linux-kernel - Re: [PATCH v9 7/8] xfs: support CoW in fsdax mode

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Date:   Tue, 21 Sep 2021 10:14:14 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        linux-xfs@...r.kernel.org, dan.j.williams@...el.com,
        david@...morbit.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
        rgoldwyn@...e.de, viro@...iv.linux.org.uk, willy@...radead.org
Subject: Re: [PATCH v9 7/8] xfs: support CoW in fsdax mode

On Fri, Sep 17, 2021 at 08:33:04AM -0700, Darrick J. Wong wrote:
> > More importantly before we can merge this series we also need the VM
> > level support for reflink-aware reverse mapping.  So while this series
> > here is no in a good enough shape I don't see how we could merge it
> > without that other series as we'd have to disallow mmap for reflink+dax
> > files otherwise.
> 
> I've forgotten why we need mm level reverse mapping again?  The pmem
> poison stuff can use ->media_failure (or whatever it was called,
> memory_failure?) to find all the owners and notify them.  Was there
> some other accounting reason that fell out of my brain?
> 
> I'm more afraid of 'sharing pages between files needs mm support'
> sparking another multi-year folioesque fight with the mm people.

Because of the way page->mapping is used by DAX.  But I think this is
mostly under control in the other series.