linux-ext4 - Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

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Date:   Fri, 3 Apr 2020 09:05:44 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Dave Chinner <david@...morbit.com>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

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