From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f200.google.com (mail-io0-f200.google.com [209.85.223.200]) by kanga.kvack.org (Postfix) with ESMTP id B5FAB6B0069 for ; Thu, 15 Sep 2016 19:07:58 -0400 (EDT) Received: by mail-io0-f200.google.com with SMTP id c31so17128755iod.2 for ; Thu, 15 Sep 2016 16:07:58 -0700 (PDT) Received: from ipmail05.adl6.internode.on.net (ipmail05.adl6.internode.on.net. [150.101.137.143]) by mx.google.com with ESMTP id i10si25893823oif.106.2016.09.15.16.07.56 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: Christoph Hellwig , Linux MM , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" , Nicholas Piggin , XFS Developers , linux-fsdevel 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org