erofs: get rid of magical Z_EROFS_MAPPING_STAGING

Previously, we played around with magical page->mapping for short-lived
temporary pages since we need to identify different types of pages in
the same pcluster but both invalidated and short-lived temporary pages
can have page->mapping == NULL. It was considered as safe because that
temporary pages are all non-LRU / non-movable pages.

This patch tends to use specific page->private to identify short-lived
pages instead so it won't rely on page->mapping anymore. Details are
described in "compress.h" as well.

Link: https://lore.kernel.org/r/20201208095834.3133565-1-hsiangkao@redhat.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/compress.h

@@ -26,30 +26,55 @@ struct z_erofs_decompress_req {
 	bool inplace_io, partial_decoding;
 };
 
+/* some special page->private (unsigned long, see below) */
+#define Z_EROFS_SHORTLIVED_PAGE		(-1UL << 2)
+
 /*
- * - 0x5A110C8D ('sallocated', Z_EROFS_MAPPING_STAGING) -
+ * For all pages in a pcluster, page->private should be one of
- * used to mark temporary allocated pages from other
+ * Type                         Last 2bits      page->private
- * file/cached pages and NULL mapping pages.
+ * short-lived page             00              Z_EROFS_SHORTLIVED_PAGE
+ * cached/managed page          00              pointer to z_erofs_pcluster
+ * online page (file-backed,    01/10/11        sub-index << 2 | count
+ *              some pages can be used for inplace I/O)
+ *
+ * page->mapping should be one of
+ * Type                 page->mapping
+ * short-lived page     NULL
+ * cached/managed page  non-NULL or NULL (invalidated/truncated page)
+ * online page          non-NULL
+ *
+ * For all managed pages, PG_private should be set with 1 extra refcount,
+ * which is used for page reclaim / migration.
  */
-#define Z_EROFS_MAPPING_STAGING         ((void *)0x5A110C8D)
 
-/* check if a page is marked as staging */
+/*
-static inline bool z_erofs_page_is_staging(struct page *page)
+ * short-lived pages are pages directly from buddy system with specific
+ * page->private (no need to set PagePrivate since these are non-LRU /
+ * non-movable pages and bypass reclaim / migration code).
+ */
+static inline bool z_erofs_is_shortlived_page(struct page *page)
 {
-	return page->mapping == Z_EROFS_MAPPING_STAGING;
+	if (page->private != Z_EROFS_SHORTLIVED_PAGE)
-}
-
-static inline bool z_erofs_put_stagingpage(struct list_head *pagepool,
-					   struct page *page)
-{
-	if (!z_erofs_page_is_staging(page))
 		return false;
 
-	/* staging pages should not be used by others at the same time */
+	DBG_BUGON(page->mapping);
-	if (page_ref_count(page) > 1)
+	return true;
+}
+
+static inline bool z_erofs_put_shortlivedpage(struct list_head *pagepool,
+					      struct page *page)
+{
+	if (!z_erofs_is_shortlived_page(page))
+		return false;
+
+	/* short-lived pages should not be used by others at the same time */
+	if (page_ref_count(page) > 1) {
 		put_page(page);
-	else
+	} else {
+		/* follow the pcluster rule above. */
+		set_page_private(page, 0);
 		list_add(&page->lru, pagepool);
+	}
 	return true;
 }
 

fs/erofs/decompressor.c

@@ -76,7 +76,7 @@ static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
 			victim = erofs_allocpage(pagepool, GFP_KERNEL);
 			if (!victim)
 				return -ENOMEM;
-			victim->mapping = Z_EROFS_MAPPING_STAGING;
+			set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
 		}
 		rq->out[i] = victim;
 	}

fs/erofs/zdata.c

@@ -226,11 +226,8 @@ int erofs_try_to_free_all_cached_pages(struct erofs_sb_info *sbi,
 
 		/* barrier is implied in the following 'unlock_page' */
 		WRITE_ONCE(pcl->compressed_pages[i], NULL);
-		set_page_private(page, 0);
+		detach_page_private(page);
-		ClearPagePrivate(page);
-
 		unlock_page(page);
-		put_page(page);
 	}
 	return 0;
 }
@@ -254,10 +251,8 @@ int erofs_try_to_free_cached_page(struct address_space *mapping,
 		}
 		erofs_workgroup_unfreeze(&pcl->obj, 1);
 
-		if (ret) {
+		if (ret)
-			ClearPagePrivate(page);
+			detach_page_private(page);
-			put_page(page);
-		}
 	}
 	return ret;
 }
@@ -648,12 +643,12 @@ hitted:
 
 retry:
 	err = z_erofs_attach_page(clt, page, page_type);
-	/* should allocate an additional staging page for pagevec */
+	/* should allocate an additional short-lived page for pagevec */
 	if (err == -EAGAIN) {
 		struct page *const newpage =
 				alloc_page(GFP_NOFS | __GFP_NOFAIL);
 
