From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753559AbcAOOez (ORCPT ); Fri, 15 Jan 2016 09:34:55 -0500 Received: from mail-pf0-f195.google.com ([209.85.192.195]:34024 "EHLO mail-pf0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752077AbcAOOex (ORCPT ); Fri, 15 Jan 2016 09:34:53 -0500 Date: Fri, 15 Jan 2016 23:34:34 +0900 From: Minchan Kim To: Junil Lee Cc: ngupta@vflare.org, sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160115143434.GA25332@blaptop.local> References: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 15, 2016 at 04:39:11PM +0900, Junil Lee wrote: > 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. If you verified it solved your problem, we should mark this patch as stable. > > Signed-off-by: Junil Lee Acked-by: Minchan Kim Below comment. > --- > 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() */ I want to make comment more clear. /* * record_obj updates handle's value to free_obj and it will invalidate * lock bit(ie, HANDLE_PIN_BIT) of handle, which breaks synchronization * using pin_tag(e,g, zs_free) so let's keep the lock bit. */ Thanks. > + free_obj |= BIT(HANDLE_PIN_BIT); > record_obj(handle, free_obj); > unpin_tag(handle); > obj_free(pool, class, used_obj); > -- > 2.6.2 > -- Kind regards, Minchan Kim