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

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Date:	Wed, 2 Dec 2015 11:00:49 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	mauricio.porto@....com, Linux MM <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

On Wed, Dec 2, 2015 at 11:26 AM, Toshi Kani <toshi.kani@....com> wrote:
> 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@...el.com> wrote:
>> > On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani <toshi.kani@....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.
>>
>> 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.

That works for __get_user_pages but not __get_user_pages_fast where we
don't have access to the vma.
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