exclusive = (!cur && ((split <= 1) || tight));
 	if (cur)
 		tight &= (fe->mode >= Z_EROFS_PCLUSTER_FOLLOWED);
 
@@ -1050,8 +1047,6 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 		goto out;
 
 	z_erofs_onlinepage_split(page);
-	/* bump up the number of spiltted parts of a page */
-	++spiltted;
 	if (fe->pcl->pageofs_out != (map->m_la & ~PAGE_MASK))
 		fe->pcl->multibases = true;
 	if (fe->pcl->length < offset + end - map->m_la) {