linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Message-ID: <20181208181258.GA13075@infradead.org>
Date:   Sat, 8 Dec 2018 10:12:58 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, Jan Kara <jack@...e.cz>,
        tom@...pey.com, Al Viro <viro@...iv.linux.org.uk>, benve@...co.com,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        rcampbell@...dia.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

On Sat, Dec 08, 2018 at 10:09:26AM -0800, Dan Williams wrote:
> Another fix that may put pressure 'struct page' is resolving the
> untenable situation of dax being incompatible with reflink, i.e.
> reflink currently requires page-cache pages. Dave has talked about
> silently establishing page-cache entries when a dax-page is cow'd for
> reflink, but I wonder if we could go the other way and introduce the
> mechanism of a page belonging to multiple mappings simultaneously and
> managed by the filesystem.

FYI, I had a a prototype for DAX + reflink that didn't require
the page cache, although it badly reimplemented parts of it.  But
that was a long time ago, before we started requiring struct page
for the DAX memory.