linux-kernel - Re: [PATCH 2/7] mm: Free device private pages have zero refcount

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Message-ID: <YzG42766BJSxro0R@nvidia.com>
Date:   Mon, 26 Sep 2022 11:36:11 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Alex Sierra <alex.sierra@....com>,
        John Hubbard <jhubbard@...dia.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 2/7] mm: Free device private pages have zero refcount

On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple wrote:
> Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page
> refcount") device private pages have no longer had an extra reference
> count when the page is in use. However before handing them back to the
> owning device driver we add an extra reference count such that free
> pages have a reference count of one.
> 
> This makes it difficult to tell if a page is free or not because both
> free and in use pages will have a non-zero refcount. Instead we should
> return pages to the drivers page allocator with a zero reference count.
> Kernel code can then safely use kernel functions such as
> get_page_unless_zero().
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c       | 1 +
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 +
>  drivers/gpu/drm/nouveau/nouveau_dmem.c   | 1 +
>  lib/test_hmm.c                           | 1 +
>  mm/memremap.c                            | 5 -----
>  mm/page_alloc.c                          | 6 ++++++
>  6 files changed, 10 insertions(+), 5 deletions(-)

I think this is a great idea, but I'm surprised no dax stuff is
touched here?

Jason