logfs: update page reference count for pined pages

LogFS sets PG_private flag to indicate a pined page. We assumed that
marking a page as private is enough to ensure its existence. But
instead it is necessary to hold a reference count to the page.

The change resolves the following BUG

BUG: Bad page state in process flush-253:16  pfn:6a6d0
page flags: 0x100000000000808(uptodate|private)
Suggested-and-Acked-by: Joern Engel <joern@logfs.org>
Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>

fs/logfs/readwrite.c

@@ -560,8 +560,13 @@ static void inode_free_block(struct super_block *sb, struct logfs_block *block)
 static void indirect_free_block(struct super_block *sb,
 		struct logfs_block *block)
 {
-	ClearPagePrivate(block->page);
-	block->page->private = 0;
+	struct page *page = block->page;
+
+	if (PagePrivate(page)) {
+		ClearPagePrivate(page);
+		page_cache_release(page);
+		set_page_private(page, 0);
+	}
 	__free_block(sb, block);
 }
 
@@ -650,8 +655,11 @@ static void alloc_data_block(struct inode *inode, struct page *page)
 	logfs_unpack_index(page->index, &bix, &level);
 	block = __alloc_block(inode->i_sb, inode->i_ino, bix, level);
 	block->page = page;
+
 	SetPagePrivate(page);
-	page->private = (unsigned long)block;
+	page_cache_get(page);
+	set_page_private(page, (unsigned long) block);
+
 	block->ops = &indirect_block_ops;
 }
 
@@ -1901,8 +1909,11 @@ static void move_page_to_inode(struct inode *inode, struct page *page)
 	li->li_block = block;
 
 	block->page = NULL;
-	page->private = 0;
+	if (PagePrivate(page)) {
 		ClearPagePrivate(page);
+		page_cache_release(page);
+		set_page_private(page, 0);
+	}
 }
 
 static void move_inode_to_page(struct page *page, struct inode *inode)
@@ -1918,8 +1929,12 @@ static void move_inode_to_page(struct page *page, struct inode *inode)
 	BUG_ON(PagePrivate(page));
 	block->ops = &indirect_block_ops;
 	block->page = page;
-	page->private = (unsigned long)block;
+
+	if (!PagePrivate(page)) {
 		SetPagePrivate(page);
+		page_cache_get(page);
+		set_page_private(page, (unsigned long) block);
+	}
 
 	block->inode = NULL;
 	li->li_block = NULL;

fs/logfs/segment.c

@@ -86,7 +86,11 @@ int __logfs_buf_write(struct logfs_area *area, u64 ofs, void *buf, size_t len,
 		BUG_ON(!page); /* FIXME: reserve a pool */
 		SetPageUptodate(page);
 		memcpy(page_address(page) + offset, buf, copylen);
+
+		if (!PagePrivate(page)) {
 			SetPagePrivate(page);
+			page_cache_get(page);
+		}
 		page_cache_release(page);
 
 		buf += copylen;
@@ -110,7 +114,10 @@ static void pad_partial_page(struct logfs_area *area)
 		page = get_mapping_page(sb, index, 0);
 		BUG_ON(!page); /* FIXME: reserve a pool */
 		memset(page_address(page) + offset, 0xff, len);
+		if (!PagePrivate(page)) {
 			SetPagePrivate(page);
+			page_cache_get(page);
+		}
 		page_cache_release(page);
 	}
 }
@@ -130,7 +137,10 @@ static void pad_full_pages(struct logfs_area *area)
 		BUG_ON(!page); /* FIXME: reserve a pool */
 		SetPageUptodate(page);
 		memset(page_address(page), 0xff, PAGE_CACHE_SIZE);
+		if (!PagePrivate(page)) {
 			SetPagePrivate(page);
+			page_cache_get(page);
+		}
 		page_cache_release(page);
 		index++;
 		no_indizes--;
@@ -485,8 +495,12 @@ static void move_btree_to_page(struct inode *inode, struct page *page,
 		mempool_free(item, super->s_alias_pool);
 	}
 	block->page = page;
+
+	if (!PagePrivate(page)) {
 		SetPagePrivate(page);
-	page->private = (unsigned long)block;
+		page_cache_get(page);
+		set_page_private(page, (unsigned long) block);
+	}
 	block->ops = &indirect_block_ops;
 	initialize_block_counters(page, block, data, 0);
 }
@@ -536,8 +550,12 @@ void move_page_to_btree(struct page *page)
 		list_add(&item->list, &block->item_list);
 	}
 	block->page = NULL;
+
+	if (PagePrivate(page)) {
 		ClearPagePrivate(page);
-	page->private = 0;
+		page_cache_release(page);
+		set_page_private(page, 0);
+	}
 	block->ops = &btree_block_ops;
 	err = alias_tree_insert(block->sb, block->ino, block->bix, block->level,
 			block);
@@ -702,9 +720,12 @@ void freeseg(struct super_block *sb, u32 segno)
 		page = find_get_page(mapping, ofs >> PAGE_SHIFT);
 		if (!page)
 			continue;
+		if (PagePrivate(page)) {
 			ClearPagePrivate(page);
 			page_cache_release(page);
 		}
+		page_cache_release(page);
+	}
 }
 
 int logfs_open_area(struct logfs_area *area, size_t bytes)