Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

[Dan Williams]

On Wed, Dec 2, 2015 at 1:37 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> On Wed, 2015-12-02 at 11:57 -0800, Dan Williams wrote:
[..]
>> The whole point of __get_user_page_fast() is to avoid the overhead of
>> taking the mm semaphore to access the vma.  _PAGE_SPECIAL simply tells
>> __get_user_pages_fast that it needs to fallback to the
>> __get_user_pages slow path.
>
> I see.  Then, I think gup_huge_pmd() can simply return 0 when !pfn_valid(),
> instead of VM_BUG_ON.

Is pfn_valid() a reliable check?  It seems to be based on a max_pfn
per node... what happens when pmem is located below that point.  I
haven't been able to convince myself that we won't get false
positives, but maybe I'm missing something.
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