Re: [PATCH 2/2] dax: move writeback calls into the filesystems

[Dan Williams]

On Mon, Feb 8, 2016 at 12:18 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
[..]
>> Setting aside the current block zeroing problem you seem to assuming
>> that DAX will always be faster and that may not be true at a media
>> level.  Waiting years for some applications to determine if DAX makes
>> sense for their use case seems completely reasonable.  In the meantime
>> the apps that are already making these changes want to know that a DAX
>> mapping request has not silently dropped backed to page cache.  They
>> also want to know if they successfully jumped through all the hoops to
>> get a larger than pte mapping.
>>
>> I agree it is useful to be able to force DAX on an unmodified
>> application to see what happens, and it follows that if those
>> applications want to run in that mode they will need functional
>> fsync()...
>>
>> I would feel better if we were talking about specific applications and
>> performance numbers to know if forcing DAX on application is a debug
>> facility or a production level capability.  You seem to have already
>> made that determination and I'm curious what I'm missing.
>
> I'm not setting any policy here at all.  This whole argument is
> based around the DAX mount option doing "global fs enable or
> silently turning it off" and the application not knowing about that.
>
> The whole point of having a persistent per-inode DAX flags is that
> it is a policy mechanism, not a policy.  The application can, if it
> is DAX aware, directly control whether DAX is used on a file or not.
> The application can even query and clear that persistent inode flag
> if it is configured not to (or cannot) use DAX.
>
> If the filesystem cannot support DAX, then we can error out attempts
> to set the DAX flag and then the app knows DAX is not available.
> i.e. the attempt to set policy failed. If the flag is set, then the
> inode will *always* use DAX - there is no "fall back to page cache"
> when DAX is enabled.
>
> If the applicaiton is not DAX aware, then the admin can control the
> DAX policy by manipulating these flags themselves, and hence control
> whether DAX is used by the application or not.
>
> If you think I'm dictating policy for DAX users and application,
> then you haven't understood anything I've previously said about why
> the DAX mount option needs to die before any of this is considered
> production ready. DAX is not an opaque "all or nothing" option. XFS
> will provide apps and admins with fine-grained, persistent,
> discoverable policy flags to allow admins and applications to set
> DAX policies however they see fit. This simply cannot be done if the
> only knob you have is a mount option that may or may not stick.

I agree the mount option needs to die, and I fully grok the reasoning.
  What I'm concerned with is that a system using fully-DAX-aware
applications is forced to incur the overhead of maintaining *sync
semantics, periodic sync(2) in particular,  even if it is not relying
on those semantics.

However, like I said in my other mail, we can solve that with
alternate interfaces to persistent memory if that becomes an issue and
not require that "disable *sync" capability to come through DAX.