From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay2.corp.sgi.com [137.38.102.29]) by oss.sgi.com (Postfix) with ESMTP id C1B097CA0 for ; Thu, 15 Sep 2016 18:08:06 -0500 (CDT) Received: from cuda.sgi.com (cuda1.sgi.com [192.48.157.11]) by relay2.corp.sgi.com (Postfix) with ESMTP id 798A0304059 for ; Thu, 15 Sep 2016 16:08:03 -0700 (PDT) Received: from ipmail05.adl6.internode.on.net (ipmail05.adl6.internode.on.net [150.101.137.143]) by cuda.sgi.com with ESMTP id kpGuZ6DoE69gMLPy for ; Thu, 15 Sep 2016 16:07:57 -0700 (PDT) Date: Fri, 16 Sep 2016 09:07:48 +1000 From: Dave Chinner Subject: Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Message-ID: <20160915230748.GS30497@dastard> References: <147392246509.9873.17750323049785100997.stgit@dwillia2-desk3.amr.corp.intel.com> <147392247875.9873.4205533916442000884.stgit@dwillia2-desk3.amr.corp.intel.com> <20160915082615.GA9772@lst.de> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com To: Dan Williams Cc: "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" , Nicholas Piggin , XFS Developers , Linux MM , linux-fsdevel , Christoph Hellwig On Thu, Sep 15, 2016 at 10:01:03AM -0700, Dan Williams wrote: > On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig wrote: > > On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote: > >> The DAX property, page cache bypass, of a VMA is only detectable via the > >> vma_is_dax() helper to check the S_DAX inode flag. However, this is > >> only available internal to the kernel and is a property that userspace > >> applications would like to interrogate. > > > > They have absolutely no business knowing such an implementation detail. > > Hasn't that train already left the station with FS_XFLAG_DAX? No, that's an admin flag, not a runtime hint for applications. Just because that flag is set on an inode, it does not mean that DAX is actually in use - it will be ignored if the backing dev is not dax capable. > The other problem with hiding the DAX property is that it turns out to > not be a transparent acceleration feature. See xfs/086 xfs/088 > xfs/089 xfs/091 which fail with DAX and, as far as I understand, it is > due to the fact that DAX disallows delayed allocation behavior. Which is not a bug, nor is it something that app developers should be surprised by. i.e. Subtle differences in error reporting behaviour occur in filesystems /all the time/. Run the test on a non-dax filesystem with an extent size hint. It fails /exactly the same way as DAX/. Run it with direct IO - fails the same way as DAX. Run it with synchronous writes - it fails the same way as DAX. IOWs, if an app can't handle the way DAX reports errors, then they are /broken/. Delayed allocation requires checking the return value of fsync() or close() to capture the allocation error - many more apps get that wrong than the ones that expect the immediate errors from write()... Anyway: to domeonstrate that the nothign is actually broken, and you might sometimes need to fix tests and send patches to fstests@vger.kernel.org, this makes xfs/086 pass for me on DAX: --- a/tests/xfs/086 +++ b/tests/xfs/086 @@ -96,7 +96,8 @@ _scratch_mount echo "+ modify files" for x in `seq 1 64`; do - $XFS_IO_PROG -f -c "pwrite -S 0x62 0 ${blksz}" "${TESTFILE}.${x}" >> $seqres.full + $XFS_IO_PROG -f -c "pwrite -S 0x62 0 ${blksz}" "${TESTFILE}.${x}" \ + >> $seqres.full 2>&1 done umount "${SCRATCH_MNT}" Cheers, Dave. -- Dave Chinner david@fromorbit.com _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs