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From: Christoph Hellwig <hch@lst.de>
To: Ralph Campbell <rcampbell@nvidia.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>,
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 jglisse@redhat.com
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount
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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).