erofs: fix up inplace I/O pointer for big pcluster

When picking up inplace I/O pages, it should be traversed in reverse
order in aligned with the traversal order of file-backed online pages.
Also, index should be updated together when preloading compressed pages.

Previously, only page-sized pclustersize was supported so no problem
at all. Also rename `compressedpages' to `icpage_ptr' to reflect its
functionality.

Link: https://lore.kernel.org/r/20210407043927.10623-5-xiang@kernel.orgAcked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/zdata.c

@@ -204,7 +204,8 @@ struct z_erofs_collector {
 
 	struct z_erofs_pcluster *pcl, *tailpcl;
 	struct z_erofs_collection *cl;
-	struct page **compressedpages;
+	/* a pointer used to pick up inplace I/O pages */
+	struct page **icpage_ptr;
 	z_erofs_next_pcluster_t owned_head;
 
 	enum z_erofs_collectmode mode;
@@ -238,17 +239,19 @@ static void preload_compressed_pages(struct z_erofs_collector *clt,
 				     enum z_erofs_cache_alloctype type,
 				     struct list_head *pagepool)
 {
-	const struct z_erofs_pcluster *pcl = clt->pcl;
-	struct page **pages = clt->compressedpages;
-	pgoff_t index = pcl->obj.index + (pages - pcl->compressed_pages);
+	struct z_erofs_pcluster *pcl = clt->pcl;
 	bool standalone = true;
 	gfp_t gfp = (mapping_gfp_mask(mc) & ~__GFP_DIRECT_RECLAIM) |
 			__GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN;
+	struct page **pages;
+	pgoff_t index;
 
 	if (clt->mode < COLLECT_PRIMARY_FOLLOWED)
 		return;
 
-	for (; pages < pcl->compressed_pages + pcl->pclusterpages; ++pages) {
+	pages = pcl->compressed_pages;
+	index = pcl->obj.index;
+	for (; index < pcl->obj.index + pcl->pclusterpages; ++index, ++pages) {
 		struct page *page;
 		compressed_page_t t;
 		struct page *newpage = NULL;
@@ -360,16 +363,14 @@ int erofs_try_to_free_cached_page(struct address_space *mapping,
 }
 
 /* page_type must be Z_EROFS_PAGE_TYPE_EXCLUSIVE */
-static inline bool z_erofs_try_inplace_io(struct z_erofs_collector *clt,
+static bool z_erofs_try_inplace_io(struct z_erofs_collector *clt,
 				   struct page *page)
 {
 	struct z_erofs_pcluster *const pcl = clt->pcl;
 
-	while (clt->compressedpages <
-	       pcl->compressed_pages + pcl->pclusterpages) {
-		if (!cmpxchg(clt->compressedpages++, NULL, page))
+	while (clt->icpage_ptr > pcl->compressed_pages)
+		if (!cmpxchg(--clt->icpage_ptr, NULL, page))
 			return true;
-	}
 	return false;
 }
 
@@ -576,9 +577,8 @@ static int z_erofs_collector_begin(struct z_erofs_collector *clt,
 	z_erofs_pagevec_ctor_init(&clt->vector, Z_EROFS_NR_INLINE_PAGEVECS,
 				  clt->cl->pagevec, clt->cl->vcnt);
 
-	clt->compressedpages = clt->pcl->compressed_pages;
-	if (clt->mode <= COLLECT_PRIMARY) /* cannot do in-place I/O */
-		clt->compressedpages += clt->pcl->pclusterpages;
+	/* since file-backed online pages are traversed in reverse order */
+	clt->icpage_ptr = clt->pcl->compressed_pages + clt->pcl->pclusterpages;
 	return 0;
 }