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Date:   Fri, 3 Apr 2020 09:27:31 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Dave Chinner <david@fromorbit.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Dan Williams <dan.j.williams@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations
 V5
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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.