Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753901Ab2FJAli (ORCPT ); Sat, 9 Jun 2012 20:41:38 -0400 Received: from mail-pz0-f46.google.com ([209.85.210.46]:35715 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752857Ab2FJAlh (ORCPT ); Sat, 9 Jun 2012 20:41:37 -0400 From: Nitin Gupta To: Greg KH Cc: Dan Magenheimer , Konrad Rzeszutek Wilk , Seth Jennings , Minchan Kim , linux-mm , linux-kernel , Nitin Gupta Subject: [PATCH v2] zsmalloc documentation Date: Sat, 9 Jun 2012 17:41:14 -0700 Message-Id: <1339288874-2743-1-git-send-email-ngupta@vflare.org> X-Mailer: git-send-email 1.7.10.2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3039 Lines: 78 Documentation of various struct page fields used by zsmalloc. Signed-off-by: Nitin Gupta Changes for v2: - Regroup descriptions as suggested by Seth --- drivers/staging/zsmalloc/zsmalloc-main.c | 48 ++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c index 4496737..1db76ec 100644 --- a/drivers/staging/zsmalloc/zsmalloc-main.c +++ b/drivers/staging/zsmalloc/zsmalloc-main.c @@ -10,6 +10,54 @@ * Released under the terms of GNU General Public License Version 2.0 */ + +/* + * This allocator is designed for use with zcache and zram. Thus, the + * allocator is supposed to work well under low memory conditions. In + * particular, it never attempts higher order page allocation which is + * very likely to fail under memory pressure. On the other hand, if we + * just use single (0-order) pages, it would suffer from very high + * fragmentation -- any object of size PAGE_SIZE/2 or larger would occupy + * an entire page. This was one of the major issues with its predecessor + * (xvmalloc). + * + * To overcome these issues, zsmalloc allocates a bunch of 0-order pages + * and links them together using various 'struct page' fields. These linked + * pages act as a single higher-order page i.e. an object can span 0-order + * page boundaries. The code refers to these linked pages as a single entity + * called zspage. + * + * Following is how we use various fields and flags of underlying + * struct page(s) to form a zspage. + * + * Usage of struct page fields: + * page->first_page: points to the first component (0-order) page + * page->index (union with page->freelist): offset of the first object + * starting in this page. For the first page, this is + * always 0, so we use this field (aka freelist) to point + * to the first free object in zspage. + * page->lru: links together all component pages (except the first page) + * of a zspage + * + * For _first_ page only: + * + * page->private (union with page->first_page): refers to the + * component page after the first page + * page->freelist: points to the first free object in zspage. + * Free objects are linked together using in-place + * metadata. + * page->objects: maximum number of objects we can store in this + * zspage (class->zspage_order * PAGE_SIZE / class->size) + * page->lru: links together first pages of various zspages. + * Basically forming list of zspages in a fullness group. + * page->mapping: class index and fullness group of the zspage + * + * Usage of struct page flags: + * PG_private: identifies the first component page + * PG_private2: identifies the last component page + * + */ + #ifdef CONFIG_ZSMALLOC_DEBUG #define DEBUG #endif -- 1.7.10.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/