From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752379AbcAOHjX (ORCPT ); Fri, 15 Jan 2016 02:39:23 -0500 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:59462 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750811AbcAOHjW (ORCPT ); Fri, 15 Jan 2016 02:39:22 -0500 X-Original-SENDERIP: 156.147.1.126 X-Original-MAILFROM: junil0814.lee@lge.com X-Original-SENDERIP: 165.244.98.76 X-Original-MAILFROM: junil0814.lee@lge.com X-Original-SENDERIP: 10.168.177.123 X-Original-MAILFROM: junil0814.lee@lge.com From: Junil Lee To: , CC: sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Junil Lee Subject: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition Date: Fri, 15 Jan 2016 16:39:11 +0900 Message-ID: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com> X-Mailer: git-send-email 2.6.2 X-MIMETrack: Itemize by SMTP Server on LGEKRMHUB07/LGE/LG Group(Release 8.5.3FP6|November 21, 2013) at 2016/01/15 16:39:19, Serialize by Router on LGEKRMHUB07/LGE/LG Group(Release 8.5.3FP6|November 21, 2013) at 2016/01/15 16:39:19, Serialize complete at 2016/01/15 16:39:19 MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org To prevent unlock at the not correct situation, tagging the new obj to assure lock in migrate_zspage() before right unlock path. Two functions are in race condition by tag which set 1 on last bit of obj, however unlock succrently when update new obj to handle before call unpin_tag() which is right unlock path. summarize this problem by call flow as below: CPU0 CPU1 migrate_zspage find_alloced_obj() trypin_tag() -- obj |= HANDLE_PIN_BIT obj_malloc() -- new obj is not set zs_free record_obj() -- unlock and break sync pin_tag() -- get lock unpin_tag() Before code make crash as below: Unable to handle kernel NULL pointer dereference at virtual address 00000000 CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted: PC is at get_zspage_mapping+0x0/0x24 LR is at obj_free.isra.22+0x64/0x128 Call trace: [] get_zspage_mapping+0x0/0x24 [] zs_free+0x88/0x114 [] zram_free_page+0x64/0xcc [] zram_slot_free_notify+0x90/0x108 [] swap_entry_free+0x278/0x294 [] free_swap_and_cache+0x38/0x11c [] unmap_single_vma+0x480/0x5c8 [] unmap_vmas+0x44/0x60 [] exit_mmap+0x50/0x110 [] mmput+0x58/0xe0 [] do_exit+0x320/0x8dc [] do_group_exit+0x44/0xa8 [] get_signal+0x538/0x580 [] do_signal+0x98/0x4b8 [] do_notify_resume+0x14/0x5c and for test, print obj value after pin_tag() in zs_free(). Sometimes obj is even number means break synchronization. After patched, crash is not occurred and obj is only odd number in same situation. Signed-off-by: Junil Lee --- mm/zsmalloc.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index e7414ce..a24ccb1 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -1635,6 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class, free_obj = obj_malloc(d_page, class, handle); zs_object_copy(free_obj, used_obj, class); index++; + /* Must not unlock before unpin_tag() */ + free_obj |= BIT(HANDLE_PIN_BIT); record_obj(handle, free_obj); unpin_tag(handle); obj_free(pool, class, used_obj); -- 2.6.2