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From:   Jerome Glisse <j.glisse@gmail.com>
Date:   Thu, 19 Aug 2021 23:33:50 -0700
Message-ID: <CAH3drwYwFoPOmStOkimdmJds=0AnCjXviLOA3mxZPkSow3qTFA@mail.gmail.com>
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Christoph Hellwig <hch@lst.de>, Alex Sierra <alex.sierra@amd.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kuehling, Felix" <Felix.Kuehling@amd.com>, linux-mm@kvack.org,
        Ralph Campbell <rcampbell@nvidia.com>,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>, jgg@nvidia.com,
        Jerome Glisse <jglisse@redhat.com>
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Note that you do not want GUP to succeed on device page, i do not see
where that is handled in the new code.

On Sun, Aug 15, 2021 at 1:40 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 8/15/21 8:37 AM, Christoph Hellwig wrote:
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index 8ae31622deef..d48a1f0889d1 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -1218,7 +1218,7 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page, int refs,
> >>   static inline __must_check bool try_get_page(struct page *page)
> >>   {
> >>      page = compound_head(page);
> >> -    if (WARN_ON_ONCE(page_ref_count(page) <= 0))
> >> +    if (WARN_ON_ONCE(page_ref_count(page) < (int)!is_zone_device_page(page)))
> >
> > Please avoid the overly long line.  In fact I'd be tempted to just not
> > bother here and keep the old, more lose check.  Especially given that
> > John has a patch ready that removes try_get_page entirely.
> >
>
> Yes. Andrew has accepted it into mmotm.
>
> Ralph's patch here was written well before my cleanup that removed
> try_grab_page() [1]. But now that we're here, if you drop this hunk then
> it will make merging easier, I think.
>
>
> [1] https://lore.kernel.org/r/20210813044133.1536842-4-jhubbard@nvidia.com
>
> thanks,
> --
> John Hubbard
> NVIDIA
>