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

[Dave Hansen]

On 12/02/2015 12:54 PM, Dan Williams wrote:
> 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.

With sparsemem at least, it makes sure that you're looking at a valid
_section_.  See the pfn_valid() at ~include/linux/mmzone.h:1222.
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