Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751990AbdCTCI5 (ORCPT ); Sun, 19 Mar 2017 22:08:57 -0400 Received: from mga07.intel.com ([134.134.136.100]:36996 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751345AbdCTCIz (ORCPT ); Sun, 19 Mar 2017 22:08:55 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.36,191,1486454400"; d="scan'208";a="78241042" From: "Huang\, Ying" To: Rafael Aquini Cc: "Huang\, Ying" , Andrew Morton , Andi Kleen , Dave Hansen , Shaohua Li , Rik van Riel , Tim Chen , Michal Hocko , Mel Gorman , Aaron Lu , "Kirill A. Shutemov" , Gerald Schaefer , , Subject: Re: [PATCH 1/5] mm, swap: Fix comment in __read_swap_cache_async References: <20170317064635.12792-1-ying.huang@intel.com> <20170317124244.GF956@xps> Date: Mon, 20 Mar 2017 10:07:24 +0800 In-Reply-To: <20170317124244.GF956@xps> (Rafael Aquini's message of "Fri, 17 Mar 2017 08:42:45 -0400") Message-ID: <878to0d7kz.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3071 Lines: 79 Hi, Rafeal, Rafael Aquini writes: > On Fri, Mar 17, 2017 at 02:46:19PM +0800, Huang, Ying wrote: >> From: Huang Ying >> >> The commit cbab0e4eec29 ("swap: avoid read_swap_cache_async() race to >> deadlock while waiting on discard I/O completion") fixed a deadlock in >> read_swap_cache_async(). Because at that time, in swap allocation >> path, a swap entry may be set as SWAP_HAS_CACHE, then wait for >> discarding to complete before the page for the swap entry is added to >> the swap cache. But in the commit 815c2c543d3a ("swap: make swap >> discard async"), the discarding for swap become asynchronous, waiting >> for discarding to complete will be done before the swap entry is set >> as SWAP_HAS_CACHE. So the comments in code is incorrect now. This >> patch fixes the comments. >> >> The cond_resched() added in the commit cbab0e4eec29 is not necessary >> now too. But if we added some sleep in swap allocation path in the >> future, there may be some hard to debug/reproduce deadlock bug. So it >> is kept. >> > > ^ this is a rather disconcerting way to describe why you left that part > behind, and I recollect telling you about it in a private discussion. > > The fact is that __read_swap_cache_async() still races against get_swap_page() > with a way narrower window due to the async fashioned SSD wear leveling > done for swap nowadays and other changes made within __read_swap_cache_async()'s > while loop thus making that old deadlock scenario very improbable to strike again. Thanks for your comments! Could you tell me which kind of race remaining? > All seems legit, apart from that last paragraph in the commit log > message > > > Acked-by: Rafael Aquini Thanks! Best Regards, Huang, Ying >> Cc: Shaohua Li >> Cc: Rafael Aquini >> Signed-off-by: "Huang, Ying" >> --- >> mm/swap_state.c | 12 +----------- >> 1 file changed, 1 insertion(+), 11 deletions(-) >> >> diff --git a/mm/swap_state.c b/mm/swap_state.c >> index 473b71e052a8..7bfb9bd1ca21 100644 >> --- a/mm/swap_state.c >> +++ b/mm/swap_state.c >> @@ -360,17 +360,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, >> /* >> * We might race against get_swap_page() and stumble >> * across a SWAP_HAS_CACHE swap_map entry whose page >> - * has not been brought into the swapcache yet, while >> - * the other end is scheduled away waiting on discard >> - * I/O completion at scan_swap_map(). >> - * >> - * In order to avoid turning this transitory state >> - * into a permanent loop around this -EEXIST case >> - * if !CONFIG_PREEMPT and the I/O completion happens >> - * to be waiting on the CPU waitqueue where we are now >> - * busy looping, we just conditionally invoke the >> - * scheduler here, if there are some more important >> - * tasks to run. >> + * has not been brought into the swapcache yet. >> */ >> cond_resched(); >> continue; >> -- >> 2.11.0 >>