From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id BCA3D6B0038 for ; Thu, 15 Sep 2016 13:01:28 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id x203so5399101oia.3 for ; Thu, 15 Sep 2016 10:01:28 -0700 (PDT) Received: from mail-oi0-x22b.google.com (mail-oi0-x22b.google.com. [2607:f8b0:4003:c06::22b]) by mx.google.com with ESMTPS id 34si1943715oto.123.2016.09.15.10.01.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 15 Sep 2016 10:01:04 -0700 (PDT) Received: by mail-oi0-x22b.google.com with SMTP id q188so78606237oia.3 for ; Thu, 15 Sep 2016 10:01:04 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20160915082615.GA9772@lst.de> 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> From: Dan Williams Date: Thu, 15 Sep 2016 10:01:03 -0700 Message-ID: Subject: Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Hellwig Cc: Linux MM , "linux-nvdimm@lists.01.org" , david , "linux-kernel@vger.kernel.org" , Nicholas Piggin , XFS Developers , linux-fsdevel 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? 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. If behavior changes I think we should indicate that to userspace and VM_DAX is certainly more useful to userspace than some of the other vm internals we already export in those flags. -- 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