linux-ext4 - Re: [PATCH v6 02/13] mm: remove extra ZONE

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Date:   Fri, 20 Aug 2021 06:56:27 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, hch@....de, jgg@...dia.com,
        jglisse@...hat.com
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page
 refcount

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).