linux-kernel - Re: [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for ZONE

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Date:   Thu, 16 Jan 2020 01:37:12 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for
 ZONE_DEVICE pages

On Wed, Jan 15, 2020 at 01:19:41PM -0800, John Hubbard wrote:
> On 1/15/20 7:23 AM, Christoph Hellwig wrote:
> ...
> > 
> > I'm really not sold on this scheme.  Note that I think it is
> > particularly bad, but it also doesn't seem any better than what
> > we had before, and it introduced quite a bit more code.
> > 
> 
> Hi Christoph,
> 
> All by itself, yes. But the very next patch (which needs a little 
> rework for other reasons, so not included here) needs to reuse some of 
> these functions within __unpin_devmap_managed_user_page():

Well, then combine it with the series that actually does the change.

Also my vaguely recollection is that we had some idea on how to get rid
of the off by one refcounting for the zone device pages, which would be
a much better outcome.