From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f170.google.com (mail-pf0-f170.google.com [209.85.192.170]) by kanga.kvack.org (Postfix) with ESMTP id 5B9FF828DF for ; Fri, 15 Jan 2016 09:34:54 -0500 (EST) Received: by mail-pf0-f170.google.com with SMTP id n128so118558256pfn.3 for ; Fri, 15 Jan 2016 06:34:54 -0800 (PST) Received: from mail-pf0-x241.google.com (mail-pf0-x241.google.com. [2607:f8b0:400e:c00::241]) by mx.google.com with ESMTPS id dg7si16832983pad.75.2016.01.15.06.34.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 15 Jan 2016 06:34:53 -0800 (PST) Received: by mail-pf0-x241.google.com with SMTP id e65so8631900pfe.0 for ; Fri, 15 Jan 2016 06:34:53 -0800 (PST) Date: Fri, 15 Jan 2016 23:34:34 +0900 From: Minchan Kim 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org