-		newpage->mapping = Z_EROFS_MAPPING_STAGING;
+		set_page_private(newpage, Z_EROFS_SHORTLIVED_PAGE);
 		err = z_erofs_attach_page(clt, newpage,
 					  Z_EROFS_PAGE_TYPE_EXCLUSIVE);
 		if (!err)
@@ -710,6 +705,11 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
 		queue_work(z_erofs_workqueue, &io->u.work);
 }
 
+static bool z_erofs_page_is_invalidated(struct page *page)
+{
+	return !page->mapping && !z_erofs_is_shortlived_page(page);
+}
+
 static void z_erofs_decompressqueue_endio(struct bio *bio)
 {
 	tagptr1_t t = tagptr_init(tagptr1_t, bio->bi_private);
@@ -722,7 +722,7 @@ static void z_erofs_decompressqueue_endio(struct bio *bio)
 		struct page *page = bvec->bv_page;
 
 		DBG_BUGON(PageUptodate(page));
-		DBG_BUGON(!page->mapping);
+		DBG_BUGON(z_erofs_page_is_invalidated(page));
 
 		if (err)
 			SetPageError(page);
@@ -795,9 +795,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
 
 		/* all pages in pagevec ought to be valid */
 		DBG_BUGON(!page);
-		DBG_BUGON(!page->mapping);
+		DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-		if (z_erofs_put_stagingpage(pagepool, page))
+		if (z_erofs_put_shortlivedpage(pagepool, page))
 			continue;
 
 		if (page_type == Z_EROFS_VLE_PAGE_TYPE_HEAD)
@@ -831,9 +831,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
 
 		/* all compressed pages ought to be valid */
 		DBG_BUGON(!page);
-		DBG_BUGON(!page->mapping);
+		DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-		if (!z_erofs_page_is_staging(page)) {
+		if (!z_erofs_is_shortlived_page(page)) {
 			if (erofs_page_is_managed(sbi, page)) {
 				if (!PageUptodate(page))
 					err = -EIO;
@@ -858,7 +858,7 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
 			overlapped = true;
 		}
 
-		/* PG_error needs checking for inplaced and staging pages */
+		/* PG_error needs checking for all non-managed pages */
 		if (PageError(page)) {
 			DBG_BUGON(PageUptodate(page));
 			err = -EIO;
@@ -897,8 +897,8 @@ out:
 		if (erofs_page_is_managed(sbi, page))
 			continue;
 
-		/* recycle all individual staging pages */
+		/* recycle all individual short-lived pages */
-		(void)z_erofs_put_stagingpage(pagepool, page);
+		(void)z_erofs_put_shortlivedpage(pagepool, page);
 
 		WRITE_ONCE(compressed_pages[i], NULL);
 	}
@@ -908,10 +908,10 @@ out:
 		if (!page)
 			continue;
 
-		DBG_BUGON(!page->mapping);
+		DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-		/* recycle all individual staging pages */
+		/* recycle all individual short-lived pages */
-		if (z_erofs_put_stagingpage(pagepool, page))
+		if (z_erofs_put_shortlivedpage(pagepool, page))
 			continue;
 
 		if (err < 0)
@@ -1011,13 +1011,17 @@ repeat:
 	mapping = READ_ONCE(page->mapping);
 
 	/*
-	 * unmanaged (file) pages are all locked solidly,
+	 * file-backed online pages in plcuster are all locked steady,
 	 * therefore it is impossible for `mapping' to be NULL.
 	 */
 	if (mapping && mapping != mc)
 		/* ought to be unmanaged pages */
 		goto out;
 
+	/* directly return for shortlived page as well */
+	if (z_erofs_is_shortlived_page(page))
+		goto out;
+
 	lock_page(page);
 
 	/* only true if page reclaim goes wrong, should never happen */
@@ -1062,8 +1066,8 @@ repeat:
 out_allocpage:
 	page = erofs_allocpage(pagepool, gfp | __GFP_NOFAIL);
 	if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
-		/* non-LRU / non-movable temporary page is needed */
+		/* turn into temporary page if fails */
-		page->mapping = Z_EROFS_MAPPING_STAGING;
+		set_page_private(page, Z_EROFS_SHORTLIVED_PAGE);
 		tocache = false;
 	}
 
@@ -1080,8 +1084,9 @@ out_allocpage:
 	}
 
 	if (tocache) {
-		set_page_private(page, (unsigned long)pcl);
+		attach_page_private(page, pcl);
-		SetPagePrivate(page);
+		/* drop a ref added by allocpage (then we have 2 refs here) */
+		put_page(page);
 	}
 out:	/* the only exit (for tracing and debugging) */
 	return page;

fs/erofs/zdata.h

@@ -173,6 +173,7 @@ static inline void z_erofs_onlinepage_endio(struct page *page)
 
 	v = atomic_dec_return(u.o);
 	if (!(v & Z_EROFS_ONLINEPAGE_COUNT_MASK)) {
+		set_page_private(page, 0);
 		ClearPagePrivate(page);
 		if (!PageError(page))
 			SetPageUptodate(page);