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

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Date:   Thu, 2 Apr 2020 13:55:19 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        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 Thu, Apr 02, 2020 at 10:53:27AM +0200, Christoph Hellwig wrote:
> On Wed, Apr 01, 2020 at 12:25:11PM +0200, Jan Kara wrote:
> > >  - Applications must call statx to discover the current S_DAX state.
> > > 
> > >  - There exists an advisory file inode flag FS_XFLAG_DAX that can be
> > >    changed on files that have no blocks allocated to them.  Changing
> > >    this flag does not necessarily change the S_DAX state immediately
> > >    but programs can query the S_DAX state via statx.
> > 
> > I generally like the proposal but I think the fact that toggling
> > FS_XFLAG_DAX will not have immediate effect on S_DAX will cause quite some
> > confusion and ultimately bug reports. I'm thinking whether we could somehow
> > improve this... For example an ioctl that would try to make set inode flags
> > effective by evicting the inode (and returning EBUSY if the eviction is
> > impossible for some reason)?
> 
> 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.

Ira