From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751418AbeBUG2L (ORCPT ); Wed, 21 Feb 2018 01:28:11 -0500 Received: from mail-qt0-f194.google.com ([209.85.216.194]:33535 "EHLO mail-qt0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750716AbeBUG2K (ORCPT ); Wed, 21 Feb 2018 01:28:10 -0500 X-Google-Smtp-Source: AH8x225sbeAEmlpkhgBaCEYIarINMFQdO5YMkSdtArfBiXcVKbmhkKPzsJZGbTV+s2y93JUjuco0Toy1AGjjbTP0hjA= MIME-Version: 1.0 In-Reply-To: <20180220153857.dee8f7d5c904cd7219529312@linux-foundation.org> References: <20180213014220.2464-1-ying.huang@intel.com> <20180213154123.9f4ef9e406ea8365ca46d9c5@linux-foundation.org> <87fu64jthz.fsf@yhuang-dev.intel.com> <20180216153823.ad74f1d2c157adc67ed2c970@linux-foundation.org> <20180220153857.dee8f7d5c904cd7219529312@linux-foundation.org> From: huang ying Date: Wed, 21 Feb 2018 14:28:08 +0800 Message-ID: Subject: Re: [PATCH -mm -v5 RESEND] mm, swap: Fix race between swapoff and some swap operations To: Andrew Morton 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 21, 2018 at 7:38 AM, Andrew Morton wrote: > 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? TBH, I think my original fix patch which uses a reference count in swap_info_struct is easier to be understood. But I understand it has its own drawbacks too. Anyway, unless there are some better ideas to resolve this, I will send out a new version with more document. Best Regards, Huang, Ying