From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f43.google.com (mail-wm0-f43.google.com [74.125.82.43]) by kanga.kvack.org (Postfix) with ESMTP id E75086B0253 for ; Mon, 18 Jan 2016 09:11:01 -0500 (EST) Received: by mail-wm0-f43.google.com with SMTP id b14so124896219wmb.1 for ; Mon, 18 Jan 2016 06:11:01 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id c9si25573068wmh.52.2016.01.18.06.11.00 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 18 Jan 2016 06:11:00 -0800 (PST) Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition References: <1453095596-44055-1-git-send-email-junil0814.lee@lge.com> <20160118063611.GC7453@bbox> <20160118065434.GB459@swordfish> <20160118071157.GD7453@bbox> <20160118073939.GA30668@swordfish> <569C9A1F.2020303@suse.cz> <20160118082000.GA20244@bbox> <569CD817.7090309@suse.cz> <20160118140955.GB20244@bbox> From: Vlastimil Babka Message-ID: <569CF272.3020001@suse.cz> Date: Mon, 18 Jan 2016 15:10:58 +0100 MIME-Version: 1.0 In-Reply-To: <20160118140955.GB20244@bbox> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Sergey Senozhatsky , Junil Lee , ngupta@vflare.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 01/18/2016 03:09 PM, Minchan Kim wrote: > > True. > Let's go with this. I hope it's the last. > Thanks, guys. > > From 389bbcbad9aba7d86a575b8c6ea3b8985cc801ea Mon Sep 17 00:00:00 2001 > From: Junil Lee > Date: Mon, 18 Jan 2016 23:01:29 +0900 > Subject: [PATCH v5] zsmalloc: fix migrate_zspage-zs_free race condition > > record_obj() in migrate_zspage() does not preserve handle's > HANDLE_PIN_BIT, set by find_aloced_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 > > This patch keeps the lock bit in migration path and update > value atomically. > > Signed-off-by: Junil Lee > Signed-off-by: Minchan Kim Acked-by: Vlastimil Babka > Cc: [4.1+] > --- > mm/zsmalloc.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index e7414cec220b..2d7c4c11fc63 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -309,7 +309,12 @@ static void free_handle(struct zs_pool *pool, unsigned long handle) > > static void record_obj(unsigned long handle, unsigned long obj) > { > - *(unsigned long *)handle = obj; > + /* > + * lsb of @obj represents handle lock while other bits > + * represent object value the handle is pointing so > + * updating shouldn't do store tearing. > + */ > + WRITE_ONCE(*(unsigned long *)handle, obj); > } > > /* zpool driver */ > @@ -1635,6 +1640,13 @@ 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++; > + /* > + * 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. > + */ > + free_obj |= BIT(HANDLE_PIN_BIT); > record_obj(handle, free_obj); > unpin_tag(handle); > obj_free(pool, class, used_obj); > -- 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