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

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Date:   Mon, 10 Aug 2020 12:16:57 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ruan Shiyang <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 Mon, Aug 10, 2020 at 04:15:50PM +0800, Ruan Shiyang wrote:
> 
> 
> On 2020/8/7 下午9:38, Matthew Wilcox wrote:
> > 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.
> > 
> 
> The memory-failure needs to track owners info from a dax page, so I should
> associate the owner with this page.  In this version, I associate the block
> device to the dax page, so that the memory-failure is able to iterate the
> owners by the query interface provided by filesystem.

No, it doesn't need to track owner info from a DAX page.  What it needs to
do is ask the filesystem.