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 10:06:36 -0800 [thread overview]
Message-ID: <CAPcyv4j1vA6eAtjsE=kGKeF1EqWWfR+NC7nUcRpfH_8MRqpM8Q@mail.gmail.com> (raw)
In-Reply-To: <CAPcyv4ikJ73nzQTCOfnBRThkv=rZGPM76S7=6O3LSB4kQBeEpw@mail.gmail.com>
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.
next prev parent reply other threads:[~2015-12-02 18:06 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 [this message]
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
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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