Return-Path: Received: from mx2.suse.de ([195.135.220.15]:60972 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727483AbeJDQ4l (ORCPT ); Thu, 4 Oct 2018 12:56:41 -0400 Date: Thu, 4 Oct 2018 12:04:06 +0200 From: Johannes Thumshirn To: Jan Kara Cc: Dan Williams , Christoph Hellwig , Dave Jiang , linux-nvdimm , Linux MM , linux-fsdevel , linux-ext4 , linux-xfs , Linux API Subject: Re: Problems with VM_MIXEDMAP removal from /proc//smaps Message-ID: <20181004100406.GE6682@linux-x5ow.site> References: <20181002143713.GA19845@infradead.org> <20181002144412.GC4963@linux-x5ow.site> <20181002145206.GA10903@infradead.org> <20181002153100.GG9127@quack2.suse.cz> <20181003125056.GA21043@quack2.suse.cz> <20181003150658.GC24030@quack2.suse.cz> <20181003164407.GK24030@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20181003164407.GK24030@quack2.suse.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Oct 03, 2018 at 06:44:07PM +0200, Jan Kara wrote: > On Wed 03-10-18 08:13:37, Dan Williams wrote: > > On Wed, Oct 3, 2018 at 8:07 AM Jan Kara wrote: > > > WRT per-inode DAX property, AFAIU that inode flag is just going to be > > > advisory thing - i.e., use DAX if possible. If you mount a filesystem with > > > these inode flags set in a configuration which does not allow DAX to be > > > used, you will still be able to access such inodes but the access will use > > > page cache instead. And querying these flags should better show real > > > on-disk status and not just whether DAX is used as that would result in an > > > even bigger mess. So this feature seems to be somewhat orthogonal to the > > > API I'm looking for. > > > > True, I imagine once we have that flag we will be able to distinguish > > the "saved" property and the "effective / live" property of DAX... > > Also it's really not DAX that applications care about as much as "is > > there page-cache indirection / overhead for this mapping?". That seems > > to be a narrower guarantee that we can make than what "DAX" might > > imply. > > Right. So what do people think about my suggestion earlier in the thread to > use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success > when DAX is in use, failure otherwise. Later we could extend it to be also > used as a hint for caching policy for the inode... Hmm apart from Dan's objection that it can't really be used for a query, isn't madvise(2) for mmap(2)? But AFAIU (from looking at the xfs code, so please correct me if I', wrong), DAX can be used for the traditional read(2)/write(2) interface as well. There is at least: xfs_file_read_iter() `-> if (IS_DAX(inode)) `-> xfs_file_dax_read() `->dax_iomap_rw() So IMHO something on an inode granularity would make more sens to me. Byte, Johannes -- Johannes Thumshirn Storage jthumshirn@suse.de +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 N?rnberg GF: Felix Imend?rffer, Jane Smithard, Graham Norton HRB 21284 (AG N?rnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850