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Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()
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From: John Hubbard <jhubbard@nvidia.com>
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Date: Tue, 19 Jun 2018 18:34:12 -0700
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>, Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <john.hubbard@gmail.com>, Michal Hocko <mhocko@kernel.org>, Christopher Lameter <cl@linux.com>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, linux-rdma <linux-rdma@vger.kernel.org>

On 06/19/2018 06:24 PM, Dan Williams wrote:
> On Tue, Jun 19, 2018 at 11:11 AM, John Hubbard <jhubbard@nvidia.com> wrote:
>> On 06/19/2018 03:41 AM, Jan Kara wrote:
>>> On Tue 19-06-18 02:02:55, Matthew Wilcox wrote:
>>>> On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote:
> [..]
>>> And then there's the aspect that both these approaches are a bit too
>>> heavyweight for some get_user_pages_fast() users (e.g. direct IO) - Al Viro
>>> had an idea to use page lock for that path but e.g. fs/direct-io.c would have
>>> problems due to lock ordering constraints (filesystem ->get_block would
>>> suddently get called with the page lock held). But we can probably leave
>>> performance optimizations for phase two.
>>
>>
>> So I assume that phase one would be to apply this approach only to
>> get_user_pages_longterm. (Please let me know if that's wrong.)
> 
> I think that's wrong, because get_user_pages_longterm() is only a
> filesystem-dax avoidance mechanism, it's not trying to address all the
> problems that Jan is talking about. I don't see any viable half-step
> solutions.
> 

OK, but in that case, I'm slightly confused by Jan's comment above, about leaving 
performance optimizations until phase two. Because that *is* a half-step approach: 
phase one, phase two.  

Are you disagreeing with Jan, or are you suggesting "fix get_user_pages first, and
leave get_user_pages_fast alone for now?" 

Or something else?