staging: erofs: decompress asynchronously if PG_readahead page at first

For the case of nr_to_read == lookahead_size, it is better to
decompress asynchronously as well since no page will be needed immediately.
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/unzip_vle.c

@@ -1345,8 +1345,8 @@ static int z_erofs_vle_normalaccess_readpages(struct file *filp,
 {
 	struct inode *const inode = mapping->host;
 	struct erofs_sb_info *const sbi = EROFS_I_SB(inode);
-	const bool sync = __should_decompress_synchronously(sbi, nr_pages);
 
+	bool sync = __should_decompress_synchronously(sbi, nr_pages);
 	struct z_erofs_vle_frontend f = VLE_FRONTEND_INIT(inode);
 	gfp_t gfp = mapping_gfp_constraint(mapping, GFP_KERNEL);
 	struct page *head = NULL;
@@ -1364,6 +1364,13 @@ static int z_erofs_vle_normalaccess_readpages(struct file *filp,
 		prefetchw(&page->flags);
 		list_del(&page->lru);
 
+		/*
+		 * A pure asynchronous readahead is indicated if
+		 * a PG_readahead marked page is hitted at first.
+		 * Let's also do asynchronous decompression for this case.
+		 */
+		sync &= !(PageReadahead(page) && !head);
+
 		if (add_to_page_cache_lru(page, mapping, page->index, gfp)) {
 			list_add(&page->lru, &pagepool);
 			continue;