From: Dan Williams <dan.j.williams@intel.com>
To: Toshi Kani <toshi.kani@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Matthew Wilcox <willy@linux.intel.com>,
Ross Zwisler <ross.zwisler@linux.intel.com>,
mauricio.porto@hpe.com, Linux MM <linux-mm@kvack.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping
Date: Wed, 2 Dec 2015 11:57:55 -0800 [thread overview]
Message-ID: <CAPcyv4hvX_s3xN9UZ69v7npOhWVFehfGDPZG1MsDmKWBk4Gq1A@mail.gmail.com> (raw)
In-Reply-To: <1449087125.31589.45.camel@hpe.com>
On Wed, Dec 2, 2015 at 12:12 PM, Toshi Kani <toshi.kani@hpe.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@hpe.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@intel.com>
>> > > > wrote:
>> > > > > 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.
>> > > >
>> > > > 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.
next prev parent reply other threads:[~2015-12-02 19:57 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-23 20:04 [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping Toshi Kani
2015-11-23 20:53 ` Dan Williams
2015-11-23 22:15 ` Toshi Kani
2015-11-30 22:08 ` Dan Williams
2015-12-02 2:19 ` Toshi Kani
2015-12-02 3:45 ` Dan Williams
2015-12-02 17:43 ` Toshi Kani
2015-12-02 17:01 ` Dan Williams
2015-12-02 18:06 ` Dan Williams
2015-12-02 19:26 ` Toshi Kani
2015-12-02 19:00 ` Dan Williams
2015-12-02 20:02 ` Toshi Kani
2015-12-02 20:12 ` Toshi Kani
2015-12-02 19:57 ` Dan Williams [this message]
2015-12-02 21:37 ` Toshi Kani
2015-12-02 20:54 ` Dan Williams
2015-12-02 21:55 ` Toshi Kani
2015-12-03 23:43 ` Dan Williams
2015-12-04 16:55 ` Toshi Kani
2015-12-02 22:00 ` Dave Hansen
2015-12-02 22:03 ` Dan Williams
2015-12-02 22:09 ` Dave Hansen
2015-12-03 0:21 ` Toshi Kani
2015-12-02 23:33 ` Dan Williams
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