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

[Dan Williams]

On Wed, Dec 2, 2015 at 1:55 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> On Wed, 2015-12-02 at 12:54 -0800, 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.
>
> I believe we use the version of pfn_valid() in linux/mmzone.h.

Talking this over with Dave we came to the conclusion that it would be
safer to be explicit about the pmd not being mapped.  He points out
that unless a platform can guarantee that persistent memory is always
section aligned we might get false positive pfn_valid() indications.
Given the get_user_pages_fast() path is arch specific we can simply
have an arch specific pmd bit and not worry about generically enabling
a "pmd special" bit for now.
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