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Date: Tue, 29 Dec 2015 11:23:26 +0800
From: Bob Liu <bob.liu@oracle.com>
MIME-Version: 1.0
Subject: Re: [-mm PATCH v4 00/18] get_user_pages() for dax pte and pmd mappings
References: <20151221054406.34542.64393.stgit@dwillia2-desk3.jf.intel.com> <CAA_GA1f44ADq7dw7LUM=rEex8m0vMXvGeOdW1YKkisbv51iuKw@mail.gmail.com> <CAPcyv4j5QRAy-pM=TcCVrY8tH8H7iOL36KojZOeHKuLdBOcwDg@mail.gmail.com>
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Bob Liu <lliubbo@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Dave Hansen <dave@sr71.net>, David Airlie <airlied@linux.ie>, Dave Hansen <dave.hansen@linux.intel.com>, Dave Chinner <david@fromorbit.com>, Linux-MM <linux-mm@kvack.org>, "H. Peter Anvin" <hpa@zytor.com>, Christoph Hellwig <hch@lst.de>, Andrea Arcangeli <aarcange@redhat.com>, kbuild test robot <lkp@intel.com>, "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>, X86 ML <x86@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>, Matthew Wilcox <willy@linux.intel.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, Alexander Viro <viro@zeniv.linux.org.uk>, Thomas Gleixner <tglx@linutronix.de>, Christoffer Dall <christoffer.dall@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Logan Gunthorpe <logang@deltatee.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>


On 12/28/2015 02:55 AM, Dan Williams wrote:
> On Sun, Dec 27, 2015 at 12:33 AM, Bob Liu <lliubbo@gmail.com> wrote:
>> Hey Dan,
>>
> [..]
>> What about space for page tables?
>> Page tables(mapping all memory in PMEM to virtual address space) may
>> also consume significantly DRAM space if  huge page is not enabled or
>> split.
>> Should we also consider to allocate pte page tables from PMEM in future?
> 
> On x86_64 these ranges are covered by gigabyte pages by default (see
> init_memory_mapping()).  I don't see much incremental benefit from
> allocating pte's from pmem.
> 

Oh, that's the direct mapping. I mean ptes consumed in:
__dax_fault > dax_insert_mapping > insert_pfn > __get_locked_pte

In some bad situations e.g mmap a lot of large ext4 files exist in pmem but
hugepage can't be used because of fragmentation, then the consumption of pte
page tables may can't be ignored?
Anyway, this is not a blocker of these patches.
Thanks,
Bob

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