From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756407AbcBBRfG (ORCPT ); Tue, 2 Feb 2016 12:35:06 -0500 Received: from mga09.intel.com ([134.134.136.24]:27356 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756314AbcBBRfE (ORCPT ); Tue, 2 Feb 2016 12:35:04 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.22,385,1449561600"; d="scan'208";a="739262498" Date: Tue, 2 Feb 2016 10:34:56 -0700 From: Ross Zwisler To: Dan Williams Cc: Jan Kara , Dave Chinner , Matthew Wilcox , Ross Zwisler , Christoph Hellwig , "linux-kernel@vger.kernel.org" , Alexander Viro , Andrew Morton , Jan Kara , linux-fsdevel , linux-nvdimm Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Message-ID: <20160202173456.GB23963@linux.intel.com> Mail-Followup-To: Ross Zwisler , Dan Williams , Jan Kara , Dave Chinner , Matthew Wilcox , Christoph Hellwig , "linux-kernel@vger.kernel.org" , Alexander Viro , Andrew Morton , Jan Kara , linux-fsdevel , linux-nvdimm References: <1454009704-25959-2-git-send-email-ross.zwisler@linux.intel.com> <20160128213858.GA29114@infradead.org> <20160129182815.GB5224@linux.intel.com> <20160130052833.GY2948@linux.intel.com> <20160201145147.GD13740@quack.suse.cz> <20160201214730.GR20456@dastard> <20160202111723.GD12574@quack.suse.cz> <20160202164642.GE12574@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Tue, Feb 02, 2016 at 09:10:24AM -0800, Dan Williams wrote: > On Tue, Feb 2, 2016 at 8:46 AM, Jan Kara wrote: > > On Tue 02-02-16 08:33:56, Dan Williams wrote: > >> On Tue, Feb 2, 2016 at 3:17 AM, Jan Kara wrote: > >> [..] > >> > I see, thanks for explanation. So I'm OK with changing what is stored in > >> > the radix tree to accommodate this use case but my reservation that we IHMO > >> > have other more pressing things to fix remains... > >> > >> We don't need pfns in the radix to support XFS RT configurations. > >> Just call get_blocks() again and use the sector, or am I missing > >> something? > > > > You are correct. But if you decide to pay the cost of additional > > get_block() call, you only need the dirty tag in the radix tree and nothing > > else. So my understanding was that the whole point of games with radix tree > > is avoiding this extra get_block() calls for fsync(). > > > > DAX-fsync() is already a potentially expensive operation to cover data > durability guarantees for DAX-unaware applications. A DAX-aware > application is going to skip fsync, and the get_blocks() cost, to do > cache management itself. > > Willy pointed out some other potential benefits, assuming a suitable > replacement for the protections afforded by the block-device > percpu_ref counter can be found. However, optimizing for the > DAX-unaware-application case seems the wrong motivation to me. Oh, no, the primary issue with calling get_block() in the fsync path isn't performance. It's that we don't have any idea what get_block() function to call. The fault handler calls all come from the filesystem directly, so they can easily give us an appropriate get_block() function pointer. But the dax_writeback_mapping_range() calls come from the generic code in mm/filemap.c, and don't know what get_block() to pass in. During one iteration I had the calls to dax_writeback_mapping_range() happening in the filesystem fsync code so that it could pass in get_block(), but Dave Chinner pointed out that this misses other paths in the filesystem that need to have things flushed via a call to filemap_write_and_wait_range(). In yet another iteration of this series I tried adding get_block() to struct inode_operations so that I could access it from what is now dax_writeback_mapping_range(), but this was shot down as well.