linux-kernel - Re: [RFC 0/8] Define coherent device memory node

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Message-Id: <878tt96nxr.fsf@linux.vnet.ibm.com>
Date:   Fri, 28 Oct 2016 10:59:52 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Jerome Glisse <j.glisse@...il.com>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        js1304@...il.com, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, akpm@...ux-foundation.org,
        bsingharora@...il.com
Subject: Re: [RFC 0/8] Define coherent device memory node

Jerome Glisse <j.glisse@...il.com> writes:

> On Wed, Oct 26, 2016 at 04:39:19PM +0530, Aneesh Kumar K.V wrote:
>> Jerome Glisse <j.glisse@...il.com> writes:
>> 
>> > On Tue, Oct 25, 2016 at 09:56:35AM +0530, Aneesh Kumar K.V wrote:
>> >> Jerome Glisse <j.glisse@...il.com> writes:
>> >> 
>> >> > On Mon, Oct 24, 2016 at 10:01:49AM +0530, Anshuman Khandual wrote:
>> >> >
>> >> I looked at the hmm-v13 w.r.t migration and I guess some form of device
>> >> callback/acceleration during migration is something we should definitely
>> >> have. I still haven't figured out how non addressable and coherent device
>> >> memory can fit together there. I was waiting for the page cache
>> >> migration support to be pushed to the repository before I start looking
>> >> at this closely.
>> >> 
>> >
>> > The page cache migration does not touch the migrate code path. My issue with
>> > page cache is writeback. The only difference with existing migrate code is
>> > refcount check for ZONE_DEVICE page. Everything else is the same.
>> 
>> What about the radix tree ? does file system migrate_page callback handle
>> replacing normal page with ZONE_DEVICE page/exceptional entries ?
>> 
>
> It use the exact same existing code (from mm/migrate.c) so yes the radix tree
> is updated and buffer_head are migrated.
>

I looked at the the page cache migration patches shared and I find that
you are not using exceptional entries when we migrate a page cache page to
device memory. But I am now not sure how a read from page cache will
work with that.

ie, a file system read will now find the page in page cache. But we
cannot do a copy_to_user of that page because that is now backed by an
unaddressable memory right ?

do_generic_file_read() does
      page = find_get_page(mapping, index);
      ....
      ret = copy_page_to_iter(page, offset, nr, iter);

which does
	void *kaddr = kmap_atomic(page);
	size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
	kunmap_atomic(kaddr);


-aneesh