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

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Sep 15, 2016 at 10:36 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Thu, Sep 15, 2016 at 07:04:27PM -0700, Dan Williams wrote:
>> 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"
>
> Filesystem mounts checks all the various stuff that determines
> whether DAX can be used. It logs to the console that it is "Dax
> capable". Any file that then has FS_XFLAG_DAX set will result in DAX
> being used. There is no other possibility when these two things are
> reported.
>
> /me points at runtime diagnostic tracepoints like
> trace_xfs_file_dax_read() and notes that dax is sadly lacking in
> diagnostic tracepoints.
>
> Besides, userspace can't do anything useful with this information,
> because the FS_XFLAG_DAX can be changed /at any time/ by an admin.
> And the filesystem is free to remove it at any time, too, if it
> needs to (e.g. file gets reflinked or snapshotted).
>
> That's right, an inode can dynamically change from DAX to non-DAX
> underneath the application, and the application /will not notice/.
> That's because changing the flag will sync and invalidate the
> existing mappings and the next application access will simply fault
> it back in using whatever mechanism the inode is now configured
> with.
>
> Plain and simple: userspace has absolutely no fucking idea of
> whether DAX is enabled or not, and whatever the kernel returns to
> userspace above the DAX configuration is stale before it even got
> out of the kernel....

smaps is already known to be an ephemeral interface, but we output
useful information there nonetheless.

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