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

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Message-ID: <6335fd87adc7f_f6c9d29474@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Thu, 29 Sep 2022 13:18:15 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Alistair Popple <apopple@...dia.com>,
        Jason Gunthorpe <jgg@...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

Alistair Popple wrote:
> 
> Jason Gunthorpe <jgg@...dia.com> writes:
> 
> > 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?
> 
> free_zone_device_page() shouldn't be called for pgmap->type ==
> MEMORY_DEVICE_FS_DAX so I don't think we should have to worry about DAX
> there. Except that the folio code looks like it might have introduced a
> bug. AFAICT put_page() always calls
> put_devmap_managed_page(&folio->page) but folio_put() does not (although
> folios_put() does!). So it seems folio_put() won't end up calling
> __put_devmap_managed_page_refs() as I think it should.
> 
> I think you're right about the change to __init_zone_device_page() - I
> should limit it to DEVICE_PRIVATE/COHERENT pages only. But I need to
> look at Dan's patch series more closely as I suspect it might be better
> to rebase this patch on top of that.

Apologies for the delay I was travelling the past few days. Yes, I think
this patch slots in nicely to avoid the introduction of an init_mode
[1]:

https://lore.kernel.org/nvdimm/166329940343.2786261.6047770378829215962.stgit@dwillia2-xfh.jf.intel.com/

Mind if I steal it into my series?