From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id B2FB96B0038 for ; Fri, 15 Dec 2017 05:04:47 -0500 (EST) Received: by mail-wr0-f199.google.com with SMTP id l33so4791898wrl.5 for ; Fri, 15 Dec 2017 02:04:47 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id k9si4336574wmi.254.2017.12.15.02.04.46 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 15 Dec 2017 02:04:46 -0800 (PST) Date: Fri, 15 Dec 2017 11:04:43 +0100 From: Michal Hocko Subject: Re: [PATCH -mm -V2] mm, swap: Fix race between swapoff and some swap operations Message-ID: <20171215100443.GX16951@dhcp22.suse.cz> References: <20171214133832.11266-1-ying.huang@intel.com> <20171214151718.GS16951@dhcp22.suse.cz> <20171214124246.ceebc9c955bd32601c01a28b@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171214124246.ceebc9c955bd32601c01a28b@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: "Huang, Ying" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Minchan Kim , Johannes Weiner , Tim Chen , Shaohua Li , Mel Gorman , =?iso-8859-1?B?Suly9G1l?= Glisse , Andrea Arcangeli , David Rientjes , Rik van Riel , Jan Kara , Dave Jiang , Aaron Lu On Thu 14-12-17 12:42:46, Andrew Morton wrote: > On Thu, 14 Dec 2017 16:17:18 +0100 Michal Hocko wrote: > > > > as fast as possible, SRCU instead of reference count is used to > > > implement get/put_swap_device(). From get_swap_device() to > > > put_swap_device(), the reader side of SRCU is locked, so > > > synchronize_srcu() in swapoff() will wait until put_swap_device() is > > > called. > > > > It is quite unfortunate to pull SRCU as a dependency to the core kernel. > > Different attempts to do this have failed in the past. This one is > > slightly different though because I would suspect that those tiny > > systems do not configure swap. But who knows, maybe they do. > > > > Anyway, if you are worried about performance then I would expect some > > numbers to back that worry. So why don't simply start with simpler > > ref count based and then optimize it later based on some actual numbers. > > Btw. have you considered pcp refcount framework. I would suspect that > > this would give you close to SRCU performance. > > Or use stop_kernel() ;) well, stop_kernel is a _huge_ hammer. I think we can do much better without a large complexity. A simple ref counting (or pcp refcounting if the former has measurable complexity) should do just fine. -- Michal Hocko SUSE Labs -- 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