From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753547AbXJILWN (ORCPT ); Tue, 9 Oct 2007 07:22:13 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752111AbXJILV7 (ORCPT ); Tue, 9 Oct 2007 07:21:59 -0400 Received: from mtagate1.de.ibm.com ([195.212.29.150]:22791 "EHLO mtagate1.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751788AbXJILV6 (ORCPT ); Tue, 9 Oct 2007 07:21:58 -0400 From: Christian Borntraeger To: Andrew Morton Subject: [PATCH] ramdisk: fix zeroed ramdisk pages on memory pressure Date: Tue, 9 Oct 2007 13:21:46 +0200 User-Agent: KMail/1.9.7 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Martin Schwidefsky MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200710091321.46891.borntraeger@de.ibm.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org From: Christian Borntraeger We have seen ramdisk based install systems, where some pages of mapped libraries and programs were suddendly zeroed under memory pressure. This should not happen, as the ramdisk avoids freeing its pages by keeping them dirty all the time. It turns out that there is a case, where the VM makes a ramdisk page clean, without telling the ramdisk driver. On memory pressure shrink_zone runs and it starts to run shrink_active_list. There is a check for buffer_heads_over_limit, and if true, pagevec_strip is called. pagevec_strip calls try_to_release_page. If the mapping has no releasepage callback, try_to_free_buffers is called. try_to_free_buffers has now a special logic for some file systems to make a dirty page clean, if all buffers are clean. Thats what happened in our test case. The solution is to provide a noop-releasepage callback for the ramdisk driver. This avoids try_to_free_buffers for ramdisk pages. To trigger the problem, you have to make buffer_heads_over_limit true, which means: - lower max_buffer_heads or - have a system with lots of high memory Andrew, if there are no objections - please apply. The patch applies against 2.6.23-rc9. Signed-off-by: Christian Borntraeger --- drivers/block/rd.c | 13 +++++++++++++ 1 files changed, 13 insertions(+) Index: linux-2.6/drivers/block/rd.c =================================================================== --- linux-2.6.orig/drivers/block/rd.c +++ linux-2.6/drivers/block/rd.c @@ -189,6 +189,18 @@ static int ramdisk_set_page_dirty(struct return 0; } +/* + * releasepage is called by pagevec_strip/try_to_release_page if + * buffers_heads_over_limit is true. Without a releasepage function + * try_to_free_buffers is called instead. That can unset the dirty + * bit of our ram disk pages, which will be eventually freed, even + * if the page is still in use. + */ +static int ramdisk_releasepage(struct page *page, gfp_t dummy) +{ + return 0; +} + static const struct address_space_operations ramdisk_aops = { .readpage = ramdisk_readpage, .prepare_write = ramdisk_prepare_write, @@ -196,6 +208,7 @@ static const struct address_space_operat .writepage = ramdisk_writepage, .set_page_dirty = ramdisk_set_page_dirty, .writepages = ramdisk_writepages, + .releasepage = ramdisk_releasepage, }; static int rd_blkdev_pagecache_IO(int rw, struct bio_vec *vec, sector_t sector,