Date: Tue, 26 Sep 2017 13:35:22 -0500
From: Reza Arbab <arbab@linux.vnet.ibm.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: 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>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.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 04:32:41PM +0000, Michal Hocko wrote:
>On Tue 26-09-17 09:47:10, 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();
>
>OK, but do all the HMM devices have concept of NUMA affinity? From the
>code you are pasting they do not have to...

I don't know the definitive answer here, but as Jerome said PCIE devices 
should, and we are heading that way with NVLink/CAPI as well. It seems 
the default is just the nearest node.

>> 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.
>
>I am just worried that we allow information which is not generally
>sensible and I am also not sure what the userspace can actually do with
>that information.

As mentioned, it is minimally useful, e.g. for verifying migration, so 
returning the nid seems sensible to me. Alternatively, we might at least 
change the documentation to say 

-EFAULT
    This is a zero page, a device page, or the memory area is not mapped by the process.
			 ^^^^^^^^^^^^^

-- 
Reza Arbab