From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 837E16B0008 for ; Tue, 20 Feb 2018 18:39:01 -0500 (EST) Received: by mail-wr0-f198.google.com with SMTP id 5so9074048wrt.12 for ; Tue, 20 Feb 2018 15:39:01 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id k8si17906865wrf.484.2018.02.20.15.38.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 20 Feb 2018 15:39:00 -0800 (PST) Date: Tue, 20 Feb 2018 15:38:57 -0800 From: Andrew Morton Subject: Re: [PATCH -mm -v5 RESEND] mm, swap: Fix race between swapoff and some swap operations Message-Id: <20180220153857.dee8f7d5c904cd7219529312@linux-foundation.org> In-Reply-To: References: <20180213014220.2464-1-ying.huang@intel.com> <20180213154123.9f4ef9e406ea8365ca46d9c5@linux-foundation.org> <87fu64jthz.fsf@yhuang-dev.intel.com> <20180216153823.ad74f1d2c157adc67ed2c970@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: huang ying Cc: "Huang, Ying" , linux-mm@kvack.org, LKML , Hugh Dickins , "Paul E . McKenney" , Minchan Kim , Johannes Weiner , Tim Chen , Shaohua Li , Mel Gorman , jglisse@redhat.com, Michal Hocko , Andrea Arcangeli , David Rientjes , Rik van Riel , Jan Kara , Dave Jiang , Aaron Lu On Sun, 18 Feb 2018 09:06:47 +0800 huang ying wrote: > >> >> +struct swap_info_struct *get_swap_device(swp_entry_t entry) > >> >> +{ > >> >> + struct swap_info_struct *si; > >> >> + unsigned long type, offset; > >> >> + > >> >> + if (!entry.val) > >> >> + goto out; > >> >> + type = swp_type(entry); > >> >> + if (type >= nr_swapfiles) > >> >> + goto bad_nofile; > >> >> + si = swap_info[type]; > >> >> + > >> >> + preempt_disable(); > >> > > >> > This preempt_disable() is later than I'd expect. If a well-timed race > >> > occurs, `si' could now be pointing at a defunct entry. If that > >> > well-timed race include a swapoff AND a swapon, `si' could be pointing > >> > at the info for a new device? > >> > >> struct swap_info_struct pointed to by swap_info[] will never be freed. > >> During swapoff, we only free the memory pointed to by the fields of > >> struct swap_info_struct. And when swapon, we will always reuse > >> swap_info[type] if it's not NULL. So it should be safe to dereference > >> swap_info[type] with preemption enabled. > > > > That's my point. If there's a race window during which there is a > > parallel swapoff+swapon, this swap_info_struct may now be in use for a > > different device? > > Yes. It's possible. And the caller of get_swap_device() can live > with it if the swap_info_struct has been fully initialized. For > example, for the race in the patch description, > > do_swap_page > swapin_readahead > __read_swap_cache_async > swapcache_prepare > __swap_duplicate > > in __swap_duplicate(), it's possible that the swap device returned by > get_swap_device() is different from the swap device when > __swap_duplicate() call get_swap_device(). But the struct_info_struct > has been fully initialized, so __swap_duplicate() can reference > si->swap_map[] safely. And we will check si->swap_map[] before any > further operation. Even if the swap entry is swapped out again for > the new swap device, we will check the page table again in > do_swap_page(). So there is no functionality problem. That's rather revolting. Can we tighten this up? Or at least very loudly document it? -- 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