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

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani <toshi.kani@hpe.com> 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.

End result is that DAX pmd mappings must be fully enabled through the
get_user_pages paths with _PAGE_DEVMAP or turned off completely.  In
general I think the "page less" DAX implementation was a good starting
point, but we need to shift to page-backed by default until we can
teach more of the kernel to operate on bare pfns.  That "default" will
need to be enforced by userspace tooling.

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