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Date:   Fri, 20 Aug 2021 06:56:27 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     Felix Kuehling <felix.kuehling@amd.com>,
        Alex Sierra <alex.sierra@amd.com>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-ext4@vger.kernel.org,
        linux-xfs@vger.kernel.org, amd-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org, hch@lst.de, jgg@nvidia.com,
        jglisse@redhat.com
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page
 refcount
Message-ID: <20210820045627.GA27083@lst.de>
References: <20210813063150.2938-1-alex.sierra@amd.com> <20210813063150.2938-3-alex.sierra@amd.com> <7b821150-af18-f786-e419-ec245b8cfb1e@nvidia.com> <393e9815-838d-5fe6-d6ab-bfe7b543fef6@amd.com> <e155ed59-8c3c-4046-e731-f082ee4b10bb@nvidia.com>
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On Wed, Aug 18, 2021 at 12:28:30PM -0700, Ralph Campbell wrote:
> Did you test on a system without CONFIG_ARCH_HAS_PTE_SPECIAL defined?
> In that case, mmap() of a DAX device will call insert_page() which calls
> get_page() which would trigger VM_BUG_ON_PAGE().

__vm_insert_mixed still ends up calling insert_pfn for the
!CASE_ARCH_HAS_PTE_SPECIAL if pfn_t_devmap() is true, which it should
be for DAX.  (and as said in my other mail, I suspect we should disallow
that case anyway, as no one can test it in practice).