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

[Christoph Hellwig]

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.