From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1768113AbcHROQB (ORCPT ); Thu, 18 Aug 2016 10:16:01 -0400 Received: from mx2.suse.de ([195.135.220.15]:52698 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1767991AbcHROPz (ORCPT ); Thu, 18 Aug 2016 10:15:55 -0400 Date: Thu, 18 Aug 2016 16:15:50 +0200 From: Jan Kara To: Ross Zwisler Cc: Jan Kara , linux-kernel@vger.kernel.org, "Theodore Ts'o" , Alexander Viro , Andreas Dilger , Andrew Morton , Dan Williams , Dave Chinner , Jan Kara , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@ml01.01.org Subject: Re: [PATCH 5/7] dax: lock based on slot instead of [mapping, index] Message-ID: <20160818141550.GA2382@quack2.suse.cz> References: <20160815190918.20672-1-ross.zwisler@linux.intel.com> <20160815190918.20672-6-ross.zwisler@linux.intel.com> <20160816092816.GE27284@quack2.suse.cz> <20160817202556.GA13009@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160817202556.GA13009@linux.intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 17-08-16 14:25:56, Ross Zwisler wrote: > On Tue, Aug 16, 2016 at 11:28:16AM +0200, Jan Kara wrote: > > On Mon 15-08-16 13:09:16, Ross Zwisler wrote: > > > DAX radix tree locking currently locks entries based on the unique > > > combination of the 'mapping' pointer and the pgoff_t 'index' for the entry. > > > This works for PTEs, but as we move to PMDs we will need to have all the > > > offsets within the range covered by the PMD to map to the same bit lock. > > > To accomplish this, lock based on the 'slot' pointer in the radix tree > > > instead of [mapping, index]. > > > > I'm not convinced this is safe. What makes the slot pointer still valid > > after you drop tree_lock? At least radix_tree_shrink() or > > radix_tree_expand() could move your slot without letting the waiter know > > and he would be never woken. > > > > Honza > > Yep, you're right, thanks for catching that. > > Given that we can't rely on 'slot' being stable, my next idea is to use a > combination of [mapping, index], but tweak 'index' so that it's always the > beginning of the entry. So for 4k entries we'd leave it alone, but for 2MiB > entries we'd mask it down to the appropriate 2MiB barrier. > > Let me hack on that for a bit, unless you've a better idea. No, that's what I'd do as well. Honza -- Jan Kara SUSE Labs, CR