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

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Date:   Thu, 2 Apr 2020 10:53:27 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...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 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.