linux-kernel - Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink

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Date:   Mon, 27 Apr 2020 05:28:36 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Shiyang Ruan <ruansy.fnst@...fujitsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, darrick.wong@...cle.com,
        dan.j.williams@...el.com, david@...morbit.com, hch@....de,
        rgoldwyn@...e.de, qi.fuli@...itsu.com, y-goto@...itsu.com
Subject: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink

On Mon, Apr 27, 2020 at 04:47:42PM +0800, Shiyang Ruan wrote:
> This patchset is a try to resolve the shared 'page cache' problem for
> fsdax.
> 
> In order to track multiple mappings and indexes on one page, I
> introduced a dax-rmap rb-tree to manage the relationship.  A dax entry
> will be associated more than once if is shared.  At the second time we
> associate this entry, we create this rb-tree and store its root in
> page->private(not used in fsdax).  Insert (->mapping, ->index) when
> dax_associate_entry() and delete it when dax_disassociate_entry().

Do we really want to track all of this on a per-page basis?  I would
have thought a per-extent basis was more useful.  Essentially, create
a new address_space for each shared extent.  Per page just seems like
a huge overhead.