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

[Darrick J. Wong]

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