From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 16 Sep 2016 11:24:59 +1000 From: Dave Chinner To: Dan Williams Cc: Christoph Hellwig , Linux MM , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" , Nicholas Piggin , XFS Developers , linux-fsdevel Subject: Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Message-ID: <20160916012458.GW22388@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> <20160915230748.GS30497@dastard> 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: On Thu, Sep 15, 2016 at 05:16:42PM -0700, Dan Williams wrote: > On Thu, Sep 15, 2016 at 4:07 PM, Dave Chinner wrote: > > 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. > > What's the point of an admin flag if an admin can't do cat /proc/ of interest>/smaps, or some other mechanism, to validate that the > setting the admin cares about is in effect? Sorry, I don't follow - why would you be looking at mapping file regions in /proc to determine if some file somewhere in a filesystem has a specific flag set on it or not? FS_XFLAG_DAX is an inode attribute flag, not something you can query or administrate through mmap: I.e. # xfs_io -c "lsattr" -c "chattr +x" -c lsattr -c "chattr -x" -c "lsattr" foo --------------- foo --------------x foo --------------- foo # What happens when that flag is set on an inode is determined by a whole bunch of other things that are completely separate to the management of the inode flag itself. 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