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

[Dan Williams]

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.
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