linux-kernel - Re: [PATCH v2 2/3] mm, dax: add VM

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Message-ID: <20160916012458.GW22388@dastard>
Date:   Fri, 16 Sep 2016 11:24:59 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>, Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        XFS Developers <xfs@....sgi.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs

On Thu, Sep 15, 2016 at 05:16:42PM -0700, Dan Williams wrote:
> On Thu, Sep 15, 2016 at 4:07 PM, Dave Chinner <david@...morbit.com> 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 <hch@....de> 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/<pid
> 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@...morbit.com