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Message-ID: <1449093339.9855.1.camel@hpe.com>
Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping
From: Toshi Kani <toshi.kani@hpe.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
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Date: Wed, 02 Dec 2015 14:55:39 -0700
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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.

Thanks,
-Toshi