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

[Matthew Wilcox]

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.