linux-kernel - Re: [PATCH 7/8] mm: remove the extra ZONE

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Message-ID: <20220209122956.GI49147@ziepe.ca>
Date:   Wed, 9 Feb 2022 08:29:56 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>, nouveau@...ts.freedesktop.org,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/8] mm: remove the extra ZONE_DEVICE struct page refcount

On Wed, Feb 09, 2022 at 07:23:45AM +0100, Christoph Hellwig wrote:
> On Tue, Feb 08, 2022 at 07:30:11PM -0800, Dan Williams wrote:
> > Interesting. I had expected that to really fix the refcount problem
> > that fs/dax.c would need to start taking real page references as pages
> > were added to a mapping, just like page cache.
> 
> I think we should do that eventually.  But I think this series that
> just attacks the device private type and extends to the device coherent
> and p2p enhacements is a good first step to stop the proliferation of
> the one off refcount and to allow to deal with the fsdax pages in another
> more focuessed series.

It is nice, but the other series are still impacted by the fsdax mess
- they still stuff pages into ptes without proper refcounts and have
to carry nonsense to dance around this problem.

I certainly would be unhappy if the amd driver, for instance, gained
the fsdax problem as well and started pushing 4k pages into PMDs.

Jason