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

Darrick J. Wong
2020-04-03T16:06:09Z

On Fri, Apr 03, 2020 at 09:27:31AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 02, 2020 at 01:55:19PM -0700, Ira Weiny wrote:
> > > I'd just return an error for that case, don't play silly games like
> > > evicting the inode.
> >
> > I think I agree with Christoph here. But I want to clarify. I was heading in
> > a direction of failing the ioctl completely. But we could have the flag change
> > with an appropriate error which could let the user know the change has been
> > delayed.
> >
> > But I don't immediately see what error code is appropriate for such an
> > indication. Candidates I can envision:
> >
> > EAGAIN
> > ERESTART
> > EUSERS
> > EINPROGRESS
> >
> > None are perfect but I'm leaning toward EINPROGRESS.
>
> I really, really dislike that idea. The whole point of not forcing
> evictions is to make it clear - no this inode is "busy" you can't
> do that. A reasonably smart application can try to evict itself.
>
> But returning an error and doing a lazy change anyway is straight from
> the playbook for arcane and confusing API designs.

Agreed. That's why I wrote that applications can set FS_XFLAG_DAX and
then query statx for STATX_ATTR_DAX to find out if it actually took
effect, and that if applications require it immediately they can either
create a file in a FS_XFLAG_DAX directory, or the admin can mount with
dax=always. No magic return values required or desired anywhere.

I don't know what "try to evict the inode" magic means, but I'm fairly
sure I don't want to. ;)

--D