From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f52.google.com (mail-pa0-f52.google.com [209.85.220.52]) by kanga.kvack.org (Postfix) with ESMTP id 5330C6B0005 for ; Sun, 17 Jan 2016 23:13:31 -0500 (EST) Received: by mail-pa0-f52.google.com with SMTP id ho8so166121691pac.2 for ; Sun, 17 Jan 2016 20:13:31 -0800 (PST) Received: from mail-pa0-x243.google.com (mail-pa0-x243.google.com. [2607:f8b0:400e:c03::243]) by mx.google.com with ESMTPS id gy5si36721368pac.83.2016.01.17.20.13.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 17 Jan 2016 20:13:30 -0800 (PST) Received: by mail-pa0-x243.google.com with SMTP id gi1so39428539pac.2 for ; Sun, 17 Jan 2016 20:13:30 -0800 (PST) Date: Mon, 18 Jan 2016 13:14:40 +0900 From: Sergey Senozhatsky Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160118041440.GA415@swordfish> References: <1453079732-44198-1-git-send-email-junil0814.lee@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1453079732-44198-1-git-send-email-junil0814.lee@lge.com> Sender: owner-linux-mm@kvack.org List-ID: To: Junil Lee Cc: minchan@kernel.org, ngupta@vflare.org, sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, sergey.senozhatsky@gmail.com, Vlastimil Babka Cc Vlastimil, Hello, On (01/18/16 10:15), 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() Junil, can something like this be a bit simpler problem description? --- record_obj() in migrate_zspage() does not preserve handle's HANDLE_PIN_BIT, set by find_alloced_obj()->trypin_tag(), and implicitly (accidentally) un-pins the handle, while migrate_zspage() still performs an explicit unpin_tag() on the that handle. This additional explicit unpin_tag() introduces a race condition with zs_free(), which can pin that handle by this time, so the handle becomes un-pinned. Schematically, it goes like this: CPU0 CPU1 migrate_zspage find_alloced_obj trypin_tag set HANDLE_PIN_BIT zs_free() pin_tag() obj_malloc() -- new object, no tag record_obj() -- remove HANDLE_PIN_BIT set HANDLE_PIN_BIT unpin_tag() -- remove zs_free's HANDLE_PIN_BIT The race condition may result in a NULL pointer dereference: 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 Fix the race by removing explicit unpin_tag() from migrate_zspage(). --- > 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 I believe Vlastimil deserves a credit here (at least Suggested-by) Suggested-by: Vlastimil Babka now, can the compiler re-order record_obj(handle, free_obj); obj_free(pool, class, used_obj); ? -ss > --- > mm/zsmalloc.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index e7414ce..0acfa20 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -1635,8 +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++; > + /* This also effectively unpins the handle */ > record_obj(handle, free_obj); > - unpin_tag(handle); > obj_free(pool, class, used_obj); > } > > -- > 2.6.2 > -- 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