linux-kernel - Re: [RFC PATCH 0/8] fsdax: introduce FS query interface to support reflink

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Message-ID: <20200807133857.GC17456@casper.infradead.org>
Date:   Fri, 7 Aug 2020 14:38:57 +0100
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] fsdax: introduce FS query interface to support
 reflink

On Fri, Aug 07, 2020 at 09:13:28PM +0800, Shiyang Ruan wrote:
> This patchset is a try to resolve the problem of tracking shared page
> for fsdax.
> 
> Instead of per-page tracking method, this patchset introduces a query
> interface: get_shared_files(), which is implemented by each FS, to
> obtain the owners of a shared page.  It returns an owner list of this
> shared page.  Then, the memory-failure() iterates the list to be able
> to notify each process using files that sharing this page.
> 
> The design of the tracking method is as follow:
> 1. dax_assocaite_entry() associates the owner's info to this page

I think that's the first problem with this design.  dax_associate_entry is
a horrendous idea which needs to be ripped out, not made more important.
It's all part of the general problem of trying to do something on a
per-page basis instead of per-extent basis.