Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs

From	Dan Williams <dan.j.williams@intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs
Date	2016-09-16 04:10 +0200
Message-ID	<shQLf-6I2-1@gated-at.bofh.it> (permalink)
References	(2 earlier) <shAds-4BB-21@gated-at.bofh.it> <shIkF-1p0-1@gated-at.bofh.it> <shNX4-4Wt-21@gated-at.bofh.it> <shP2N-5Du-5@gated-at.bofh.it> <shQ8x-6fA-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On Thu, Sep 15, 2016 at 6:24 PM, Dave Chinner <david@fromorbit.com> wrote:
> 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@fromorbit.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@lst.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.

Right, I understand that, but how does an admin audit those "bunch of
other things" that actually gate whether DAX ends up being used in
practice?  There's currently no way for userspace to observe that a
file with FS_XFLAG_DAX actually results in a change in mmap behavior.

Thread

[PATCH v2 0/3] mm,  dax: export dax capabilities and mapping size info to userspace Dan Williams <dan.j.williams@intel.com> - 2016-09-15 09:00 +0200
  [PATCH v2 1/3] mm, dax: add VM_SYNC flag for device-dax VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-15 09:00 +0200
  [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-15 09:00 +0200
    Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Christoph Hellwig <hch@lst.de> - 2016-09-15 10:30 +0200
      Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-15 19:10 +0200
        Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs "Darrick J. Wong" <darrick.wong@oracle.com> - 2016-09-15 19:20 +0200
          Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-15 19:50 +0200
        Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dave Chinner <david@fromorbit.com> - 2016-09-16 01:10 +0200
          Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-16 01:20 +0200
          Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-16 02:20 +0200
            Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dave Chinner <david@fromorbit.com> - 2016-09-16 03:30 +0200
              Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-16 04:10 +0200
                Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-16 05:50 +0200
                Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dave Chinner <david@fromorbit.com> - 2016-09-16 07:40 +0200
                Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs Dan Williams <dan.j.williams@intel.com> - 2016-09-16 12:50 +0200

csiph-web