erofs: allocate extra bvec pages directly instead of retrying

If non-bootstrap bvecs cannot be kept in place (very rarely), an extra
short-lived page is allocated.

Let's just allocate it immediately rather than do unnecessary -EAGAIN
return first and retry as a cleanup.  Also it's unnecessary to use
__GFP_NOFAIL here since we could gracefully fail out this case instead.
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Link: https://lore.kernel.org/r/20230526201459.128169-2-hsiangkao@linux.alibaba.com

fs/erofs/zdata.c

@@ -242,12 +242,17 @@ static int z_erofs_bvec_enqueue(struct z_erofs_bvec_iter *iter,
 				struct z_erofs_bvec *bvec,
 				struct page **candidate_bvpage)
 {
-	if (iter->cur == iter->nr) {
-		if (!*candidate_bvpage)
-			return -EAGAIN;
+	if (iter->cur >= iter->nr) {
+		struct page *nextpage = *candidate_bvpage;
 
+		if (!nextpage) {
+			nextpage = alloc_page(GFP_NOFS);
+			if (!nextpage)
+				return -ENOMEM;
+			set_page_private(nextpage, Z_EROFS_SHORTLIVED_PAGE);
+		}
 		DBG_BUGON(iter->bvset->nextpage);
-		iter->bvset->nextpage = *candidate_bvpage;
+		iter->bvset->nextpage = nextpage;
 		z_erofs_bvset_flip(iter);
 
 		iter->bvset->nextpage = NULL;
@@ -906,10 +911,8 @@ static bool z_erofs_collector_end(struct z_erofs_decompress_frontend *fe)
 	z_erofs_bvec_iter_end(&fe->biter);
 	mutex_unlock(&pcl->lock);
 
-	if (fe->candidate_bvpage) {
-		DBG_BUGON(z_erofs_is_shortlived_page(fe->candidate_bvpage));
+	if (fe->candidate_bvpage)
 		fe->candidate_bvpage = NULL;
-	}
 
 	/*
 	 * if all pending pages are added, don't hold its reference
@@ -1054,24 +1057,13 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 	if (cur)
 		tight &= (fe->mode >= Z_EROFS_PCLUSTER_FOLLOWED);
 
-retry:
 	err = z_erofs_attach_page(fe, &((struct z_erofs_bvec) {
 					.page = page,
 					.offset = offset - map->m_la,
 					.end = end,
 				  }), exclusive);
-	/* should allocate an additional short-lived page for bvset */
-	if (err == -EAGAIN && !fe->candidate_bvpage) {
-		fe->candidate_bvpage = alloc_page(GFP_NOFS | __GFP_NOFAIL);
-		set_page_private(fe->candidate_bvpage,
-				 Z_EROFS_SHORTLIVED_PAGE);
-		goto retry;
-	}
-
-	if (err) {
-		DBG_BUGON(err == -EAGAIN && fe->candidate_bvpage);
+	if (err)
 		goto out;
-	}
 
 	z_erofs_onlinepage_split(page);
 	/* bump up the number of spiltted parts of a page */