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

[Toshi Kani <toshi.kani@hpe.com>]

On Thu, 2015-12-03 at 15:43 -0800, Dan Williams wrote:
> On Wed, Dec 2, 2015 at 1:55 PM, Toshi Kani <toshi.kani@hpe.com> 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@hpe.com>
> > > 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.

Sounds good to me.  Thanks!
-Toshi

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