linux-kernel - Re: Question about the "EXPERIMENTAL" tag for dax in XFS

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Date:   Fri, 26 Feb 2021 11:24:53 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     "ruansy.fnst@...itsu.com" <ruansy.fnst@...itsu.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "jack@...e.cz" <jack@...e.cz>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        "david@...morbit.com" <david@...morbit.com>,
        "hch@....de" <hch@....de>, "rgoldwyn@...e.de" <rgoldwyn@...e.de>,
        "y-goto@...itsu.com" <y-goto@...itsu.com>,
        "qi.fuli@...itsu.com" <qi.fuli@...itsu.com>,
        "fnstml-iaas@...fujitsu.com" <fnstml-iaas@...fujitsu.com>
Subject: Re: Question about the "EXPERIMENTAL" tag for dax in XFS

On Fri, Feb 26, 2021 at 11:05 AM Darrick J. Wong <djwong@...nel.org> wrote:
>
> On Fri, Feb 26, 2021 at 09:45:45AM +0000, ruansy.fnst@...itsu.com wrote:
> > Hi, guys
> >
> > Beside this patchset, I'd like to confirm something about the
> > "EXPERIMENTAL" tag for dax in XFS.
> >
> > In XFS, the "EXPERIMENTAL" tag, which is reported in waring message
> > when we mount a pmem device with dax option, has been existed for a
> > while.  It's a bit annoying when using fsdax feature.  So, my initial
> > intention was to remove this tag.  And I started to find out and solve
> > the problems which prevent it from being removed.
> >
> > As is talked before, there are 3 main problems.  The first one is "dax
> > semantics", which has been resolved.  The rest two are "RMAP for
> > fsdax" and "support dax reflink for filesystem", which I have been
> > working on.
>
> <nod>
>
> > So, what I want to confirm is: does it means that we can remove the
> > "EXPERIMENTAL" tag when the rest two problem are solved?
>
> Yes.  I'd keep the experimental tag for a cycle or two to make sure that
> nothing new pops up, but otherwise the two patchsets you've sent close
> those two big remaining gaps.  Thank you for working on this!
>
> > Or maybe there are other important problems need to be fixed before
> > removing it?  If there are, could you please show me that?
>
> That remains to be seen through QA/validation, but I think that's it.
>
> Granted, I still have to read through the two patchsets...

I've been meaning to circle back here as well.

My immediate concern is the issue Jason recently highlighted [1] with
respect to invalidating all dax mappings when / if the device is
ripped out from underneath the fs. I don't think that will collide
with Ruan's implementation, but it does need new communication from
driver to fs about removal events.

[1]: http://lore.kernel.org/r/CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@mail.gmail.com