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

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Date:	Wed, 2 Dec 2015 11:57:55 -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 12:12 PM, Toshi Kani <toshi.kani@....com> wrote:
> On Wed, 2015-12-02 at 13:02 -0700, Toshi Kani wrote:
>> On Wed, 2015-12-02 at 11:00 -0800, Dan Williams wrote:
>> > 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.
>>
>> __get_user_page_fast already refers current->mm, so we should be able to get
>> the vma, and pass it down to gup_pud_range().
>
> Alternatively, we can obtain the vma from current->mm in gup_huge_pmd() when the
> !pfn_valid() condition is met, so that we do not add the code to the main path
> of __get_user_pages_fast.

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.
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