From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:53884 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751046AbeFEAq3 (ORCPT ); Mon, 4 Jun 2018 20:46:29 -0400 Date: Mon, 4 Jun 2018 20:46:28 -0400 From: Mike Snitzer To: Ross Zwisler , Toshi Kani , dm-devel@redhat.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode Message-ID: <20180605004558.GB6898@redhat.com> References: <20180529195106.14268-1-ross.zwisler@linux.intel.com> <20180529195106.14268-6-ross.zwisler@linux.intel.com> <20180601220443.GB18712@redhat.com> <20180604232416.GB10666@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180604232416.GB10666@linux.intel.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Jun 04 2018 at 7:24pm -0400, Ross Zwisler wrote: > On Fri, Jun 01, 2018 at 06:04:43PM -0400, Mike Snitzer wrote: > > On Tue, May 29 2018 at 3:51pm -0400, > > Ross Zwisler wrote: > > > > > The DM_TYPE_DAX_BIO_BASED dm_queue_mode was introduced to prevent DM > > > devices that could possibly support DAX from transitioning into DM devices > > > that cannot support DAX. > > > > > > For example, the following transition will currently fail: > > > > > > dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw] > > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > > > but these will both succeed: > > > > > > dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw] > > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > > > > I fail to see how this succeeds given > > drivers/md/dm-ioctl.c:is_valid_type() only allows transitions from: > > > > DM_TYPE_BIO_BASED => DM_TYPE_DAX_BIO_BASED > > Right, sorry, that was a typo. What I meant was: > > > For example, the following transition will currently fail: > > > > dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw] > > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED > > > > but these will both succeed: > > > > dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw] > > DM_TYPE_BIO_BASED DM_TYPE_BIO_BASED > > > > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem] > > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED > > So we allow 2 of the 3 transitions, but the reason that we disallow the third > isn't fully clear to me. > > > > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem] > > > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED > > > > > > This seems arbitrary, as really the choice on whether to use DAX happens at > > > filesystem mount time. There's no guarantee that the in the first case > > > (double fsdax pmem) we were using the dax mount option with our file > > > system. > > > > > > Instead, get rid of DM_TYPE_DAX_BIO_BASED and all the special casing around > > > it, and instead make the request queue's QUEUE_FLAG_DAX be our one source > > > of truth. If this is set, we can use DAX, and if not, not. We keep this > > > up to date in table_load() as the table changes. As with regular block > > > devices the filesystem will then know at mount time whether DAX is a > > > supported mount option or not. > > > > If you don't think you need this specialization that is fine.. but DM > > devices supporting suspending (as part of table reloads) so is there any > > risk that there will be inflight IO (say if someone did 'dmsetup suspend > > --noflush').. and then upon reload the device type changed out from > > under us.. anyway, I don't have all the PMEM DAX stuff paged back into > > my head yet. > > > > But this just seems like we really shouldn't be allowing the > > transition from what was DM_TYPE_DAX_BIO_BASED back to DM_TYPE_BIO_BASED > > I admit I don't fully understand all the ways that DM supports suspending and > resuming devices. Is there actually a case where we can change out the DM > devices while I/O is running, and somehow end up trying to issue a DAX I/O to > a device that doesn't support DAX? Yes, provided root permissions, it's very easy to dmsetup suspend/load/resume to replace any portion of the DM device's logical address space to map to an entirely different DM target (with a different backing store). It's pretty intrusive to do such things, but easily done and powerful. Mike