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

[Toshi Kani]

On Wed, 2015-12-02 at 10:06 -0800, Dan Williams wrote:
> On Wed, Dec 2, 2015 at 9:01 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> > > Oh, I see.  I will setup the memmap array and run the tests again.
> > > 
> > > But, why does the PMD mapping depend on the memmap array?  We have 
> > > observed major performance improvement with PMD.  This feature should 
> > > always be enabled with DAX regardless of the option to allocate the memmap
> > > array.
> > > 
> > 
> > Several factors drove this decision, I'm open to considering
> > alternatives but here's the reasoning:
> > 
> > 1/ DAX pmd mappings caused crashes in the get_user_pages path leading
> > to commit e82c9ed41e8 "dax: disable pmd mappings".  The reason pte
> > mappings don't crash and instead trigger -EFAULT is due to the
> > _PAGE_SPECIAL pte bit.
> > 
> > 2/ To enable get_user_pages for DAX, in both the page and huge-page
> > case, we need a new pte bit _PAGE_DEVMAP.
> > 
> > 3/ Given the pte bits are hard to come I'm assuming we won't get two,
> > i.e. both _PAGE_DEVMAP and a new _PAGE_SPECIAL for pmds.  Even if we
> > could get a _PAGE_SPECIAL for pmds I'm not in favor of pursuing it.
> 
> Actually, Dave says they aren't that hard to come by for pmds, so we
> could go add _PMD_SPECIAL if we really wanted to support the limited
> page-less DAX-pmd case.
> 
> But I'm still of the opinion that we run away from the page-less case
> until it can be made a full class citizen with O_DIRECT for pfn
> support.

I may be missing something, but per vm_normal_page(), I think _PAGE_SPECIAL can
be substituted by the following check when we do not have the memmap.

	if ((vma->vm_flags & VM_PFNMAP) ||
	    ((vma->vm_flags & VM_MIXEDMAP) && (!pfn_valid(pfn)))) {

This is what I did in this patch for follow_trans_huge_pmd(), although I missed
the pfn_valid() check.

Thanks,
-Toshi
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