Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

Ira Weiny
2020-04-03T04:39:07Z

On Wed, Apr 01, 2020 at 04:00:21AM +0000, Darrick J. Wong wrote:
> On Mon, Mar 16, 2020 at 10:55:09AM +0100, Christoph Hellwig wrote:
> > On Mon, Mar 16, 2020 at 10:52:24AM +0100, Jan Kara wrote:
> > > > This sounds reasonable to me.
> > > >
> > > > As for deprecating the mount option, I think at a minimum it needs to
> > > > continue be accepted as an option even if it is ignored to not break
> > > > existing setups.
> > >
> > > Agreed. But that's how we usually deprecate mount options. Also I'd say
> > > that statx() support for reporting DAX state and some education of
> > > programmers using DAX is required before we deprecate the mount option
> > > since currently applications check 'dax' mount option to determine how much
> > > memory they need to set aside for page cache before they consume everything
> > > else on the machine...
> >
> > I don't even think we should deprecate it. It isn't painful to maintain
> > and actually useful for testing. Instead we should expand it into a
> > tristate:
> >
> > dax=off
> > dax=flag
> > dax=always
> >
> > where the existing "dax" option maps to "dax=always" and nodax maps
> > to "dax=off". and dax=flag becomes the default for DAX capable devices.
>
> That works for me. In summary:
>
> - Applications must call statx to discover the current S_DAX state.
>
> - There exists an advisory file inode flag FS_XFLAG_DAX that can be
> changed on files that have no blocks allocated to them. Changing
> this flag does not necessarily change the S_DAX state immediately
> but programs can query the S_DAX state via statx.
>
> If FS_XFLAG_DAX is set and the fs is on pmem then it will always
> enable S_DAX at inode load time; if FS_XFLAG_DAX is not set, it will
> never enable S_DAX. Unless overridden...
>
> - There exists a dax= mount option. dax=off means "never set S_DAX,
> ignore FS_XFLAG_DAX"; dax=always means "always set S_DAX (at least on
> pmem), ignore FS_XFLAG_DAX"; and dax=iflag means "follow FS_XFLAG_DAX"
> and is the default. "dax" by itself means "dax=always". "nodax"
> means "dax=off".
>
> - There exists an advisory directory inode flag FS_XFLAG_DAX that can
> be changed at any time. The flag state is copied into any files or
> subdirectories created within that directory. If programs require
> that file access runs in S_DAX mode, they'll have to create those
> files themselves inside a directory with FS_XFLAG_DAX set, or mount
> the fs with dax=always.

One other thing to add here. They _can_ set the FS_XFLAG_DAX on a file with
data and force an eviction to get S_DAX to change.

I think that is a nice reason to have a different error code returned.

>
> Ok? Let's please get this part finished for 5.8, then we can get back
> to arguing about fs-rmap and reflink and dax and whatnot.

I'm happy to see you motivated to get this in.

I'm starting with a new xfstest to make sure we agree on the semantics prior to
more patches. I hope to have the xfstest patch sent tomorrow sometime.

Ira

>
> --D