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Date: Tue, 26 Sep 2017 09:19:42 -0700
From: Jerome Glisse <jglisse@redhat.com>
To: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Matthew Wilcox <willy@linux.intel.com>,
        Hugh Dickins <hughd@google.com>, Huang Ying <ying.huang@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        James Morse <james.morse@arm.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Minchan Kim <minchan@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
        Will Deacon <will.deacon@arm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat
 device memory
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On Tue, Sep 26, 2017 at 09:47:10AM -0500, Reza Arbab wrote:
> On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
> > On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> > > The move_pages() syscall can be used to find the numa node where a page
> > > currently resides. This is not working for device public memory pages,
> > > which erroneously report -EFAULT (unmapped or zero page).
> > > 
> > > Enable by adding a FOLL_DEVICE flag for follow_page(), which
> > > move_pages() will use. This could be done unconditionally, but adding a
> > > flag seems like a safer change.
> > 
> > I do not understand purpose of this patch. What is the numa node of a
> > device memory?
> 
> Well, using hmm_devmem_pages_create() it is added to this node:
> 
> 	nid = dev_to_node(device);
> 	if (nid < 0)
> 		nid = numa_mem_id();
> 
> I understand it's minimally useful information to userspace, but the memory
> does have a nid and move_pages() is supposed to be able to return what that
> is. I ran into this using a testcase which tries to verify that user
> addresses were correctly migrated to coherent device memory.
> 
> That said, I'm okay with dropping this if you don't think it's worthwhile.

Just to add a data point, PCIE devices are tie to one CPU (architecturaly PCIE
lane are connected to CPU at least on x86/ppc AFAIK) and thus to one numa node.


Right now i am traveling but i want to check that this patch does not allow
user to inadvertaly pin device memory page. I will look into it once i am
back.

Cheers,
J?r?me